Smear microscopy misses many TB cases: AIIMS study

A significant number of Tuberculosis (TB) cases go undetected due to the limited sensitivity of smear microscopy, the most common diagnostic test for TB in India.

This finding was highlighted in a recent study conducted by the All India Institute of Medical Sciences (AIIMS), Gorakhpur, and published in the International Journal of Mycobacteriology.

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The study raises concerns about the continued reliance on smear microscopy, particularly in patients with low bacterial load, early-stage disease, extra-pulmonary TB, or co-morbid conditions such as HIV and diabetes. These groups are at a higher risk of being missed by conventional diagnostic methods, leading to delayed treatment and ongoing transmission of the disease.

According to the study, which was released here on Sunday, an analysis of 4,249 pulmonary and extra-pulmonary samples showed that smear microscopy detected TB in only 4.3% of cases.

“In comparison, the TrueNat Mycobacterium TB/rifampicin (RIF) test identified the infection in 13.7% of the samples, indicating its improved susceptibility. This study reinforces the need to expand rapid molecular testing under the national TB programmes to aid in timely diagnosis and get closer to India’s TB elimination goal.”

“Smear microscopy is affordable and easily available, but its sensitivity is limited, especially in patients with early disease, extra-pulmonary TB, HIV co-infection or diabetes,” said Dr Ashutosh Dubey, senior TB disease specialist at the Civil Hospital.

He further added that TrueNat, which is a chip-based real-time PCR test made in India and supported by WHO, can rapidly detect TB and identify rifampicin resistance, which was found in 5.6% of confirmed cases in the study.

According to Dr. Ashutosh, early detection of drug resistance is crucial to initiate appropriate treatment and prevent further infection. He emphasised that combining conventional microscopy with molecular diagnostics can significantly strengthen TB control efforts in resource-limited, high-burden areas.

The study strongly reinforces the need to expand access to rapid molecular testing under the National Tuberculosis Elimination Programme (NTEP). While smear microscopy will continue to play a role in TB diagnosis, experts emphasise that it should be complemented with molecular diagnostic tools to improve case detection and treatment outcomes.

“Combining conventional microscopy with molecular diagnostics can significantly strengthen TB control efforts, especially in rural and underserved areas,” Dr Dubey added. “Scaling up tests like TrueNat is essential if India is to meet its goal of eliminating TB.”

As India works towards its ambitious TB elimination targets, the study’s findings serve as an important reminder that improved diagnostic strategies are central to success. Enhancing early detection, identifying drug resistance promptly, and ensuring timely treatment can help reduce transmission, improve patient outcomes, and bring the country closer to a TB-free future.