A new frontier for TB diagnosis

In a marked departure from the current methods for active TB diagnosis that are based on the presence of live bacteria in sputum samples, a rapid blood test that relies on two proteins for diagnosis and quantification of the severity of active TB has shown promise.

The blood test accurately detects minute levels of two biomarkers — CFP-10 and ESAT-6 — that TB bacteria release only during active infections. In a pilot study, the new blood test was able to diagnose active TB cases with “high sensitivity and specificity”. It was able to diagnose active TB even in people co-infected with HIV.

It was able to diagnose both pulmonary and extra-pulmonary TB cases with high sensitivity — over 91% in the case of culture-positive pulmonary TB (PTB) and above 92% extra-pulmonary TB (EPTB), and 82% in culture-negative PTB and 75% in EPTB in HIV-positive patients. In the case of HIV coinfected cases, the sensitivity was 87.5% for PTB and 85.7% for EPTB cases.

Obtaining sputum samples is not always easy. Biopsy samples are needed in the case of EPTB cases. Even Gene Xpert, introduced a few years ago to improve sensitivity and specificity, relies on sputum samples, and as per a 2014 WHO update, Xpert has “very low quality evidence” for EPTB diagnosis.

As per the results of the study published on Monday in the Proceedings of the National Academy of Sciences, the two peptides that are actively secreted by virulent Mycobacterium help in early diagnosis of active TB.

For the first time, quantitative results can be obtained that will help knowing the severity of active TB and in monitoring treatment outcomes.

It takes about four hours for the NanoDisk-MS assay to accurately detect the presence of the two peptides in the blood.