Cord factor detection and macroscopic evaluation of mycobacterial colonies: an efficient combined screening test for the presumptive identification of Mycobacterium tuberculosis complex on solid media.
OBJECTIVE
The rapid differentiation between Mycobacterium tuberculosis and nontuberculous mycobacteria is fundamental for patients co-infected with tuberculosis and HIV. To that end, we use two methods in our laboratory: detection of cord factor and PCR-restriction enzyme analysis (PRA). The objective of this study was to evaluate the accuracy of a screening test on solid medium as a rapid method for the presumptive identification of M. tuberculosis complex, considering costs and turnover time.


METHODS
A total of 152 strains were submitted to a combined screening test, consisting of the detection of cord factor under microscopy (Ziehl-Neelsen staining) and evaluation of the macroscopic aspect of colonies, as well as to PRA, which was used as the gold standard. Costs were estimated by calculating the price of all of the materials needed for each test.


RESULTS
The overall accuracy of cord factor detection alone was 95.4% (95% CI: 90.7-98.1%), and that of the combined screening test was 99.3% (95% CI: 96.4-100%). Cord factor detection costs US$ 0.25, whereas the PRA costs US$ 7.00. Results from cord factor detection are ready in 2 days, whereas PRA requires 4 days to yield results.


CONCLUSIONS
The presumptive identification of M. tuberculosis using the macroscopic evaluation of colonies combined with cord factor detection under microscopy is a simple, rapid and inexpensive test. We recommend the combined screening test to rapidly identify M. tuberculosis in resource-poor settings and in less well-equipped laboratories while awaiting a definite identification by molecular or biochemical methods.
Simeão
Fernanda Cristina dos Santos
FC
Instituto Adolfo Lutz, São Paulo, Brazil.
Chimara
Erica
E
Oliveira
Rosângela Siqueira
RS
Yamauchi
Jonas Umeoka
JU
Latrilha
Fábio Oliveira
FO
Telles
Maria Alice da Silva
MA
D015373:Bacterial Typing Techniques / Q000191:economics / Q000379:methods / Q000592:standards*; D003311:Cord Factors / Q000032:analysis*; D003470:Culture Media; D009169:Mycobacterium tuberculosis / Q000302:isolation & purification*; D016133:Polymerase Chain Reaction / Q000191:economics / Q000379:methods
D003311:Cord Factors; D003470:Culture Media