Pneumocystosis and quantitative PCR

Objective: Pneumocystis pneumonia (PCP) is now predominantly observed in immunosuppressed non-HIV-infected patients. The sensitivity of the PCR is here higher than direct examination (DE) of respiratory secretions because the infection is caused by a lower inoculum of Pneumocystis jirovecii (P. jirovecii). The objective of our retrospective study was to assess the contribution of quantitative PCR (qPCR) in the diagnosis of PCP.

Patients and methods: All patients hospitalized for PCP suspicion with a positive qPCR were included. Irrespective of the qPCR value, patients were initially classified into two groups (infection and colonization [PCP ruled out]) based on clinical, radiological, and microbiological data. Both groups were then compared based on the qPCR value.

Results: Between 2013 and 2016, 150 patients were included; 75% of them were not infected with HIV. The diagnosis of PCP was retained for 129 patients and rejected for 21 patients. The DE was negative in 60% of PCP cases. The median value of qPCR was 76,650copies/mL among infected patients and 3220copies/mL among colonized patients. The threshold corresponding to a specificity of 100% was 56,000copies/mL. The optimal value to distinguish an infection from a colonization was 10,100copies/mL.

Conclusion: Our study confirms the diagnostic value of the qPCR in immunosuppressed patients, especially when the DE is negative. When the qPCR is˂56,000copies/mL, the result should be interpreted based on the clinical context and paraclinical examinations.