Private interferon-gamma (IFN-y) testing for bovine tuberculosis (TB) in England
Private cattle IFN-y testing, including when to test, how to apply for permission, carry out testing and submit samples to APHA.
Applies to England
The interferon-gamma (IFN-y) test is a supplementary test used to improve the on-farm diagnosis of bovine tuberculosis (TB). It’s used on veterinary advice mostly in herds with:
- severe breakdowns
- persistent or recurrent TB problems
- breakdowns in areas where TB incidence is low
Private IFN-y testing must be authorised by the Animal and Plant Health Agency (APHA) and paid for by the herd owner. It allows private vets to submit blood samples to an APHA laboratory for IFN-y testing of cattle that are not eligible for government-funded testing.
When private vets can apply for private IFN-y testing
IFN-y testing can only be carried out in certain circumstances. For example, as:
- supplementary pre or post-movement testing of high value cattle that are not subject to, or have passed, a compulsory skin test
- more sensitive TB screening of animals joining high-value herds, including pedigree bulls entering semen collection centres
- a marketing tool to add value to a herd or animals intended for sale
- additional testing needed following a negative routine or tracing skin test
- rapid re-testing of inconclusive skin test reactors (IRs) where no government-funded IFN-y blood test is planned and the keeper does not want to wait 60 days for the skin re-test. An IR skin re-test with a negative result is always required before the animal (or herd) TB movement restrictions can be lifted.
- testing of resolved IRs that are permanently restricted to their farm of detection in England, to release them from lifelong restrictions
APHA may also approve private IFN-y testing for supplementary testing of non-reactor cattle in TB breakdown herds with Officially Tuberculosis Free (OTF) status suspended or withdrawn. For example, animals that have passed a short interval skin test and do not qualify for a government-funded IFN-y test.
When private IFN-y testing cannot be carried out
IFN-y testing cannot be used on its own to maintain or regain OTF status for a particular cattle herd.
You also cannot carry out private IFN-y testing on:
- cattle from herds under TB restriction and undergoing government-funded IFN-y testing
- skin test reactors, IFN-y test-positive animals and other cattle awaiting slaughter for TB-control purposes
- animals in Scotland, Wales or Northern Ireland at the time of testing
- herds under TB movement restrictions for overdue tuberculin skin testing
- cattle less than 6 months old
Available IFN-y test types
The standard IFN-y blood test is a comparative test. It uses bovine (PPDB) and avian (PPDA) tuberculin as antigens to challenge live white blood cells. Sensitised white blood cells from infected cattle produce IFN-y in response, and this is measured using the Bovigam ELISA test kit.
A positive result is recorded where the IFN-y response to PPDB is greater than the response to PPDA. Each blood sample has its own positive and negative quality controls to ensure the test results are valid.
The comparative TB skin test has an average sensitivity of 81% (95% CI: 77.3 to 84.7) and specificity of 99.98% (95% CI: 99.9 to 100).
There are 2 types of private IFN-y test available.
High specificity test for cattle from OTF herds only
This test can be used for:
- movement
- sales
- inconclusive reactors pending retest (including those in restricted non-breakdown herds subject to the 3 year rule)
- resolved IRs in non-breakdown herds
- additional surveillance following negative routine skin testing
This test includes an additional specific peptide cocktail alongside the standard test, which must also be positive in order for a positive overall test result to be issued. This increases the diagnostic specificity of the test by reducing the likelihood of false-positive tests, at the expense of a lower sensitivity when compared to the basic IFN-y test, as follows:
- sensitivity 74% (95% CI: 70.4 to 83.0)
- specificity 99.2% (95% CI: 98.6 to 99.8)
High sensitivity test for cattle in TB breakdown herds that do not qualify for a government-funded IFN-y test (including IRs and resolved IRs)
The higher test sensitivity is required for infected herds coming to the end of breakdown testing. This increases the likelihood of identifying any residual infected skin test-negative cattle.
This test uses the standard IFN-y test format and carries the highest diagnostic sensitivity at the expense of a lower sensitivity, as follows:
- sensitivity 90% (95% CI: 87.2 to 92.8)
- specificity 96.5% (95% CI: 95.3 to 97.7)
How to decide which type of test to use
Contact APHA if you are not sure which type of test to use. They will decide based on the circumstances of the animal or herd being tested.
The results from the high specificity test will give both the test result and the interpretation of the test at high sensitivity. The interpretation may identify additional positive animals but will not be acted on by APHA or Defra, it just provides additional information that the cattle keeper may wish to consider.
Submitting blood samples to APHA
Before you carry out the test
- All IFN-y tests must use a heparinised blood sample.
- Read about packaging blood samples for IFN-y testing in the submission of blood samples for private TB interferon gamma testing of cattle (PRIV01) form and the guidance on private interferon-gamma (IFN-y) test sample delivery box specifications.
- Apply for permission to test using the application for approval to undertake private TB interferon gamma testing of cattle in England (PRIV02) form. You must also get approval before resampling visits to the same herd following a ‘resample’ or ‘reject’ outcome for any of the tested animals.
- Submit application form PRIV03 if you’re also carrying out a private non-validated or non-Defra approved TB test.
- Contact the APHA IFN-y testing laboratory at Newcastle to book a test.
After you have completed the test
- Use the PRIV01 form to submit samples for testing.
- Read the guidance on IFN-y test delivery box specifications.
- Send the samples to the APHA IFN-y laboratory in Newcastle in suitable temperature-controlled packaging.
APHA will check the temperature of the samples on arrival to the laboratory and will reject any samples that are below 17°C or above 27°C on arrival.
Private tests are only carried out on Thursdays. You must take the sample on a Wednesday and send it to the laboratory by either 4pm on the day of sampling, or by 9am the following Thursday morning. Samples taken earlier or incorrectly transported will not be suitable for testing. Rejected samples will incur an administrative fee.
IFN-y test submission costs
Fees are accurate as of July 2024.
| Cost per test | 1 to 4 blood samples | 5 or more blood samples |
|---|---|---|
| High specificity format (TC0751) | £28.30 | £23.10 |
| High sensitivity format (TC0651) | £20.50 | £16.70 |
There is also a box return fee (TC0251) of £23.60 per box.
Costs for additional submissions
Approximately 5% of IFN-y test samples may not give a valid result on the first submission. This is known as a ‘resample’ result. If this happens, you will need to submit a new blood sample from the animal to try to obtain a valid result.
Samples following a ‘resample’ result or rejection on arrival to the laboratory are charged as new submissions.
Getting your test results
All test results will be sent directly to the submitting veterinary practice and copied to the appropriate APHA team.
If there is a positive result on a private IFN-y test:
- most animals giving the positive result will be compulsory slaughtered
- the herd owner will be offered statutory compensation
- the herd will be put under TB movement restrictions (if they are not already in place)
- low-risk animals may be retested at APHA’s discretion and expense
Private non-validated or unapproved TB tests for cattle in England
Private veterinary surgeons can also consider using non-validated or unapproved TB tests on a voluntary basis, at the herd owner’s expense, in cattle herds in England.
This is subject to certain conditions, and it requires previous authorisation from APHA.
Private veterinary surgeons must also submit an application form PRIV03 if they want to carry out any additional non-validated or unapproved test in conjunction with a private IFN-y tests.
Read the guidance on private non-validated or unapproved TB tests for cattle in England.