Falsely elevated parathyroid hormone in a patient with osteoporosis: a case report and review

BACKGROUND

Parathyroid hormone (PTH) measurements can be falsely elevated due to the hormone binding to other molecules (macro-PTH) or immunoassay interference with heterophile, human anti-animal or other antibodies. This is rare but could lead to incorrect diagnosis, unnecessary investigations or avoidance of teriparatide treatment. We report a case of falsely high PTH levels due to assay interference and review the literature on cases of spuriously elevated PTH.

CASE REPORT

An 87-year-old woman attending our bone health clinic with osteoporosis had persistently elevated PTH (383-784 pg/ml) using the Roche Cobas e801 immunoassay despite having normal serum calcium, phosphate, 25 hydroxyvitamin D (> 50 nmol/l) and eGFR (> 60 ml/min). To rule out falsely elevated PTH, a polyethylene glycol precipitation (PEG) test was performed which recovered less than 10% of the hormone resulting in a normal level. PTH was also tested on a different assay (Atellica Siemens) that identified a result of 27 pg/ml. The findings were consistent with immunoassay interference likely due to heterophile antibodies giving rise to a spuriously high PTH.

DISCUSSION

The presence of unexpectedly high PTH levels should alert physicians to the possibility of false results due to assay interference or macro-PTH. This highlights the importance of clinically correlating results and of good communication with the testing laboratory. Here, we present the case of an 87-year-old woman with spuriously elevated PTH levels due to immunoassay interference likely mediated by heterophile antibodies. The presence of unexpectedly high PTH levels should prompt consideration of the possibility of false results due to assay interference or macro-PTH.

Interference in ELISA

ELISA is a well-established technique used worldwide to quantify analytes present in a diverse milieu of biological samplings. It is especially important to clinicians who rely on the accuracy and precision of the test to administer patient care. Those results are to be held with great scrutiny since the assay is subject to error caused by interfering substances found in the sample matrix. In this chapter, we examine the nature of such interferences and discuss approaches to identify and offer remedies to remove the interference and validate the assay.

Falsely elevated D-dimer partially caused by heterophilic antibodies: A case report

Plasma D-dimer is a screening marker for thrombosis and hemostasis disorders. Falsely elevated D-dimer may result in misdiagnosis and potentially invasive investigations. Therefore, identifying falsely elevated D-dimer is of great value in laboratory practice. D-dimer is determined by immunoassay and vulnerable to heterophilic antibody interference. Here, we reported a case of falsely elevated D-dimer partially caused by heterophilic antibodies. Dilution test, polyethylene glycol precipitation, heterophilic blocking reagent and method comparison were used to identify the heterophilic antibody interference. All these methods, except for the heterophilic blocking reagent, revealed the existence of heterophilic antibody interference. Polyethylene glycol precipitation failed to reduce the D-dimer to its reference interval. Therefore, we conclude that the falsely elevated D-dimer is partially caused by heterophilic antibody interference.