Real-time PCR is more specific than conventional PCR for induced sputum diagnosis of Pneumocystis pneumonia in immunocompromised patients without HIV infection

The diagnosis of Pneumocystis pneumonia (PCP) is based on microscopic examination of respiratory specimens. PCP patients without AIDS have a lower burden of P. jiroveci than those with AIDS, which leads to difficulty in detecting the organisms. Although conventional PCR (c-PCR) has been used to detect the DNA, it is frequently positive in patients with colonization. Real-time PCR (r-PCR), a method to detect the DNA quantitatively, might be helpful in distinguishing between infection and colonization. We investigated the utility of real-time PCR in the diagnosis of PCP in non-AIDS patients. Induced sputum samples obtained from 86 non-HIV immunocompromized patients with clinical symptoms of pulmonary infection were evaluated for the presence of Pneumocystis jiroveci-specific DNA using c-PCR and r-PCR. The diagnosis of PCP was confirmed by typical clinical and radiological findings and response to treatment. Of the 86 patients, 17 were diagnosed as having PCP. Twenty-eight samples were positive for c-PCR, but the false-positive rate was high (46.4%). Sensitivity, specificity and positive predictive values (PPV) of c-PCR were 88.2%, 81.2% and 53.6%, respectively. Concentrations of the DNA detected by r-PCR were significantly higher in PCP patients than in non-PCP patients. Using 30 copies per tube as a cut-off value for the diagnosis of PCP, the sensitivity (82.4%) of r-PCR was almost equal to c-PCR. Notably, its specificity and PPV were higher than c-PCR (98.6% and 93.3%, respectively). r-PCR on induced sputum is more useful for diagnosing PCP than c-PCR in non-HIV immunocompromized patients, especially in terms of distinguishing between colonization and infection.