In the hospital referred to in this case, the assay used was a Roche Elecsys PTH STAT assay. This is an antibody-based assay that uses two murine antibodies – a “capture” antibody and a “signal” antibody. In patients who have been previously exposed to murine tissue, anti-mouse antibodies can be present in the serum that bind to both of these antibodies giving a false positive result on the assay. In this case, the patient had received OKT3 in the past which has been associated with a prevalence of anti-mouse antibodies of up to 26%. Her PTH level was repeated using an alternative assay – her pre-op PTH was 18 pg/ml and her post-op PTH was 5 pg/ml. Unfortunately, she required long-term treatment with vitamin D and calcium supplementation. In retrospect, the normal calcium and phosphate in a patient with this degree of apparent hyperparathyroidism and normal kidney function was a clue that there may have been an issue with the test although, of course, it is easy to say that knowing the answer.
Congratulations to “anonymous” for getting the correct answer. The complete case report can be found here.