Novel method to quickly detect active TB in smear-negative patients

WASHINGTON - A new method developed by European researchers can now quickly detect active tuberculosis (TB) in patients with negative sputum tests.

TB is the seventh-leading cause of death worldwide, and the diagnosis of active TB can be rapidly established when the bacteria can be identified on sputum microscopy.

However, in almost half of all cases, the TB bacterium cannot be detected, making another diagnostic option critical in efforts to control the spread of TB.

“In this study, we showed that a differentiation between active pulmonary tuberculosis and LTBI is possible by the ELISpot test,” said principal investigator of the Tuberculosis Network European Trialsgroup (TBNET), Dr. Christoph Lange.

Only 10 to 20 percent of world’s population is infected with M. tuberculosis go on to develop active TB, while the rest have latent TB infection, recently redefined as ‘lasting tuberculosis immune responses’ or LBTI, and are at risk for developing active TB at any time.

The researchers found that immune cells specific to the TB bacilli are concentrated in the airways of patients with active tuberculosis and these cells could be readily identified with an enzyme-linked immunospot assay (ELISpot) and the results of the test are available in one day.

The ELISpot test is then able to distinguish between LTBI and active TB by comparing the frequencies of TB-specific T-lymphocytes in the blood versus in the lung.

The identification of tuberculosis bacilli by culture, considered the gold standard takes several weeks.

“Because bronchoalveolar lavage is routinely performed in this situation for other diagnostic purposes, the ELISpot does not result in an extra procedure for the patient,” explained Lange

To determine the sensitivity and specificity of the ELISpot test, the researchers recruited 347 patients suspected of having TB, but who were either unable to produce sputum, or who had had three consecutive negative acid-fast bacilli sputum culture results.

Their blood was drawn and bronchoaveolar lavage (BAL) was performed for use in ELISpot testing.

Of the 347 patients, 71 were diagnosed with active pulmonary TB. In patients with active TB, ELISpot results were positive in 65 cases (91.5 percent).

“These findings show us that positive result in the BAL ELISpot was highly indicative of and actual case of active TB. And a negative BAL ELISpot result almost excludes active tuberculosis,” said Lange.

The study has been published in the American Thoracic Society’s American Journal of Respiratory and Critical Care Medicine. (ANI)