Microcolony detection in 7H11 thin layer culture is an alternative for rapid diagnosis of Mycobacterium tuberculosis infection

SETTING: Radiometric technology and molecular biology are used in rapid diagnosis of tuberculosis in laboratories around the world. However, these technologies increase costs and are not available in laboratories where economic resources are limited.

OBJECTIVE: To compare sensitivity and time for detection of positive cultures in a microcolony method, Middlebrook 7H11 thin layer agar plate (TL7H11), and a conventional culture, Löwenstein-Jensen (L-J).

DESIGN: A total of 761 clinical samples were processed using acid-fast smear and culture on TL7H11 plates and L-J tubes. TL7H11 plates were checked microscopically for microcolony growth twice weekly for 4 weeks, and L-J tubes were checked once a week for 8 weeks.

RESULTS: Overall positivity was 11.0%. More than 60% of the positive samples were detected within the first 10 days on TL7H11, and none on L-J. After 2 weeks, more than 80% were positive on TL7H11 compared to 10% on L-J. In paucibacillary samples, TL7H11 detected 2.18% and L-J 4.57% (P < 0.001). Microcolony morphology was 100% distinctive for Mycobacterium tuberculosis on TL7H11. The calculated cost of TL7H11 prepared in the laboratory was US$2.90 per unit.

CONCLUSION: The TL7H11 method is an inexpensive, rapid and reliable alternative for diagnosing M. tuberculosis infection. It is therefore a valuable option for laboratories in low income countries.