Introduction The 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.

Hypothesis Levels of interferon-γ in TB2 should always be greater than TB1

Methods QFT-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.

Results Data 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 no TB2 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.

Conclusion These 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.