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

BACKGROUND AND OBJECTIVE: 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.

METHODS: 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.

RESULTS: 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).

CONCLUSIONS: 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.