A comparison of direct microscopy, the concentration method and the Mycobacteria Growth Indicator Tube for the examination of sputum for acid-fast bacilli.

SETTING: In countries with high human immunodeficiency virus prevalence, laboratory diagnosis of pulmonary tuberculosis with the standard Ziehl-Neelsen (ZN) technique is characterised by low sensitivity.
OBJECTIVE: To compare test characteristics of direct microscopy, the concentration method and the Mycobacteria Growth Indicator Tube (MGIT).
DESIGN: Three hundred specimens from patients diagnosed with pulmonary tuberculosis were tested for the presence of mycobacteria. Specimens were stained with ZN, decontaminated by adding 4% NaOH, concentrated by centrifuging and processed in MGIT broth. The gold standard was defined as a positive MGIT culture or a positive acid-fast bacilli smear of material obtained from a negative culture after 42 days.
RESULTS: A total of 44 (14.7%) specimens were contaminated. Of 256 valid specimens, 234 (91.4%) were positive according to the gold standard definition. Decontamination and concentration of the sample increased the sensitivity of direct microscopy from 67.5% to 87.1%. Specificity remained unchanged (95.5%). The overall median time to detection of MGIT culture-positive specimens was 5 days, ranging from 4 (direct smear-positive specimens) to 12 days (concentration smear-negative specimens).
CONCLUSION: The concentration method substantially increases the sensitivity of direct microscopy without much extra input. The MGIT culture technique has considerable advantages, but its relatively high contamination rate and its high cost make it a less recommendable option for widespread use in routine district laboratories.