Microbiology

QuantiFERON

QuantiferonTB is an in vitro laboratory test for the detecting immune responses to tuberculosis infection. It is an indirect test for Mycobacterium tuberculosis and is an alternative to the tuberculin skin test. It is not affected by a previous BCG vaccination.

It is intended for use as a diagnostic aid for latent M. tuberculosis complex infection. Like the skin test it cannot distinguish between latent and active infection and is intended for use in conjunction with risk assessment, radiography and other medical and diagnostic evaluations.

In individuals suspected of having active TB culture remains the gold standard. Routine use in the diagnosis of active TB is discouraged, due to the phenomenon of false negative results in overwhelming infection.

Sample required:

  • 5 ml blood in dark green Lithium Heparin blood collection bottle.
    Dark green liquid heparin bottle
  • Samples must be kept at room temperature once taken- do not refrigerate.
  • NB. the sample must reach the Microbiology laboratory within 16 hours of collection in order to be processed.

The following results are possible, with the corresponding interpretative comments that are provided on the laboratory result.

QuantiFERON-TB Gold test POSITIVE

Positive results have an automatically generated laboratory comment added to the report. This reads “QuantiFERON-TB Gold test POSITIVE. INTERFERON GAMMA RELEASE ASSAYS SHOULD NOT BE USED IN THE DIAGNOSIS OF POSSIBLE ACTIVE TUBERCULOSIS. This result may be due to exposure to M.tuberculosis complex (except M.bovis BCG), M.kansasii, M.szulgai or M.marinum. a positive test is consistent with latent OR active tuberculosis. Clinical, radiological and microbiological assessment should be undertaken when TB suspected.”

QuantiFERON-TB Gold test NEGATIVE

“QuantiFERON-TB Gold test NEGATIVE. INTERFERON GAMMA RELEASE ASSAYS SHOULD NOT BE USED IN THE DIAGNOSIS OF POSSIBLE ACTIVE TUBERCULOSIS. This result indicates that the patient does not have evidence of cellular immunity to M.tuberculosis. Therefore latent tuberculosis is UNLIKELY in an immunocompetent patient. A negative result SHOULD NOT BE USED TO RULE OUT ACTIVE TUBERCULOSIS. If active tuberculosis likely, take cultures and refer to TB physician.”

QuantiFERON-TB Gold test INDETERMINATE

“Valid result NOT obtained. The mitogen control in the test indicated this patient has impaired cellular immune function (anergy) at the time the sample was obtained. The possibility of M.tuberculosis infection cannot be excluded. NB: Overwhelming infection with M.tuberculosis is itself a cause of anergy.”