Implications of false positive serology of Toxoplasma gondii in a pre-transplant patient

Patient: A 21-year-old white male with cystic fibrosis. Chief Complaint: Pre-transplant workup in preparation for bilateral lung transplant. Past Medical History: Cystic fibrosis diagnosed at age 3, onset of insulin-dependent diabetes around age 20, and multiple hospitalizations for pulmonary and gastrointestinal complications. Family and Social History: The patient lives with his father and stepmother, has a pet bearded dragon, and has multiple tattoos and piercings. His stepmother has a cat, but he does not clean the litter box. Principal Laboratory Findings: The pre-transplant workup included several tests for infectious diseases, tests of organ function, radiology studies, and markers of malignancy. The only significant finding was a positive Toxoplasma gondii (T. gondii) IgM titer (≥1:40) (reference values for IgM: negative; <1:40, positive; ≥1:40) and IgG (1:2048) (reference values for IgG: negative; < 1:16, equivocal; ≥1:16 - <1:256, positive; ≥1:256). Testing was done by indirect immunofluorescence assay (IFA) in April 2012 in our hospital laboratory. The patient was treated with sulfadiazine, leucovorin, and pyrimethamine. Three months later (July), he returned for follow-up testing. Real-time polymerase chain reaction (PCR) for T. gondii DNA performed by a reference laboratory was negative. One month later (August), Toxoplasma serology was performed by enzyme-linked immunosorbent assay (ELISA) by a different reference laboratory and showed an elevated IgM of 0.95 IU/mL (reference values: negative; <0.55 IU/mL, equivocal; ≥ 0.55-< 0.65 IU/mL, positive; ≥ 0.65 IU/mL) and a normal level of IgG (<4 IU/mL). At this time, PCR was repeated and was negative. An additional month later (September), the patient's serology studies were performed at a third reference laboratory and showed an elevated IgM of 1.32 IU/mL (reference values: negative; 0.89, equivocal; 0.90 - 1.09, positive; >1.10) and a normal IgG.