Atypical pneumonia testing in transplant recipients.

BACKGROUND: The incidence of atypical pneumonia among immunocompromised patients is not well characterized. Establishing a diagnosis of atypical pneumonia is challenging as positive tests must be carefully interpreted. We aimed to assess the test positivity rate and incidence of atypical pneumonia in transplant recipients.

METHODS: A retrospective cohort study was conducted at the Yale New Haven Health System in Connecticut. Adults with solid organ transplant, hematopoietic stem cell transplant (HSCT), or chimeric antigen receptor T-cell, who underwent testing for atypical pathogens of pneumonia (Legionella pneumophilia, Mycoplasma pneumoniae, Chlamydia pneumoniae, and Bordetella pertussis) between January 2016 and August 2022 were included. Positive results were adjudicated in a clinical context using pre-defined criteria. A cost analysis of diagnostic testing was performed.

RESULTS: Note that, 1021 unique tests for atypical pathogens of pneumonia were performed among 481 transplant recipients. The testing positivity rate was 0.7% (n = 7). After clinical adjudication, there were three cases of proven Legionella and one case of possible Mycoplasma infection. All cases of legionellosis were in transplant recipients within 1-year post-transplantation with recently augmented immunosuppression and lymphopenia. The possible case of Mycoplasma infection was in an HSCT recipient with augmented immunosuppression. The cost of all tests ordered was $50,797.73.

CONCLUSION: The positivity rate of tests for atypical pneumonia was very low in this transplant cohort. An algorithmic approach that targets testing for those with compatible host, clinical, radiographic, and epidemiologic factors, and provides guidance on test selection and test interpretation, may improve the diagnostic yield and lead to substantial cost savings.