gms | German Medical Science

Objectives: Whereas tuberculosis-specific interferon gamma-release assays (TIGRA) are known to have predictive value in contact tracing [1], the progression rate to active tuberculosis (TB) in immunocompromised (IC) individuals with a positive TIGRA result is not well known.

Methods: After ethical approval, 460 IC patients were prospectively enrolled and tested with the third generation Quantiferon Gold in tube assay (QFT Cellestis, Australia). We retrospectively followed up patients with positive QFT tests (mean follow up time: 2,1 years) by reviewing patient charts for diagnosis of active TB disease. We compared TB events per 100 patient years (pys) for different cut-off values of QFT. Logistic regression was computed in SPSS Version 15 for determination of risk factors for indeterminate (presented elsewhere (Lange et al. European Respiratory Journal. 2010 (in press)) and positive results.

Results: 38/460 (8,3%) patients had a positive QFT result (25 organ transplant, 6 autoimmune disease, 4 HIV positive, 3 other immunodeficiencies). Two patients taking TB prophylaxis after positive QFT were excluded. Risk factors for a positive QFT result included immigrant status and old age (p<0.05, backward logistic regression). Four patients had been treated for TB previously. In these and in 30 further patients there was no indication of active TB when QFT was performed or at follow up. One patient with chronic sialadenitis was treated empirically for presumed TB sialadenitis based on the positive QFT, no tissue culture was obtained. One HIV-positive patient with a history of M. chelonae skin infection had no evidence of mycobacterial disease when QFT was positive. One year later he developed culture proven tuberculous lymphadenitis.

Progression rate assessed retrospectively (including only the culture-proven case) was 1,3/100 pys or excluding known history of TB 1,4/100pys for a cut off of 0,35 IU/ml. Using a cut off of 1 IU/ml progression rate would be 3,2/100 pys or excluding known history of TB 3,4/100pys.

Conclusions: Keeping in mind the limits of a retrospective chart assessment to determine TB status, these results show that IC patients with a positive QFT result should be carefully evaluated. Prophylaxis should be recommended in light of anamnestic data (immigrant status, age) as well as quantity of the TB specific immune response.