Endocrine Abstracts

A 28-year-old man presented to his GP in Turkey complaining of fatigue. As part of his work up, serum PTH was assessed and was found to be markedly elevated at 29.9 pmol/l (normal range 1.6–6.8), with normal calcium, phosphate, alkaline phosphatase, renal function, 1,25- and 25-hydroxy vitamin D3 levels. A DEXA scan was normal and a Sestamibi scan did not reveal any parathyroid adenoma. Prior to any further investigations or treatment, he moved to the UK.

He represented to a local UK GP explaining his ‘hyperparathyroidism’ and previous investigations in Turkey. Repeat measurement of PTH was however normal at 1.4 pmol/l (normal range 0.5–4.4). He was reassured but insisted on further investigations, which showed a high PTH of 35.4 pmol/l. He was then referred on to our unit.

The patient’s history and examination were normal and an erroneous result was suspected. Our laboratory had recently replaced its analytical equipment in between the UK based PTH measurements (Siemens Immulite 2500 replaced by Roche Cobas 6000) and we investigated the possibility that analytical interference could explain the discrepancy between the two assays.

Analysis after PEG precipitation or incubation in a heterophilic antibody blocking tube yielded normal PTH levels, indicating the presence of heterophilic or anti-animal antibodies. An explanation of the sequence of events was made to the patient and he was reassured. He was also made aware that antibody interference could potentially affect other blood tests.

Antibody interference can cause falsely high or low results. It should be considered when results do not fit the clinical picture, and particularly when discrepant results are obtained using different analytical methods. Although this is well described in the context of thyroid function tests, to our knowledge this is the first case described in the UK of antibody interference in a PTH assay.