Rationale: Immunological ex-vivo assays to diagnose tuberculosis (TB) have great potential, but have largely been blood-based and poorly evaluated in active TB. Lung sampling enables combined microbiological and immunological testing and utilises higher frequency antigen-specific responses than in blood.
Objective and Methods: Prospective evaluation of a flow-cytometric assay measuring the percentage of interferon-gamma synthetic CD4+ lymphocytes following stimulation with purified protein derivate of M. tuberculosis (PPD) in broncho-alveolar lavage fluid from 250 sputum smear negative individuals with possible TB. A positive assay was defined as >1.5%.
Results: Of those who underwent lavage and were diagnosed with active TB 95% (106 of 111) had a positive immunoassay (95% confidence intervals 89%, 98%). In 139 individuals deemed not to have active TB, 76% (105 of 139) were immunoassay negative (95% confidence intervals 68%, 82%). Among the remaining 24% (34 cases) with a positive immunoassay a substantial proportion had evidence of untreated TB - in 2 of these active TB was subsequently diagnosed. Assay performance was unaffected by HIV status, disease site or BCG vaccination. In culture-positive pulmonary cases, response to purified protein derivative was more sensitive than nucleic acid amplification testing (94% versus 73%). The use of early secretory antigen target-6 (ESAT-6) responses in 71 subjects was no better than PPD; and 19% of those with culture-confirmed TB and a positive PPD-immunoassay had no detectable response to ESAT-6.
Conclusions: These data suggest lung-orientated immunological investigation is a potentially powerful tool in diagnosing individuals with sputum-smear negative active TB regardless of HIV sero-status.
- immune response