CF laboratory guide pathways: text versions
Updated 20 December 2024
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This publication is available at https://www.gov.uk/government/publications/cystic-fibrosis-screening-laboratory-handbook/cf-laboratory-guide-pathways-text-versions
The information below provides a text description of each of the pathways (figures 1, 2 and 3) shown in the cystic fibrosis screening laboratory handbook.
Figure 1: cystic fibrosis (CF) screening algorithm
Blood spot sample immunoreactive trypsin (IRT) assay. The routine newborn blood spot (NBS) sample should be taken on day 5 (day if birth is day 0). Assay in singleton.
1. Is the IRT above or below the cut-off A?
IRT above or equal to cut-off A: re-assay in duplicate. Go to question 2.
IRT below cut-off A: CF not suspected. End of pathway.
2. Is the IRT above or below cut-off B?
IRT above or equal to cut-off B: carry out DNA analysis (4 panel) Go to question 3.
IRT below cut-off B: CF not suspected. End of pathway.
3. Were any CFTR variants detected?
Yes: Go to question 6.
No: go to question 4.
4. Is the IRT above or below 120µg/L?
IRT above or equal to 120µg/L: IRT on repeat blood spot sample (assay in duplicate). Go to question 5.
IRT below 120 µg/L: CF not suspected. End of pathway.
5. Is the IRT above or below cut-off A?
IRT above or equal to cut-off A: CF suspected. End of pathway.
IRT below cut-off A: CF not suspected. End of pathway.
6. How many CFTR variants detected?
One: Carry out DNA analysis (50-100 panel). Go to question 7.
Two: CF suspected. End of pathway.
7. How many CFTR variants detected?
One: IRT on repeat blood spot sample (assay in duplicate). Go to question 8.
Two: CF suspected. End of pathway.
8. Is the IRT above or below cut-off A?
IRT above or equal to cut-off A: CF suspected. End of pathway.
IRT below cut-off A: Probable carrier. End of pathway.
End of text description for figure 1 pathway.
Figure 2: Handling of IRT results from the first screening specimen
The average of 3 IRT results is above or equal to cut-off B. (Note that cut-off A should be used if the sample was collected on or after day 21 (day of birth counted as day 0).
1. Are the results discrepant?
Yes: Go to question 2.
No: Accept the average result. End of pathway.
2. Are 2 results above or equal to cut-off B? (Note that cut-off A should be used if the sample was collected on or after day 21 (day of birth counted as day 0).
Yes: Go to question 3.
No: Report CF not suspected. End of pathway.
3. Is the average IRT above or equal to 120µg/L?
Yes: Request a repeat sample. If the repeat sample cannot be taken before day 21, immediate referral is indicated. End of pathway.
No: Send for genetic analysis. End of pathway.
End of text description for figure 2 pathway.
Figure 3: handling of late first IRT samples
Late first IRT sample. Establish the age of the baby (day of birth is counted as day 0).
1.Is the baby aged day 6 to 20 inclusive?
Yes: Follow the standard CF screening protocol. End of pathway.
No: Go to question 2.
2. Is the baby aged day 21 to 56 inclusive?
Yes: Assess IRT against cut-off A and follow the standard CF screening protocol. End of pathway.
No: Go to question 3.
3. Is the baby aged over 56 days (8 weeks) and the sample has been taken but not yet analysed?
Yes: Report as not screened for CF. Inform the GP and child health records department (CHRD). Only refer if the baby has clinical signs of CF. End of pathway.
No: Go to question 4.
4. Is the baby aged over 56 days (8 weeks) and the sample has been taken and unavoidably analysed, with an IRT above or equal to cut-off A?
Yes: Refer as CF suspected and arrange genetic analysis. End of pathway.
No: Report as not screened for CF. Inform the GP and child health records department (CHRD). Only refer if the baby has clinical signs of CF. End of pathway.
End of text description for figure 3 pathway.