The presence of pretreatment cavitations and the bacterial load on smears predict tuberculosis infectivity negative conversion judged on sputum smear or culture.

OBJECTIVE There are no rules to predict the time to infectivity negative conversion based on sputum smear conversion or culture conversion during chemotherapy in smear-positive tuberculosis patients. We aimed to develop and validate rules of sputum smear grades and cavitations in the lungs due to tuberculosis. METHODS This study was a retrospective cohort study that consisted of development (n=158, 64 ± 19 years of age) and validation (n=214, 63 ± 20 years of age) steps in different cohorts. We developed a rule with pretreatment smear grades and the existence of cavitations in the lungs due to tuberculosis as independent variables and the duration to infectivity negative conversion as the dependent variable in a Cox hazard model. The smear grade indicating the pretreatment bacterial load was classified into four grades according to the Japanese guidelines. The presence of cavitations was assessed on X-ray. Infectivity conversion was defined according to the criteria of the Japanese Ministry of Health, Labour and Welfare: a patient is currently receiving proper treatment and shows clinical improvement and sputum smears and cultures in any combination are consecutively negative three times. RESULTS We developed an "Infectivity Conversion Score" classifying each patient with a smear grade of I though IV and assessing the existence of cavitations in the lungs due to tuberculosis. The median time to infectivity conversion in the validation cohort was 13, 22, 35 and 50 days for Infectivity Conversion Scores I through IV, respectively (p<0.001). CONCLUSION We developed and validated Infectivity Conversion Scores.