Pneumocystis jirovecii (PJ) can colonize the lungs, and cause pulmonary infections. It is essential to distinguish between PJ colonization and infection in the lung to avoid clinical overtreatment and delayed treatment. Many studies have shown that both the mNGS technique and serum G test can be used to aid the diagnosis of PJ pneumonia, but it is still unclear whether they have the appropriate cut-off value to distinguish PJ colonization from PJ infection.

To investigate the values of mNGS of alveolar lavage fluid and serum G test in distinguishing between infection and colonization of PJ and to assess the correlation between them.

A retrospective study design was used. Forty inpatients with pulmonary infections were recruited from Hubei No.3 People's Hospital of Jianghan University from September 2018 to May 2022, and divided into PJ infection group (n=21) and colonization group (n=19), according to the prevalence of anti-PJ treatment based on mNGS results of alveolar lavage fluid. Clinical data were collected, including general information〔gender, age, BMI, smoking history, underlying diseases (chronic kidney disease, hematological diseases, autoimmune diseases, malignancy, HIV, solid organ transplantation, chronic lung disease, the use of glucocorticoid/immunosuppressive agents) 〕, clinical symptoms (fever, cough, phlegm, dyspnea, chest pain, hemoptysis), imaging features (ground-glass shadow, interstitial change, consolidation, nodule, pleural effusion, cyst), laboratory indicators (white blood cell count, neutrophil count, lymphocyte count, PCT, CRP, LDH, PaO₂/FiO₂, CD₄⁺T lymphocytes). The receiver operating characteristic (ROC) curve of mNGS and serum G test was plotted and analyzed to assess their performance in distinguishing between PJ infection and colonization, and the correlation between the two was analyzed.

The utilization rate of glucocorticoids or immunosuppressants in PJ infection group was higher than that of colonization group (P<0.05). The prevalence of ground-glass shadow and interstitial change in PJ infection group was higher than that of colonization group (P<0.05). PJ infection group had lower number of CD₄⁺T lymphocytes, but higher number of sequences of PJ detected by mNGS and higher level of serum G measured by serum G test than colonization group (P<0.05). ROC analysis showed that in distinguishing between PJ infection and colonization, the optimal threshold of the number of sequences of PJ detected by mNGS was 24, with an AUC of 0.95, 95.2% sensitivity and 78.9% specificity, and that of serum G level measured by serum G test was 106.7 ng/L, with an AUC of 0.89, 76.2% sensitivity and 89.5% specificity. Spearman's correlation showed that the number of sequences of PJ detected by mNGS was positively correlated with serum G level measured by serum G test (r_s=0.769, P<0.001) .

The frequent use of glucocorticoid or immunosuppressive agent, imaging manifestation of typical ground-glass shadow or interstitial change, decreased number of peripheral blood CD₄⁺T lymphocytes, especially <200 /μl, and increased serum G level measured by serum G test (≥106.7 ng/L) or increased number of sequences of PJ detected by mNGS (n≥24), may be helpful for the diagnosis of PJ infection.