Rapid diagnosis of active pulmonary TB is difficult. Microscopy for identifying acid-fast bacilli is quick, but it is negative in 50% of those with active disease; nucleic acid amplification tests also have low sensitivity. In active TB, Mycobacterium tuberculosis–specific lymphocytes are activated at the site of infection. Results of a pilot study (JW Infect Dis Nov 15 2006) have suggested that response by bronchoalveolar lavage fluid mononuclear cells (BALMCs) to two M. tuberculosis antigens, ESAT-6 and CFP-10, is significantly increased in patients with active pulmonary TB but not in those with latent disease.
As a follow-up to this work, investigators (some with support from the assay manufacturer) performed an enzyme-linked immunospot (ELISpot) test on peripheral blood mononuclear cells (PBMCs) and BALMCs from 347 patients with suspected active TB. Seventy-one of the patients were eventually determined to have active TB; the remaining 276 had other conditions. ELISpot on BALMCs had a sensitivity of 91% and a specificity of 79% in diagnosing active TB (diagnostic odds ratio, 27; P<0.001). Moreover, among patients with a positive PBMC ELISpot, the BALMC ELISpot was positive in 55 of the 60 with active TB (92%) and negative in 82 of the 116 without active TB (71%; P=0.0012), demonstrating that BALMC ELISpot distinguished active from latent TB.
Comment: This assay has two drawbacks: It requires an invasive procedure (bronchoalveolar lavage), and results can be indeterminate. However, the test is clearly useful for rapidly establishing the diagnosis of active TB and has applicability in other pulmonary infections, such as the endemic mycoses.
— Neil M. Ampel, MD
Published in Journal Watch Infectious Diseases October 21, 2009