Abstract
BACKGROUND: Pneumocystis jirovecii pneumonia (PCP) is a severe opportunistic infection affecting immunocompromised patients. Quantitative polymerase chain reaction (qPCR) is widely used for the detection of P. jirovecii in respiratory samples. However, the diagnosis of PCP remains challenging and the high prevalence of P. jirovecii airway colonisation complicates the interpretation of positive results. The aim of this study was to assess the utility of P. jirovecii PCR Quantification Cycle (Cq) values in differentiating between PCP and colonisation in PCR-positive respiratory samples from immunocompromised patients. METHODS: Adult patients with P. jirovecii detected by qPCR in respiratory samples (bronchoalveolar lavage (BAL), sputum and oral wash) collected between 2017 and 2023 were retrospectively enrolled in the study. Patients were classified as having PCP or P. jirovecii colonisation and Cq values were compared between the groups. Receiver-operating characteristics (ROC) curve analyses were used to assess the performance of Cq values to distinguish between PCP and colonisation, and to establish Cq cut-off values for the different sample types. RESULT: Of 520 included participants, 247 patients (47.5%) were classified as PCP and 273 (52.5%) as colonised. The median Cq value was significantly lower in the PCP group compared to colonised patients in BAL (33.0 vs. 36.6, p < 0.001) and sputum (33.4 vs. 36.0, p < 0.0001), yielding a ROC area under the curve of 0.75 and 0.73, respectively. Cq levels for oral wash did not differ between PCP and colonisation and lacked discriminatory power with a ROC AUC of 0.45. A Cq cut-off level at 31 for BAL and sputum could predict PCP with a positive predictive value of > 85% while Cq < 38 provided a negative predictive value of 89% for BAL and 73% for sputum. CONCLUSION: Different Cq cut-off values in BAL and sputum may support discrimination between PCP and colonisation and assist physicians in their clinical management of PCP.