Detection and quantification of Pneumocystis jirovecii DNA: colonization or infection

Background and Objectives: Detection of Pneumocystis jirovecii DNA in respiratory samples is a validated method for proving presence of the organism in those clinical samples; however this approach can not differentiate P. jirovecii pneumonia (PCP) and colonization. The burden of fungal infection and in consequence the load of fungal DNA varies in different group of immunocompromised patients, so quantification of DNA load could help differentiation of colonization and pneumonia. In present study, usefulness of a quantitative real-time PCR (qPCR) assay was evaluated to identify and discriminate these entities. Materials and methods: Two hundred and three patients with the presentation of atypical pneumonia consist of a triad of; dyspnea, fever, and cough were enrolled in the study. Bronchoalveolar lavage (BAL) samples were examined using both microscopy (stained with Calcofluor-white) and the real-time PCR assay. P. jirovecii DNA was detected and quantified by a real-time PCR method, targeting the mitochondrial large subunit ribosomal RNA gene (mtLSU rRNA) of P. jirovecii. Results: Twenty-four patients led to P.jirovecii detection. In ruling out of PCP all 24 patients were considered to be colonized by Pneumocystis. The fungal copy numbers in BAL fluids were (1.4x104/ml to 1.88x106/ml). A meaningful correlation between fungal copy numbers and the severity of disease was not seen. As PCP ruled oud in all 24 cases, a reliable Cut-Off point for differentiating PCP and colonization was not finding. Conclusion: Although Pneumocystis real-time PCR method showed good performance and represents an alternative method to detect P. jirovecii, more studies need for introducing a Cut-Off point. Those studies should include large enough samples from approved PCP and non-PCP patients for differentiating PCP and colonization according to the fungal copy number.