Family Practice News (02.04.13)
Dr. Edward C. Jones López of Boston Medical Center and associates have reported on a new method of diagnosing infectious TB by directly measuring aerosols from coughs of pulmonary TB patients. Dr. Jones Lopez asserts that the analysis of cough aerosols was more accurate at predicting TB transmission than sputum smear microscopy or culture.
Using 96 adult TB patients with sputum AFB-positive culture and their 442 household contacts from May 2009 to January 2011, the researchers analyzed the number of M. tuberculosis colony forming units (CFUs) in the patients’ aerosols to determine whether the CFU number was a better prediction of new infection in contacts than an AFB-positive smear. The subjects were patients at Mulago Hospital National TB and Leprosy Programme in Kampala, Uganda, who lived with at least three household contacts. All patients had initial AFB of at least 1+ and M. Tuberculosis culture growth and had received fewer than 6 days of TB treatment or no treatment.
Of the total number of patients, 45 percent (43) produced aerosols that could be cultured, during two five-minute coughing periods. The 26 percent of total study group patients who produced high aerosols (at lest 10 CFUs) were more likely to transmit an infection to contacts than the 19 percent with low aerosols (1-9 CFUs) or 55 percent of aerosol-negative cases. Of the high-aerosol patients’ contacts, 69 percent were at-risk of tuberculin skin test (TST) conversion compared to 23 percent of low-aerosol patients and 30 percent of contacts of aerosol negative patients. New infections were diagnosed through a positive TST or interferon-gamma release assay (IGRA) with retest six weeks later for contacts who tested negative at baseline with TST and IGRA. TST conversion risk in low-aerosol and aerosol-negative patients’ contacts was similar.
The risk to high-aerosol patients’ contacts was more than five times greater than that of low-aerosol patients before adjustment. The authors concluded that high-aerosol TB patients more accurately predicted new TB infections based on risk of TST conversion. The also believe that in addition to providing a more precise marker of source infectiousness, cough aerosols may help determine the individual risk of TB infection after exposure.
The study, “Cough Aerosols of Mycobacterium Tuberculosis Predict New Infection: A Household Contact Study, was published online ahead of print in the American Journal of Respiratory Care and Critical Medicine (2013; doi: 10.1164/rccm.201208-1422OC).