Search

IntroductionThe revised QuantiFERON-TB Gold assay (QFT-Plus) has an extra tube (TB2), containing peptides thought to stimulate CD8 +T cells; TB7 antigen is no longer included. The test is designed to detect latent tuberculosis infection.HypothesisLevels of interferon-γ in TB2 should always be greater than TB1MethodsQFT-Plus was used routinely in a TB contact clinic June 2016 – January 2017. Interferon-γ values were recorded minus the negative control; a significant difference was defined as a two-fold variation. Clinical details were recorded after consent on a standard form, which included age, sex, reason for test, tuberculin response (mm) and HIV status.ResultsData was available from 259 subjects; 2 were HIV-positive. Four gave an indeterminate result. The majority (194; 75%) gave a negative test for both tubes. For 65 positive tests with a≥2 fold difference (all ≥0.7 IU/mL difference), TB1 ≥TB2 occurred in 4 (6%) and TB2 ≥TB1 occurred in 3 (5%). All 4 TB1 ≥TB2 had noTB2 response: there were 2 contacts (10 and 0.68 IU/ml); one was PPD positive with neck lymphadenopathy (4.23 IU/ml); one was PPD negative with haemoptysis after nasal surgery (1.75 IU/ml). Where TB2 was positive in the ‘grey zone’ and TB1 negative, one had Crohn’s disease (0.24, 0.01 IU/ml) and the other (0.23, 0.01 IU/ml) was a contact of non-pulmonary TB, but in none was there HIV co-infection. Three had TB2 ≥TB1: one with smear-positive lung TB had a TB2=0.6 with a TB1=0.25, but the latter was in an accepted ‘grey zone’; two had positive tests for both tubes, of which one was a contact 30 years previously and one had testicular TB.ConclusionThese data may indicate a technical failure of the TB2 tube. Alternatively, the absent immune response, associated with active disease, might prove useful in determining who could benefit from preventive treatment.Abstract P164 Table 1Highest TB1 or TB2 value (IU/ml)Difference (IU/ml)<0.2 (n=186)≥0.2 and<0.35 (n=4)>0.35 (n=65)0–0.1182231>0.1 to<0.354212≥0.350022>2-foldna27