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Viability microscopy predicted concentration of M. tuberculosis in sputum culture
Quantitative viability microscopy was accurate in predicting the concentration of Mycobacterium tuberculosis in sputum culture, according to new data published in Clinical Infectious Diseases.
“Monitoring early treatment response is difficult because quantitative culture is slow, labor-intensive, expensive and too complex for most settings,” Sumona Datta, MD, from the Wellcome Trust Centre for Global Health Research, Imperial College London, told Infectious Disease News. “Conventional sputum smear microscopy and PCR techniques cannot discriminate between live and dead TB, so they cannot indicate early treatment response. Instead these tests assess treatment response months after beginning treatment, and we feel it is unacceptable for patients to receive treatment for months before knowing whether it is working.”
Datta and colleagues with the Innovation For Health and Development (IFHAD) team at Imperial College London, Universidad Peruana Cayetano Heredia and Associacion Benefica PRISMA in Lima, Peru, evaluated sputum samples collected from 35 patients who had sputum smear microscopy-positive TB disease. Sputum samples were collected pre-treatment and on treatment days 3, 6 and 9. The samples were evaluated with quantitative viability microscopy and with quantitative culture to determine how well the results from viability microscopy predicted the culture results.
Sumona Datta
Viability microscopy predicted quantitative culture results accurately within 1 logarithm for 76% of the results (P<.0001). In 31 patients who did not have multidrug-resistant (MDR) TB, viability microscopy and quantitative culture reduced by approximately one-half each day, so by day 9 of treatment, there was a 10-fold reduction in viability microscopy in 100% of patients and in quantitative culture in 95% of patients. In contrast, the four patients who were later found to have MDR-TB, did not demonstrate significant changes in viability microscopy or quantitative culture.
According to Datta, viability staining to monitor treatment response has been studied for leprosy, but the need for fluorescent microscopes in this technique limited the tests applicability. However, with the development of cheap and safe LED microscopes, interest in fluorescence microscopy techniques, including viability staining, has increased.
“We propose to use viability microscopy as a screening tool to demonstrate those who are not responding to treatment and to prompt physicians to investigate further,” Datta said. “The beauty of viability microscopy is that it is inexpensive, uses infrastructure that is already universally available and provides results within 1 hour.”
Datta said that viability microscopy needs further evaluation with a larger study that assesses its utility in diverse settings, and they are starting that study now. She said that other research questions to explore include what can be done to increase the concentration of M. tuberculosis in a sputum sample, to potentially allow viability microscopy to assess treatment response in patients with smear-negative TB. – by Emily Shafer
Sumona Datta, MD, can be reached at: sumona.datta@ifhad.org.
Disclosure: The researchers report no relevant financial disclosures. The study was funded by the Wellcome Trust and IFHAD.