Inhibition of the polymerase chain reaction by sputum samples from tuberculosis patients after processing using a silica-guanidiniumthiocyanate DNA isolation procedure

With the objective to evaluate PCR-mediated detection of Mycobacterium tube rculosis DNA as a diagnostic procedure for diagnosis of tuberculosis in ind ividuals attending ambulatory services in Primary Health Units of the City Tuberculosis Program in Rio de Janeiro, Brazil, their sputum samples were c ollected and treated with a DNA extraction procedure using silica-guanidini umthiocyanate. This procedure has been described to be highly efficient for extraction of different kind of nucleic acids from bacteria and clinical s amples. Upon comparing PCR results with the number of acid-fast bacilli, no direct relation was observed between the number of bacilli present in the sample and PCR positivity. Part of the processed samples was therefore spik ed with pure DNA of M. tuberculosis and inhibition of the PCR reaction was verified in 22 out of 36 (61%) of the samples, demonstrating that the extra ction procedure as originally described should not be used for PCR analysis of sputum samples.