Letters To The Editor

Pulmonary tuberculosis

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To the Editor: The article by Dr. Catherine Curley, 1Rule out pulmonary tuberculosis: clinical and radiographic clues for the internist ,” was very well written, but we would like to point out two facts regarding the diagnosis of pulmonary tuberculosis, especially in high-prevalence countries like India, that might make the article more informative.

First, it has been shown conclusively that good-quality microscopy of two consecutive sputum specimens identifies the majority (95%–98%) of smear-positive tuberculosis patients. The World Health Organization (WHO) therefore revised its policy on case detection by microscopy 2 in 2007 to recommend a reduction in the number of specimens examined, from three to two in settings with appropriate external quality assurance and documented good-quality microscopy. This approach greatly reduces the workload of laboratories, a considerable advantage in countries with a high proportion of smear-negative tuberculosis patients because of human immunodeficiency virus (HIV), extrapulmonary disease, or both.

Moreover, in 2011, the WHO recommended in a policy statement that countries that have implemented the current WHO policy for two-specimen case-finding consider switching to same-day diagnosis, especially in settings where patients are likely to default from the diagnostic pathway. 3

Second, regarding the interferon-gamma-release assay, the 2011 WHO policy stated that there are not only insufficient data and low-quality evidence on the performance of this assay in low- and middle-income countries, typically those with a high tuberculosis and HIV burden, but also that the interferon-gamma-release assay and the tuberculin skin test cannot accurately predict the risk of infected individuals developing active tuberculosis. Moreover, neither the assay nor the skin test should be used for the diagnosis of active tuberculosis disease. The interferon-gamma-release assay is more costly and technically complex than the skin test. Given comparable performance but the increased cost, replacing the skin test with the interferon-gamma-release assay is not recommended as a public health intervention in resource-constrained settings. 4 The majority of tuberculosis cases (on average 85.8%) were detected with the first sputum specimen. With the second sputum specimen, the average incremental yield was 11.9%, while the incremental yield of the third specimen, when the first two specimens were negative, was 3.1%. 5

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    In reply: Pulmonary tuberculosis

    Purified protein derivative testing and the interferon-gamma-release assay are more useful in low-prevalence countries like the United States.