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Private cattle TB blood interferon-gamma testing

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APHA offer the option to private vets of additional private interferon-gamma (IFNG) blood TB testing of cattle in England under certain conditions.

Background

The introduction of private IFNG testing was one of the objectives in Defra’s current strategy for achieving Officially TB-free status (OTF) for England published in April 2014.

This facility allows private vets, with prior APHA approval and at the herd owner’s expense, to submit blood samples to an APHA laboratory for IFNG testing of cattle that are not eligible for government-funded testing. This is possible in a limited number of scenarios where herd owners and their vets seek additional tools to check/monitor the TB status of certain animals over and above those afforded by statutory TB testing.

Examples include:

  • 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
  • As a marketing tool to add value to herd/animals intended for sale
  • Ad hoc, additional testing following a negative routine or tracing skin test
  • Rapid re-testing of inconclusive skin test reactors (IRs) where no government-funded IFNG blood test is planned and the keeper does not wish 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.

In addition, supplementary testing of non-reactor cattle in TB breakdown herds with OTF status suspended or withdrawn may also be considered for APHA approval. For example, animals that have passed a short interval skin test and do not qualify for a government-funded IFNG test.

Cattle NOT eligible for private IFNG testing include the following:

  • Cattle from herds under TB restriction and undergoing government-funded IFNG testing
  • Skin test reactors, IFNG 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 that are under six months of age.

Please also note that the IFNG blood test cannot be used on its own to maintain or regain OTF status for a particular cattle herd.

IFNG test information

The standard IFNG blood test is a comparative test, like the tuberculin skin test used in the UK and Ireland.

It uses bovine (PPDB) and avian (PPDA) tuberculin as antigens to ‘challenge’ live white blood cells. Sensitised white blood cells from infected cattle produce IFNG in response to this stimulation and the IFNG so produced is measured using the BOVIGAM™ ELISA test kit. A test-positive result is recorded where the IFNG response to PPDB is greater than the IFNG response to PPDA.

Each blood sample also has its own positive and negative quality controls to ensure the test results are valid.

Two types of private IFNG test are available:

  • High Specificity Test for cattle from OTF herds only, for movement, sales, inconclusive reactors pending retest (including those in restricted non-breakdown herds subject to the three-year rule), resolved IRs in non breakdown herds and additional surveillance following negative routine skin testing. This extended IFNG test incorporates an additional specific peptide cocktail to the standard INFG test which must also be positive in order for a positive overall test result to be issued. The inclusion of this peptide cocktail increases the diagnostic specificity of the test (i.e. reduces the likelihood of false-positive results), but at the expense of a lower sensitivity (i.e. the probability of detecting infected animals) compared to the basic IFNG test, as follows:
    - sensitivity: 74.0% (95% CI: 70.4 – 83.0)
    - specificity: 99.2% (95% CI: 98.6 – 99.8)
  • High Sensitivity Test for cattle in TB-breakdown herds that do not qualify for a government-funded IFNG test, e.g. IRs and resolved IRs. Higher test sensitivity is required for infected herds coming to the end of breakdown testing to increase the likelihood of identifying any residual infected skin test-negative cattle. This test is the standard IFNG test format outlined above, and carries the highest diagnostic sensitivity (probability of detecting infection), but at the expense of a lower specificity (higher probability of false-positive results) as follows:
    - sensitivity 90.0% (95% CI: 87.2 – 92.8)
    - specificity 96.5% (95% CI: 95.3 – 97.7)

If private vets are not sure as to which IFNG test format is applicable, they should contact APHA, who will decide which format of the IFNG test should be used depending on the circumstances of the animals or herd to be privately tested. Where the High Specificity Test is applied, the results report will provide both the High Specificity test result and the interpretation of the test at High Sensitivity (which may identify additional positive animals). While this latter result will not be acted upon by APHA/Defra, it provides additional information that the customer may wish to consider.

Due to the nature of the IFNG test, approximately 5% of samples may not yield a valid result on the first submission and a new blood sample would then be required from that particular animal. Samples with a ‘Resample’ result issued against them will still be charged for. If a new sample is submitted following a ‘Resample’ result, it will be charged as a new submission.

The comparative tuberculin skin test has, on average, a sensitivity of 81.0% (95%CI: 77.3 – 84.7) and specificity of 99.98% (95%CI: 99.9 - 100).

Submission of blood samples to APHA

Private vets should contact the APHA IFNG testing laboratory at Newcastle to book their IFNG test, after obtaining prior authorisation from APHA. A heparinised blood sample is required for the IFNG test.

Samples must be couriered to the APHA testing laboratory in a suitable temperature-controlled delivery box to prevent exposure to extremes of temperature that could damage blood cell viability. Private vets should read the guidance on packaging of blood samples for interferon-gamma testing on the submission form (PRIV01) before seeking approval and arranging to collect the samples. All submissions are temperature checked upon arrival at the laboratory and any falling outside of the limits recommended by the manufacturer (within the range 22+/- 5oC) are rejected (not tested). Further information about delivery box options can be obtained from the laboratory (see below).

Private tests are only carried out on Thursdays. Samples must therefore be taken on a Wednesday and arrive at the laboratory by either 16:00 on the day of sampling or by 09:00 on Thursday morning. Samples taken earlier or incorrectly transported will not be suitable for testing. Rejected samples do not attract a laboratory testing fee but an administrative fee will be charged.

See further information on delivery box specifications.

Written approval must be obtained from APHA before the start of every testing event. This includes resampling visits to the same herd in the event of the laboratory issuing a ‘resample’ or ‘reject’ outcome for some of the tested animals. Private vets must use the IFNG test application form (PRIV02) to obtain approval from APHA and the sample submission form (PRIV01).  If the IFNG test is being conducted in conjunction with a non-validated or non-Defra approved test for TB, a separate application form (PRIV03) will also need to be submitted to TB.Advice@apha.gov.uk. These forms can be found on the submission forms page, including contact details for returning completed forms.

Private IFNG test cost (as of September 2023)


Cost per test for:

1-4 blood samples

5+ blood samples

High Specificity format (TC0751)

£27.30

£22.30

High Sensitivity format (TC0651)

£19.80

£16.10
Box return fare (TC0251) £22.80 per box

Test results

All results will be sent directly to the submitting veterinary practice and copied to the appropriate regional APHA office. Farmers should be made aware that most animals giving a positive result on a private IFNG test will be compulsory slaughtered with statutory compensation and the herd put under TB movement restrictions, if not already in place. Some low-risk animals giving a positive result may be retested at APHA’s discretion and expense.

Further information

  • Goodchild et al., 2015, Vet. Rec., doi. 10.1136/vr.102961
  • Karolemeas et al., 2012, Estimation of the relative sensitivity of the comparative tuberculin skin test in tuberculous cattle herds subjected to depopulation, PLOSone, 7(8):1-7.
  • Vordermeier et al., 2006, The BOVIGAM® assay as ancillary test to the tuberculin skin test. Govt. Vet. J., vol. 16(1): p72-80.

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