Poor performance of universal sample processing method for diagnosis of pulmonary tuberculosis by smear microscopy and culture in Uganda

Abstract

Laboratory methods to improve smear microscopy are an urgent priority for global tuberculosis control. The novel universal sample processing (USP) method has been reported to improve conventional diagnostic testing for tuberculosis while also providing inhibitor-free specimens for molecular assays. However, no studies evaluating the method in the field have been conducted. In this study, we compared the performance of the USP method to that of the standard N-acetyl-L-cysteine-NaOH (NALC) method for conventional diagnosis of tuberculosis in 252 adults admitted to Mulago Hospital in Kampala, Uganda, with a clinical suspicion of pneumonia. A single early-morning sputum specimen collected from each patient was divided into two aliquots, each of which was assigned a random identification number. One randomly numbered specimen was processed by the USP method and the other by the NALC method. Mycobacterial cultures were more frequently negative in USP compared to NALC specimen aliquots (58% versus 43%; P < 0.001). There was no difference in the proportion of contaminated mycobacterial cultures (12% versus 11%; P = 0.87). The sensitivity and specificity of smear microscopy for the USP method were 52% and 86%, respectively, and were not significantly different from those for the NALC method (56% and 86%, respectively) using mycobacterial culture results as a reference standard. These results suggest that the USP method did not provide any significant advantage over the standard NALC method for conventional diagnosis of tuberculosis in our setting and illustrate the importance of well-designed, field-level evaluations of novel diagnostic techniques.

FIG. 1.

Mycobacterial culture results (n = 207). Mycobacterial cultures were performed on Lowenstein-Jensen slants using specimen aliquots processed by the NALC and USP methods. Specimen aliquots processed by the USP method were significantly more likely to be culture negative (58% versus 43%; P < 0.001) and less likely to display confluent growth (6% versus 11%; P = 0.02). There were no significant differences in the proportion of each specimen aliquot with any other mycobacterial culture result, including contaminated cultures. *, P < 0.05 for comparison between the NALC and USP methods.

FIG. 2.

Smear microscopy results (n = 252). Results of fluorescence microscopy examination of smears prepared from specimen aliquots processed by the NALC and USP methods are shown. Smear microscopy results were classified according to the WHO/IUATLD scale. There were no significant differences in the proportion of USP- and NALC-processed specimens within any smear result category.