False-positive Histoplasma antigenemia caused by antithymocyte globulin antibodies

False-positive Histoplasma antigen results were identified in two patients who received rabbit antithymocyte globulin (ATG, Thymoglobulin(R)) to prevent allograft rejection. To determine the prevalence of false-positive results following the administration of Thymoglobulin, sequential specimens were tested from a cohort of transplant recipients. Of 107 such patients, 17 (15.9%) demonstrated false-positive tests for Histoplasma antigenemia. False antigenemia peaked at 2-4 weeks after ATG administration and cleared over the next few months. Physicians should be aware of the potential for false-positive results in specimens from patients who have received ATG.