PTH immunoassay interference due to human anti-mouse antibodies in an obese subject with normal parathyroid function.

Immunoassay interference is most often found for Prolactin measurement. Only few data exist on immunoassay interference for other hormones.A 36 years old obese female subject (BMI 31 kg/m2) regularly attended our endocrine unit for type 2 diabetes therapy. When she was included as a control subject into a study for obesity management, a detailed laboratory testing was performed including parathyroid hormone (PTH). In the absence of clinical symptoms, she presented normal values for calcium, phosphate and vitamin D, however, at the same time PTH levels of >5000 ng/l. These results were obtained by Roche Elecsys electrochemiluminescence assay. Repeated measurements with this assay (mouse antibody) led to the same result, while using an Euroimmun assay (goat antibody) the exact PTH values were measured as 18,0 ng/l. Remarkably, after pre-treatment with an heterophilc antibody blocking reagent the results of the Roche assay dropped to a normal level. This phenomenon was explained by the detection of human anti-mouse antibodies (HAMA) in the probands serum.In case of Prolactin immunoassay interference, endogenous antibodies bind to the hormone in vivo resulting in complexes of a high molecular weight which are less efficiently cleared by the kidneys and thus accumulate in the blood. In contrast, the PTH values of >5000 ng/l detected in our subject are most likely due to a specific interference of the HAMA present in the probands serum with the assay antibody resulting in an artificial stimulation of the Roche Assay detection system ex vivo.