Integrated diagnostic strategies for Pneumocystis pneumonia: bronchoalveolar lavage qPCR cycle threshold and serum β-D-glucan.

OBJECTIVES

Pneumocystis jirovecii pneumonia (PCP) is a severe opportunistic infection increasingly affecting HIV-uninfected immunocompromised patients. Although bronchoalveolar lavage fluid (BALF) PCR is highly sensitive, distinguishing infection from colonisation remains challenging. We evaluated the diagnostic performance of BALF quantitative PCR (qPCR) cycle threshold (Ct) values (BioGX, BD MAX) and an integrated diagnostic strategy combining qPCR and serum β-D-glucan (BDG) (Toxinometer MT-6500 system).

METHODS

We conducted a retrospective diagnostic accuracy study including all BALF P. jirovecii qPCR tests performed between March 2020 and December 2024 at the University Hospitals of Geneva. Each test was considered a distinct episode. PCP status was adjudicated by an expert panel according to European Organisation for Research and Treatment of Cancer/Mycoses Study Group Education and Research Consortium 2020 criteria, blinded to Ct results. Diagnostic performance of BALF qPCR Ct values and BDG was assessed using receiver operating characteristic curve analysis.

RESULTS

Among 1300 PCR tests, 774 BALF qPCR episodes were included; 68 of 774 (8.8%) were positive. After integration of PCR results, 47 of 774 (6.1%) were classified as probable/proven PCP and 28 of 774 (3.6%) as colonisation. BALF qPCR Ct values showed good diagnostic performance (Area Under the Curve (AUC) 0.85; 95% CI, 0.76-0.94), with an optimal cutoff of 30 (sensitivity 0.73, specificity 0.87). qPCR negativity showed high sensitivity (0.96; 95% CI, 0.85-0.99) for ruling out PCP. Among PCR-positive episodes, Ct ≤26 identified probable/proven PCP and Ct ≥36 colonisation. BDG levels were higher in PCP (median 119 vs. 0 pg/mL) and showed excellent discrimination (AUC, 0.96; 95% CI, 0.93-1.00), with an optimal cutoff of 9 pg/mL (sensitivity 0.97, specificity 0.90). For intermediate Ct values (26-36), BDG cutoff of 9 pg/mL further differentiated PCP from colonisation (sensitivity 0.94, specificity 0.93). Among patients with BDG ≥9 pg/mL, a qPCR Ct <36 identified PCP (sensitivity 0.94, specificity 0.96).

CONCLUSIONS

BALF qPCR Ct values reliably discriminate PCP from colonisation. Combined interpretation of Ct values and BDG enhances diagnostic confidence in clinical practice.