Heterophilic antibody as a source of error in immunoassay.

We describe our experience with a young woman believed to be hypothyroid and menopausal because of erroneously elevated TSH, LH, and FSH estimates. These errors were found to be due to the presence of antibodies to rabbit IgG in the patient's blood. We found that antibody-limited assays for TSH, LH, and FSH using antisera raised in rabbits were affected by this problem unless rabbit IgG was included, whereas antibody-excess assays were not. These problems were most simply detected by observing inappropriate results when measurements were made in dilutions of the patient's serum. The presence of endogenous antibody directed against antibodies used in immunoassays is a significant potential source of error requiring awareness on the part of both the clinician caring for such patients and the clinical laboratory making the measurements.