Weakly positive tests and chronologic variation of the QuantiFERON assay: a retrospective appraisal of usefulness

QuantiFERON Tb Gold IT (QFT) is routinely used as diagnostic tool for detecting immune reactivity towards Mycobacterium tuberculosis. When diagnosing a latent infection weakly positive results and test reproducibility must be considered.

To judge the frequency of useful information versus confusion gained by QFT in patients with immune-mediated diseases and in pulmonary outpatients.

We retrospectively analysed 1485 QFT-tests performed on 855 patients with autoimmune diseases prior to initiating biologics and on 447 pulmonary outpatients with a suspected tuberculosis infection over a period of 16 months. In 161 cases the IGRA test was erroneously repeated. After 18 months clinical, radiographic and bacteriologic evidence for tuberculosis was collected.

Of all 1485 QFT-tests 247 (=16.63%) showed a positive result, 1186 (79.87%) were negative and 52 (3.50%) turned out to be indeterminate. Using the manufacturers cut-off value for the interferon-γ response, a cohort (n = 64) of weakly positive individuals (26% of all positives) was apparent. Repeated testing within ten weeks yielded negative reactivity in 69% of these patients. Of 25 IGRA positive patients who had a repeat test after one year without tuberculosis medication, 48% showed a reversion to a negative test. The frequency of indeterminate results of the IGRA was satisfying (3.5%).

In case of only slightly elevated immune reactivity quantitative reporting of positive QFT-test results and a repeat test strategy might be of advantage for the usefulness of the test.