Increased sensitivity of acid-fast smears.

Evaluation of the centrifuges used in the mycobacteriology laboratory indicated a failure to develop sufficient relative centrifugal force for optimal concentration of acid-fast bacilli. Retrospective analysis of 14,509 specimens received between 1 July 1973 and 30 June 1975 and sedimented at 1,260 x g relative centrifugal force revealed a positive smear rate of 1.8% and a positive culture rate of 7.1%, with a correlation between positive paired smears and cultures of 25.2%. After increasing the relative centrifugal force from 1,260 x g to 3,800 x g at maximum radius, the positive smear and culture rates were 9.6 and 11.6%, respectively, with a correlation between positive paired smears and cultures of 82.4%. The sensitivity of acid-fast smears is directly related to the relative centrifugal force achieved while concentrating the specimen by centrifugation.