Evaluation of a novel concentration method for the microscopic detection of 'Mycobacterium tuberculosis' from induced sputum

Abstract

Diagnosis of tuberculosis (TB) in developing countries relies on sputum smear microscopy, which has poor sensitivity compared to culture, particularly for immune-compromised people. Induction of sputum by inhalation of saline has the potential to increase case detection but centrifugation of the sample is necessary before microscopy. A systematic review was undertaken comparing the performance of microscopy and culture on induced sputum. A study was then undertaken in Zimbabwe on the use of magnetic beads (TB Beads) coated with chemical ligands to concentrate the mycobacteria from induced sputum, which if successful would negate the requirements for a biosafety cabinet and centrifuge, and therefore increase access to TB diagnosis in peripheral clinics. Adult TB suspects who were smear-negative on conventional microscopy underwent the induction procedure. The resulting sample was divided in two: half was processed with the TB Beads method with fluorescent and ZiehlNeelsen microscopy; the other half was subjected to centrifugation followed by fluorescent smear microscopy, and culture on Lowenstein-Jensen medium. Of the 139 patients that were enrolled, 97% produced an induced sputum sample. Mild side-effects were experienced by 13 % of patients. 26 smear-positive patients were found, of which 21 were positive on centrifugation, and 13 on TB Beads. Seven patients were culture-positive. One patient was smear-negative and culture-positive. The TB Beads elution buffer was altered by the manufacturer during enrolment, so results were analysed for both buffer types combined, as well as separately. For both buffers combined, the sensitivity and specificity of microscopy compared to culture were 43 % and 95 % respectively for ZN microscopy and 43% and 94% when using fluorescent microscopy. When both ZN and fluorescent microscopy were combined the sensitivity was 57%. However, when centrifugation was used prior to fluorescent microscopy, sensitivity increased to 86 %. In conclusion the TB Beads did not perform sufficiently well to replace centrifugation.