Cryptococcal pneumonia and meningitis in a renal transplant recipient with a false negative serum cryptococcal antigen due to postzone phenomenon

Cryptococcal infection can cause significant morbidity and mortality in immunocompromised patients. We present a patient who was diagnosed with cryptococcal meningitis and pulmonary disease in the setting of a history of renal transplantation. The diagnosis was made based on growth of Cryptococcus neoformans in blood cultures and identification of cryptococcal antigen (CrAg) in cerebral spinal fluid (CSF) using a lateral flow assay (LFA). Our case is unique since the initial serum CrAg was falsely negative due to excess cryptococcal antigen preventing the formation of antigen-antibody complexes, referred to as the postzone phenomenon. This phenomenon has been reported on CSF samples but rarely reported on serum samples in patients without an HIV diagnosis.