Abstract

Background Pulmonary infection is a common and severe post-transplant complication in kidney transplant recipients (KTRs). Their long-term immunosuppression results in an extremely complex pathogen spectrum. Compared with conventional etiological detection methods, metagenomic next-generation sequencing (mNGS) enables rapid and broad-spectrum pathogen identification. However, compared with bronchoalveolar lavage fluid (BALF), research on the diagnostic value of sputum - used as a non-invasive sample - for pulmonary infections in KTRs remains limited. Methods A retrospective study included 77 kidney transplant recipients (KTRs) with pulmonary infections admitted from July 2021 to January 2025. BALF (n=37) or sputum (n=40) was collected for mNGS. Ninety-two non-immunosuppressed patients with pulmonary infections, treated during the same period and with BALF for mNGS, were also included. We compared pathogen profiles between the two groups and evaluated the diagnostic performance for KTRs pulmonary infections between BALF and sputum. Results The pathogen spectrum in KTRs was dominated by viruses (43.0%) and opportunistic fungi (20.0%), whereas bacteria (67.97%) predominated in the non-immunosuppressed group. The co-infection rate was significantly higher in KTRs than in the non-immunosuppressed group (67.57% vs. 35.87%, P<0.001). In the KTRs cohort, the sputum group had a much higher prevalence of heart disease than the BALF group (52.5% vs. 2.7%, P<0.001). The positive detection rates of sputum and BALF mNGS showed no statistical difference (97.5% vs. 91.89%, P = 0.268), but sputum mNGShad a higher concordance rate with the clinical composite diagnosis (95.0%) compared to BALF mNGS (81.08%). In both specimen types, mNGS achieved a significantly higher pathogen detection rate than conventional tests (P<0.001 for both), with poor agreement between the two approaches (Kappa < 0.2).