Guidance

Laboratory guide to screening for CHT in the UK

Updated 8 October 2024

This handbook is for laboratories that provide a newborn blood spot (NBS) screening service for congenital hypothyroidism (CHT) in the UK. It defines a framework for the pre-analytical, analytical and post-analytical steps in the newborn screening process so as to:

• improve consistency across the screening programme
• provide guidance on achieving good quality by application of standards and audit

You should use it alongside other NHS NBS screening programme guidance.

1. Screening for CHT

All babies across the UK are offered a blood test to identify CHT as part of the NBS screening programme. Screening for CHT aims to detect infants who do not produce adequate thyroxine from birth because:

• their thyroid gland has not developed normally (thyroid dysgenesis)
• the thyroid gland cannot produce active thyroid hormone due to an inherited enzyme deficiency (thyroid dyshormonogenesis)

1.1 General organisation

CHT screening is fully integrated within the existing NBS screening programme and based on the same screening laboratory populations. The initial screening test, the assay of TSH, uses blood collected on the standard newborn screening blood sample collection card. Quality assurance (QA) and performance management arrangements follow the same general principles as those for other newborn screening programmes.

With CHT, as for other blood spot screening conditions, the screening laboratory is a major communication hub. Screening results are fed back to child health records departments (CHRDs), with onward transmission of negative results to the parents. More than 99% of results are ‘CHT not suspected’ and generated promptly. However, in the case of premature infants who need repeat testing at 28 days, tests will not be completed until the baby is over a month old.

Some parents may decide that they do not want their baby to be screened for some or all of the conditions. Screening can be declined for CHT, sickle cell disease and cystic fibrosis individually, but the 6 inherited metabolic diseases (IMDs) can only be declined as a group.

Complete the blood spot card to indicate which conditions have been accepted or declined, as outlined in the NBS screening sampling guidelines.

2. About CHT

CHT is a disorder which means that not enough thyroid hormones are produced – mainly thyroxine (T4), but also tri-iodothyronine (T3) – by the thyroid gland. Thyroid hormones are essential for normal childhood growth and development. Low thyroid hormone concentrations in the first 2 years of life can have a harmful impact on neurodevelopment.

Thyroid hormone secretion is regulated by thyroid-stimulating hormone (TSH), also known as thyrotropin, which is secreted by the pituitary gland. CHT can be caused by a problem with the thyroid gland itself (primary hypothyroidism) which affects around 1 in 2,000 babies, or with the production of TSH (secondary or central hypothyroidism) which affects around 1 in 20,000 babies (Peters et al, 2018).

The NBS screening programme is designed to detect only primary CHT. Permanent primary CHT in the UK (where there is a long-term requirement for thyroid hormone treatment) is usually due to one of 2 main underlying abnormalities.

2.1 Thyroid dysgenesis

Most babies with permanent, primary CHT have thyroid dysgenesis with an absent, ectopic or hypoplastic gland. The majority of thyroid dysgenesis cases do not have an identifiable genetic cause (Peters et al, 2018).

2.2 Thyroid dyshormonogenesis (DHG)

A minority of babies with permanent, primary CHT have thyroid DHG. This is where the thyroid gland is present in its’ usual position in the neck, but cannot manufacture thyroid hormone normally. DHG cases were thought to represent around 20% of permanent primary CHT, but this figure may be an underestimate.

The following 2 studies have shown that a genetic defect can be identified in many babies with DHG if a thorough molecular genetic screening strategy is employed:

• Nicholas et al (2016)
• Peters et al (2018)

2.3 Symptoms and signs

The symptoms and signs of primary CHT can include (Grant et al, 1992):

• lethargy
• feeding difficulties
• coarse facial features
• enlarged protruding tongue
• cold mottled skin
• large posterior fontanelle
• umbilical hernia
• jaundice
• constipation

CHT differs in clinical and biochemical severity and babies at the mild end of the spectrum may be asymptomatic in the neonatal period.

The illustration below describes the classification of CHT.

2.4 Text description of classification of CHT illustration

Primary CHT

This type of the condition:

• accounts for about 90% of all cases of CHT
• is characterised by abnormal thyroid development or function
• is detected by newborn blood spot (NBS) screening

There are 2 types of primary CHT:

• permanent primary CHT
• transient primary CHT

Permanent primary CHT

This type of the condition:

• accounts for about 90% of all cases of primary CHT
• is characterised by permanent abnormality of thyroid hormone release
• usually means long-term thyroid hormone replacement treatment is needed beyond 3 years of age

There are 2 types of permanent primary CHT:

Dysgenesis: this is when there is abnormal thyroid gland development.

Dyshormonogenesis: this is when there is an enzyme defect affecting thyroid hormone release.

Transient primary CHT

This type of the condition:

• accounts for about 10% of all cases of primary CHT
• is characterised by transient abnormality of thyroid hormone release
• usually means thyroid hormone replacement treatment is needed up to around 3 years of age

Causes include:

• drug or iodine exposure
• prematurity
• dyshormonogenesis, for example THOX

Secondary CHT

This type of the condition:

• accounts for about 10% of all cases of CHT
• is characterised by abnormal pituitary thyroid stimulating hormone (TSH) secretion
• is not detected by NBS screening

End of text description.

2.5 Transient CHT

Babies can also have transient (as opposed to permanent) CHT. Transient CHT can be caused by a variety of factors, including:

• maternal anti-thyroid drug use
• perinatal iodine exposure
• antibody transfer from mother across the placenta

Transient CHT is more common in preterm babies. However, there are also preterm babies (particularly <32 weeks gestation) with low T4 and normal TSH levels in the newborn period who later display an abnormally elevated TSH and are diagnosed with primary CHT. These babies may not be detected by NBS screening using TSH measurement in early postnatal life (Mandel et al, 2000) and this is the rationale for repeat testing in preterm infants (<32 weeks gestation) as part of the NBS screening programme.

2.6 Children with a confirmed diagnosis of CHT

Children with a confirmed diagnosis of CHT should be treated with oral levothyroxine sodium without delay. The objective of treatment is to normalise TSH as soon as possible. This may take a few weeks despite adequate thyroxine treatment.

The dose of levothyroxine will need to be reduced if TSH is suppressed (below the reference range) or if the baby is showing clinical signs of overtreatment such as irritability or a fast heart rate.

Babies with significant endogenous thyroid hormone production may need smaller initial thyroxine doses than babies who cannot make any thyroid hormone at all. Once treatment starts, TSH and thyroid hormone concentrations are closely monitored so that levels are maintained within or close to local age-appropriate reference ranges. This will help to ensure normal growth and intellectual function.

The CHT initial clinical referral guidelines state that babies in whom a diagnosis of CHT has been made should start treatment by 14 days of age for patients diagnosed on the first sample.

Long term follow-up studies by Grosse and Van Vliet (2011) and Aleksander et al (2018) have documented excellent outcomes among children with CHT who are started on an appropriate dose of thyroid hormone soon after birth, and who are then monitored closely.

Confirmatory testing should be undertaken in cases where the cause or permanence of hypothyroidism has not been confirmed. This will involve stopping treatment at 2 to 3 years of age with subsequent monitoring of thyroid function. See the initial clinical referral guidelines for further information.

Children with permanent CHT will need thyroxine treatment for life and therefore need long-term monitoring and follow-up. Female patients need to be aware that an increased dose of thyroxine is typically needed during pregnancy to ensure that the foetus receives an adequate supply of thyroid hormone across the placenta.

3. Screening protocol

The CHT screening protocol is intended to:

• maximise the early detection of CHT, so that pre-symptomatic treatment can be started to reduce the long-term morbidity associated with the condition if untreated
• minimise the need for second heel pricks, and any other causes of diagnostic delay

A visual summary of the screening protocol is shown in the CHT screening protocol flowchart, below. No alternative is recommended.

3.1 Text description of the CHT screening protocol flowchart.

1. Routine newborn blood spot (NBS) screening completed

Screening is completed on day 5 in line with the newborn blood spot screening pathway. Once the sample has been received in the laboratory, perform thyroid stimulating hormone (TSH) analysis (in singleton). The analytical cut-off is 6.0 mU/L whole blood (WB).

2. Is the TSH equal to or greater than the analytical cut-off of 6.0 mU/L WB?

Yes: Re-test for TSH in duplicate from the same blood spot card, but using a different spot(s). Action is taken on the triplicate mean result. The action cut-off is 8.0 mU/L WB. Go to question 3.

No: Report ‘CHT not suspected’. No further action required. End of pathway.

3. Is the TSH equal to or greater than the action cut-off of 8.0 mU/L WB mean?

Yes: Go to question 4.

No: Report ‘CHT not suspected’. No further action required. End of pathway.

4. Is the TSH equal to or greater than 20.0 mU/L WB mean?

Yes: Report ‘CHT suspected’ and refer. End of pathway.

No: Request a ‘CHT borderline’ repeat sample. This should be taken 7 to 10 days after the previous sample was taken. Once the ‘CHT borderline’ repeat sample has been received in the laboratory, test for TSH in duplicate (ideally discs should be punched from different blood spots). Go to question 5.

5. Is the TSH equal to or greater than the action cut-off of 8.0 mU/L WB mean?

Yes: Report ‘CHT suspected’ and refer. End of pathway.

No: Report ‘CHT not suspected’. No further action required. End of pathway.

3.2 Standard protocol

The first screening specimen

TSH analysis should be carried out on a single spot from the initial dried blood sample. It needs to be performed frequently enough to permit referral of screen positive results within 2 to 4 working days of sample receipt. Samples are initially assayed in singleton.

Babies in whom the TSH concentration is <6.0 mU/L whole blood (WB) (the analytical cut-off) in the initial screening sample should be considered to have a negative screening result for CHT. They should be reported as ‘CHT not suspected’, unless they were born at less than 32 weeks gestation (less than or equal to 31 weeks + 6 days).

For babies born at less than 32 weeks gestation, a repeat request should be issued as per the CHT preterm repeat policy (see section 3.8 below), using the status code for ‘repeat sample required for CHT preterm’.

Samples with TSH greater than or equal to the analytical cut-off (≥6.0mU/L WB) are retested in duplicate from the same card but on a different spot(s) to give a more definitive result. This is to minimise effects of volumetric variability of the punched discs, day-to-day variation in TSH assay calibration, and to detect possible sample misidentification.

Action is taken on the triplicate mean result.

The set of triplicate results should be reviewed for consistency, as poor analytical performance can produce different results. There may be a spuriously high result if the blood spot is layered. Similarly, there may be a low result if there is a missing spot or poor sample.

Review of triplicate mean results

Babies in whom the triplicate mean TSH concentration in the initial screening sample is <8.0 mU/L WB (the action cut-off) should be considered to have a negative screening result for CHT. They should be reported as ‘CHT not suspected’, unless the baby was born at less than 32 weeks gestation. For babies born at less than 32 weeks gestation, a repeat request should be issued as per the CHT preterm repeat policy, using the status code for ‘repeat sample required for CHT preterm’.

Babies in whom the triplicate mean TSH concentration in the initial screening sample is ≥20.0 mU/L WB should be considered to have a positive screening result for CHT and should be reported as ‘CHT suspected’. These babies are referred, regardless of their gestational age at birth, to the paediatric endocrine team (regional specialist team) or to a clearly identified lead paediatrician with special interest in CHT or experience in managing these patients.

Babies in whom the triplicate mean TSH concentration in the initial screening sample is ≥8.0 mU/L and <20.0 mU/L WB should be considered to have a borderline result for CHT and should be reported as ‘CHT borderline’. A repeat dried blood spot sample should be requested to be taken 7 to 10 days after the initial sample and assayed for TSH in duplicate. Ideally discs should be punched from different spots.

The analytical cut-off (6.0 mU/L WB) is set below the action cut-off (8.0mU/L WB) to allow for the natural variation in the TSH assay (the coefficient of variation = 10%) and to minimise the effect of volumetric variability that occurs in dried blood spots.

Insufficient sample: no re-assay possible

If it is not possible to punch any further discs from the card (for example, when a single result is ≥6.0, but <8.0 mU/L WB) then a repeat specimen must taken as soon as possible. The reason for this request should be given as ‘insufficient sample’. If the single result is ≥8.0 then proceed as shown in the CHT screening protocol flowchart.

Only one re-assay possible

Sometimes only 2 TSH results can be obtained (for example, when only one further spot from the card is possible after the initial analysis). If this is the case, follow the steps as outlined below.

1. If the average of the 2 results is below the action cut-off (<8.0 mU/L WB), report ‘CHT not suspected’ unless the baby was born at less than 32 weeks gestation. For babies born at less than 32 weeks gestation, a repeat request should be issued as per the CHT preterm repeat policy, using the status code for ‘repeat sample required for CHT preterm’.

2. If the average of both results is ≥8.0 mU/L, then proceed as shown in the CHT screening protocol flowchart.

3. Widely discrepant results may require a repeat specimen, which should be requested and carried out as soon as possible. If one of the results is ≥20.0, then refer as per the initial clinical referral guidelines.

The second sample (repeat TSH test)

Requests for second blood samples for TSH testing will be made via locally agreed pathways defined as part of the newborn screening responsibilities.

Reasons for repeat samples

A second dried blood spot specimen for TSH screening is requested in the following situations.

When a ‘CHT borderline’ result has been reported: On detecting a borderline result, a second sample is to be taken 7 to 10 days after the initial sample. The explanation to be given to parents that a second sample is needed to confirm the result (either positive or negative).

When a preterm baby (born at <32 weeks gestation) has had a ‘CHT not suspected’ screening result on the first sample: The explanation to be given to parents is that the routine day 5 screening test may not pick up CHT in babies born at less than 32 weeks of pregnancy. It is therefore advised that these babies have another test at either 28 days of age or immediately before the baby is discharged home, whichever comes first.

The repeat request should be confirmed in writing to the appropriate health professional(s), outlining the reason for the repeat sample and when it should be completed.

Action on repeat samples

The second dried blood spot specimen is for TSH testing only. It is recommended that other screening tests are not repeated on this specimen.

The following actions are undertaken on the second sample depending on the reason for its request. If a ‘CHT borderline’ result has been reported:

• this second sample is assayed for TSH in duplicate, ideally from 2 different spots
• babies with an average TSH concentration ≥8.0 mU/L WB in this second sample should be considered to have a positive screening result for CHT – this should be referred and reported as ‘CHT suspected’
• babies with an average TSH concentration <8.0 mu/L WB in this second sample should be considered to have a negative screening result for CHT – this should be reported as ‘CHT not suspected’ unless the baby was born less than 32 weeks gestation

If the baby was born at less than 32 weeks gestation, a further repeat request should be issued as per the CHT preterm repeat policy, using the status code for ‘repeat sample required for CHT preterm’. The subsequent repeat sample should be treated the same as in the scenario below.

If a second sample is requested for a preterm baby (born at <32 weeks gestation) with a previous (unreported) screening result of ‘CHT not suspected’:

• this second sample is assayed for TSH in singleton and processed as per an initial screening sample
• babies with an average TSH concentration ≥20.0 mU/L WB in this second sample should be considered to have a positive screening result for CHT – this should be referred and reported as ‘CHT suspected’
• babies with an average TSH concentration <8.0 mu/L WB in this second sample should be considered to have a negative screening result for CHT – this should be reported as ‘CHT not suspected’
• babies with an average TSH concentration ≥8.0 mU/L WB and <20.0 mU/L WB in this second sample should be considered to have a borderline screening result for CHT – this should be reported as ‘CHT borderline’

If a borderline result is detected at this stage, a further sample is to be taken 7 to 10 days after the initial sample. The explanation to be given to parents is that another sample is needed to confirm the result (either positive or negative).

This subsequent sample is assayed for TSH in duplicate (ideally from 2 different spots). The following actions should be taken depending on the results, whereby:

• babies with an average TSH concentration ≥8.0 mU/L WB in this second sample should be considered to have a positive screening result for CHT – this should be referred and reported as ‘CHT suspected’
• babies with an average TSH concentration <8.0 mu/L WB in this second sample should be considered to have a negative screening result for CHT – this should be reported as ‘CHT not suspected’

3.3 Rationale for cut off

A comparison between the whole blood TSH cut-off levels of 6, 8 and 10mU/L was carried out by Knowles et al (2018). This study demonstrated that the screening test had optimal sensitivity and specificity to detect children with persistent CHT at 3 years of age, without a significant increase in the false positive rate, at a TSH cut-off of 8.0mU/L. False positive cases included transient CHT and babies in whom CHT was excluded at the initial clinical referral.

3.4 Sibling testing

Diagnostic testing conducted to confirm a case from newborn screening may provide additional information on recurrence. Recurrence is unusual in the case of thyroid dysgenesis, but there is likely to be autosomal recessive inheritance with a 1 in 4 recurrence risk for families of babies with thyroid dyshormogenesis.

We do not recommend ‘early’ screening for siblings (prior to day 5, counting day of birth as day 0). Due to the neonatal TSH surge in the first few hours of life, screening using this protocol cannot be accurately completed until TSH has decreased. This is usually after a few days.

The blood spot sample should be taken on day 5 for all babies regardless of medical condition, milk feeding and prematurity. This is to enable timely detection of abnormal results and initiation of appropriate treatment.

3.5 Family history and other risk factors

As discussed in section 2.0 About CHT above, babies presenting with clinical symptoms, and babies known to be at risk due to family history, should be regarded as high risk. They may need to be investigated independently according to clinical circumstances as well as being screened in the normal way.

Guidance on family history is available in the NBS screening handbook.

In cases where additional testing has been carried out before routine screening, the results should be passed on to the screening laboratory. This will help to:

• avoid duplicating follow-up or diagnostic testing
• identify whether a baby has already started treatment (which has implications for the screening result)

3.6 Unscreened babies

Screening should be offered to all eligible babies under a year of age without documented results (or declines). If screening is accepted, the sample must be taken no later than 14 calendar days after the baby’s first birthday. Once the baby is above this age, then they are no longer eligible for screening. If the family or GP have any clinical concerns after this point, a referral for paediatric assessment would be appropriate.

3.7 Previously screened babies with subsequent discrepant result

A discrepant result may occur when a repeat sample detects a raised TSH concentration of ≥8.0 mU/L WB where there is a previous ‘CHT not suspected’ result in a baby born ≥32 weeks gestation.

A discrepant result may also occur when a repeat sample is requested for another condition, or an unrequested additional repeat as been unavoidably analysed for CHT. In these situations, you should:

• be aware that contamination is unlikely
• use the CHT screening protocol flowchart

The revised screening result for the baby needs to be carefully communicated to the appropriate health professional for onwards transmission to the parents.

3.8 Preterm repeat policy

A group of experts reviewed the evidence and concluded that the optimal gestational age threshold for repeat testing is ≥32 weeks gestation. This group had representatives from the:

• British Society of Paediatric Endocrinology (BSPED)
• British Association of Perinatal Medicine (BAPM)
• UK Newborn Screening Laboratory Network (UKNSLN)

The decision on the optimal gestational age threshold was informed by published evidence, including that from a study to clarify postnatal trends in postpartum serum thyroid hormones in preterm infants (Williams et al, 2004). Physiological evidence suggests that 28 days is the postnatal age by which maturation of thyroid function has occurred in most very preterm infants.

The preterm policy therefore mandates repeat testing of all babies born at less than 32 weeks gestation (less than or equal to 31+6 days) at 28 days postnatal age (counting day of birth as day 0) or at discharge home, whichever is the sooner. This is about 1 to 2% of all babies (Moser et al, 2007).

See the CHT preterm repeat policy algorithm flowchart for further details.

The policy for CHT screening in preterm infants was implemented in all 4 UK countries in April 2012. The written policy can be found in the NBS screening sampling guidelines.

The advantages of this policy are that:

• the offer of screening is completed while the baby is an inpatient
• the responsibility for taking the sample is clear
• resources can be used more efficiently because there is less need to chase for a repeat sample in the community
• it supports audit requirements

If babies are moved to another hospital, responsibility for taking the CHT preterm repeat sample (if born at less than 32 weeks gestation) is transferred to the receiving hospital.

The flowchart below describes the preterm repeat algorithm for CHT.

3.9 Text description of the preterm repeat algorithm for CHT flowchart

1. Routine newborn blood spot (NBS) screening

Offer routine NBS screening for all preterm babies (babies born at less than 32 weeks gestation). Gain consent. Take the NBS sample on Day 5 (day of birth is counted as Day 0) in line with the newborn blood spot screening pathway.

2. Does the routine NBS sample result indicate that CHT is suspected?

Yes: Report ‘CHT suspected’ and refer. End of pathway.

No: Go to question 3.

3. Does the routine NBS sample result indicate that CHT is not suspected?

Yes: Request a ‘CHT preterm’ repeat sample. Take the ‘CHT preterm’ repeat sample on Day 28 or on discharge home (whichever is sooner). Go to question 5.

No: Request a ‘CHT borderline’ repeat sample. Take the ‘CHT borderline’ repeat sample 7 to 10 days after the previous sample was taken. Go to question 4.

4. Does the ‘CHT borderline’ sample result indicate that CHT is suspected?

Yes: Report ‘CHT suspected’ and refer. End of pathway.

No: Request a ‘CHT preterm’ repeat sample. Take the ‘CHT preterm’ repeat sample on Day 28 or on discharge home (whichever is sooner). Go to question 5.

5. Does the ‘CHT preterm’ sample result indicate that CHT is suspected?**

Yes: Report ‘CHT suspected’ and refer. End of pathway.

No: Go to question 6.

6. Does the ‘CHT preterm’ sample result indicate that CHT is not suspected?**

Yes: Report ‘CHT not suspected’. No further action required. End of pathway.

No: Request a ‘CHT borderline’ repeat sample. Take the ‘CHT borderline’ repeat sample 7 to 10 days after the previous sample was taken. Go to question 7.

7. Does the ‘CHT borderline’ sample result indicate that CHT is suspected?**

Yes: Report ‘CHT suspected’ and refer. End of pathway.

No: Report ‘CHT not suspected’. No further action required. End of pathway.

4. Laboratory quality and performance

4.1 Analytical processes

TSH in the routine newborn screening blood spot is to be assayed using a methodology that has been demonstrated to be fit-for-purpose and approved by the CHT Screening Advisory Board. Any proposal to introduce new analytical methods needs careful collective consideration by the CHT screening advisory board and must meet the recommended specification. The sensitivity and specificity of the CHT screen are crucially dependent on the performance of the TSH assay. Ideally, the reagents and instrumentation should be CE marked. The laboratory should follow the procedures detailed in the manufacturer’s instructions.

4.2 Internal quality and performance monitoring

Internal quality control samples covering at least 2, but ideally 3, TSH levels should be included with each analysis batch. Each laboratory should assign acceptable ranges for these samples. It is important to include an internal quality control (IQC) close to the borderline TSH cut-off of 8.0mU/L

Dried blood spots available from the Centers for Disease Control and Prevention (CDC) can also be used for IQC purposes to provide a third party IQC. These achieve levels of precision (coefficient of variation) of approximately 7 to 10% at TSH concentrations of approximately 8 and 20 mU/L WB. Each laboratory should assess and regularly monitor their own precision data.

Laboratories must participate in audit at local, regional and national levels, to assess the effectiveness of the national screening programme. Laboratories should publish the results and performance of their NBS screening programme in an annual report.

There must be a documented risk management policy for the laboratory aspects of the CHT screening programme as part of an overall pathway risk assessment. This should describe:

• the steps in the testing protocol where failures are more likely to happen
• the procedures that have been implemented to minimise the risk of things going wrong

4.3 External quality assessment and accreditation

Laboratories should take part in an approved external quality assessment (EQA) scheme, which assesses laboratories on the precision and accuracy of analytical steps. An example of an EQA scheme would be the United Kingdom National External Quality Assessment Service (UK NEQAS).

Laboratories undertaking NBS screening must be accredited in accordance with ISO 15189 for Medical Testing Laboratories by a competent accreditation testing service, for example, by the United Kingdom Accreditation Service (UKAS).

Following agreement from the NBS screening programme, the laboratory must release reports on screening performance. These must include external QA and accreditation assessments to agencies with a legitimate interest in the quality and safety of the programme on behalf of the public.

4.4 Stability of TSH in blood spots

TSH is relatively stable. The following 2 papers report on the stability of TSH in dried blood spot samples when stored at room temperature:

• Magalhaes et al (2018), which concludes that TSH values remained stable up to day 30 when stored at 22°C
• El Ezzi and Hakim (2003), which concludes that the results presented show that the blood spots used for neonatal screening for CHT can be stored without alteration at room temperature for up to 20 weeks

On this basis, samples greater than one month old should not be analysed to report TSH concentrations.

Both papers add that when stored at 4-8 °C or -20 °C, samples are stable for several months.

5. Pre-analytical factors

5.1 Specimen requirements

Blood spot sampling should be carried out according to the NBS screening sampling guidelines. Samples should be transported to the laboratory in the usual way and kept in a dry environment at room temperature or 4°C before analysis.

Storage after analysis should follow the guidelines provided in the NBS screening programme standards.

Venepuncture or venous/arterial sampling from an existing line is an alternative method to collect the blood spot sample. This is providing the sample is not contaminated with EDTA/heparin and the line is cleared of infusate. Anticoagulants may affect the assay. Capillary tubes (plain or heparinised) must not be used to collect blood samples.

Generic guidelines (based on the stability of all analytes) state that samples are unsuitable for testing if they arrive at the laboratory more than 14 days after being taken. This is despite the fact that TSH is relatively stable. A repeat sample must be requested as soon as possible on all samples received more than 14 days after collection. The original sample should be tested and only reported if ≥8.0 mU/L.

Samples taken when the baby is greater than 12 months and 14 days of age should not be analysed. See the earlier section on unscreened babies for further details.

5.2 Factors affecting the screening result

The newborn experiences a TSH surge post birth which peaks at 30 minutes of age. Levels decrease slowly until 5 to 7 days of age (Fisher and Klein, 1981).

Secondary CHT due to generalised pituitary failure or isolated TSH deficiency will not be detected as described in the CHT classification illustration. This accounts for approximately 10% of all CHT cases.

There are a number of factors which could cause false negative and false positive results. These may be due to process errors, contamination or interference, or have a physiological basis (for example, the presence of maternal antibodies or acute illness). If there is any suspicion about the integrity of the sample, it should be rejected and a repeat requested.

Potential for false negative results

Several factors are known to lower TSH concentrations in babies with CHT, leading to a false negative screening results.

Insufficient or inadequate samples: If the blood does not fully percolate to the reverse side of the sample paper, the measured TSH could be falsely low, leading to a false negative result.

Compressed spots: When the blood sample has been taken, the blood spot must not be compressed. Applying pressure reduces the density of blood on the card and can lead to a false negative result.

Prematurity: Preterm babies (particularly when <32 weeks gestation) may have a primary thyroid problem that escapes detection because TSH concentrations do not appear to increase in these infants in the way that they do in more mature babies. This is the rationale for the repeat testing strategy described in the earlier sections on general organisation and the preterm repeat policy.

Blood transfusion: This could cause a false negative result, and a repeat sample should be taken after a reasonable time has elapsed. At least 72 hours is recommended, as for other screening tests, to allow pre-transfusion levels to be reached.

Bear in mind that not all cases of CHT will be detected on newborn screening and all children showing appropriate symptoms should be investigated accordingly.

Potential for false positive results

A false positive is where:

• TSH result is confirmed on retest in duplicate on the original blood spot sample, or
• repeat blood spot sample (also in duplicate) is elevated (screen positive), but is not confirmed on follow-up (in other words, confirmatory diagnostic testing results are normal)

In practice it may be impossible to differentiate an incorrect or artefactual result on the screening specimen from a genuine increase of TSH which is transient and not present at diagnostic follow-up.

Possible causes of false positives are listed below.

Early sampling: Due to the neonatal TSH surge in the first few hours of life, screening using this protocol cannot be accurately completed until TSH has decreased, usually after a few days. Screening for CHT should not be undertaken prior to day 5 (counting day of birth as day 0).

Multi-layering: The CHT screen requires a good quality blood spot for the TSH assay. Specimens that are over-layered by multiple applications are likely to give falsely raised results and could lead to a false positive screening result. Nevertheless, these samples should be analysed and reported to avoid any potential delays.

Transient hypothyroidism: Approximately 10% of babies diagnosed with primary CHT have a transient abnormality that resolves by the time the baby is around 2 to 3 years. Causes of transient CHT include prematurity, exposure to drugs such as iodine, some causes of DHG and illness. See the earlier section on general organisation and the CHT classification illustration. For this reason, it is important that all babies undergo a trial off thyroxine at around 2 to 3 years of age if there is any doubt about whether the CHT is permanent or not.

Iodine deficiency: Maintenance of thyroid function depends on the availability of dietary iodine. Studies have shown that hypothyroidism related to maternal iodine deficiency is rare in Western iodine-sufficient populations, although maternal iodine status may be suboptimal (Vanderpump et al, 2011). According to a study looking at UK data, variations in iodine status do not have a major impact on blood spot TSH concentrations (Evans et al, 2014).  

6. Reporting and communicating results

6.1 Status codes

Screening laboratories and CHRDs/CHISs should use the national status codes and subcodes to record the outcomes of NBS screening. Ideally, the laboratory will send screening results to CHRDs/CHISs and the newborn blood spot failsafe solution (NBSFS) using electronic messaging.

6.2 Communicating results

‘CHT not suspected’ results should be communicated to the parents by 6 weeks of age.

‘CHT suspected’ results require follow-up/clinical referral. The laboratory should refer babies with positive screening results for CHT the same or next working day following confirmation of a positive result.

The screening laboratory should report the screen positive result, verbally and in writing, either:

• to the paediatric endocrine team (regional specialist team), or
• to a clearly identified lead paediatrician with a special interest in CHT or with experience of managing these patients

Where the referral is to a regional endocrine centre, the regional specialist team should be available to provide support and to facilitate access to diagnostic investigations where required.

The referral notification should include a link to the CHT initial clinical referral guidelines. This initiates the clinical referral of screen positive cases.

Template letters are available for the screening laboratory to notify:

• the designated clinician for a baby with suspected CHT
• the baby’s GP of the ‘CHT suspected’ result

‘CHT suspected’ results should be communicated directly to the parents by an appropriately trained healthcare professional. The ‘CHT suspected’ information can support healthcare professionals to have this conversation. Parents need to be contacted as soon as possible if their baby has a ‘CHT suspected’ result so treatment can be arranged without delay.

Reports of all screening results should have a generic disclaimer saying: ‘These tests are screening tests. No screening test is 100% reliable.’ Such a disclaimer is particularly relevant to CHT because of the variable nature of the condition, as outlined in Section 2.0 About CHT above.

Each screening laboratory should have an agreed arrangement for the follow-up and referral of all CHT suspected cases, which should be in line with the CHT initial clinical referral guidelines.

6.3 NBSFS

The NBSFS is an IT system that identifies babies, born in England, who have missed NBS screening and is used by all maternity units across England. The system also records repeat requests and screening outcomes, to support failsafe processes.

The NBSFS user guide and the NBSFS operational level agreements for laboratory users provide more information about how to use the NBSFS.

7. Programme standards, data collection and screening safety incidents

7.1 Programme standards

NBS screening standards are a set of measures that must be met to make sure screening is safe and effective. All health care professionals involved in the NBS screening pathway have a part to play in meeting standards.

Standards data should be submitted to the NBS screening programme on an annual basis. The annual data collection template is shared with the screening laboratories via email each year, with instructions for completion and submission. Data submissions must be accurate, timely and complete, to enable performance monitoring and programme evaluation.

7.2 Clinical data collection

Clinical information should be requested from clinical referral centres on each presumptive positive case. Data on each case notified should be collated and anonymised before submission to the NBS screening programme. It is the responsibility of the designated clinician to make sure the forms are completed and returned to their respective laboratory directors. The regional endocrine centre should also be told about diagnostic outcome to enable regional and national audit.

7.3 Screening safety incidents

All safety concerns and incidents must be reported and managed in accordance with the guidance for managing safety incidents in NHS screening programmes. This details the accountabilities for reporting, investigating and managing NHS screening programme safety incidents. It covers the management of:

• safety concerns
• safety incidents
• serious incidents in screening programmes

8. Background to screening for CHT

Screening for CHT was formally introduced as a national newborn screening programme in England and Wales in June 1981. Screening was already taking place in Scotland (since 1979) and Northern Ireland (since 1980). The programme is a service that seeks to balance the interests of families where a child is identified as having CHT versus the interests of the large majority of families where children are unaffected.