Sr. Director:
Measurement of serum parathyrine (PTH) has experimented significant advances in the last ten years thanks to the introduction of two-site immunometric assays for the intact hormone. These assays now available are suitable for use in most clinical laboratories and are widely used in studies of patients with abnormalities of calcium homeostasis. However, interpretation of serum PTH measurements requires the knowledge of the effects of drugs, diseases, and preanalytical variables on circulating levels of PTH, as well as a clear understanding of possible causes of analytical artefacts that may result in spurious hormonal concentrations. Here we report a case of very high serum PTH concentrations associated to normal serum calcium due to inappropriate anatomical site of specimen collection.
A request for analysis of serum PTH concentration and other biochemical and haematological constituents in a 47 year old man with the diagnosis of multiple endocrine neoplasia type 1 was received in our laboratory in August 1997. Serum intact PTH concentration measured by an automated two-site immunochemiluminometric assay (Immulite®, Diagnostics Product Corp., Los Angeles, CA, USA)1 was > 263 pmol/l (reference range: 1 to 6.5 pmol/L), and after sample dilution (1/10-1/100) concentration was of 350 pmol/l. Serum phosphate was 0.82 mmol/l, and alkaline phosphatase was 245 IU/l; results of remaining biochemical and haematological analysis were all within reference ranges (table 1).
Although biochemical data did not suggest evidence of severe osteomalacia, total serum 25-hydroxyvitamin D concentration was measured in an attempt to find a possible explanation for the abnormal PTH concentration in a patient without renal insufficiency, as secondary hyperparathyroidism could be present in vitamin D deficient patients2,3. Serum concentration of this analyte was clearly within the reference range (table 1).
Due to the fact that circulant endogenous antibodies could simultaneously bind the two antibodies (goat polyclonal) used in the two-site immunochemiluminometric assay3,4, non-immune goat, sheep, and bovine sera (Sigma Chemical Co., Madrid, Spain) were used at a concentration of approximately 20 mg protein/l in the PTH assay to block potential interference from these antibodies. Results from this assay did not change the concentration of PTH obtained with the original assay.
As no cause for this very high serum PTH concentration was evident, the analytical result was not reported and the referral endocrinologist was consulted about medications and other diseases which may have induced increments of serum PTH concentrations in the patient. The patient had antecedents of bilateral nephrolithiasis, but presently had no elevated calcium excretion5. Neither was he on lithium or on loop diuretics, drugs that may cause elevations in PTH concentrations6-8. However, he had been parathyroidectomized on three ocasions (1985, 1986, and 1994) because of hyperparathyroidism (nodular hyperplasia), and he had also been adrenalectomized unilateraly (1995) due to a non-functional adrenal adenoma. In 1985 he had developed recurrent peptic ulcerations and was medically treated. In 1994, elevated serum gastrin concentrations were found, and in 1995 diagnosis of multiple duodenal and pancreatic gastrinomas was made by immunocytochemistry. Six months after surgical intervention of gastrinomas, an lllIn-labelled octreotide scintigraphy was performed and showed abnormal localization of tracer in mean abdominal region and in the left lung. Currently, the patient is on treatment with lanreotide. Serum calcium concentrations prior to the present sample were 2.37 mmol/l (1995), and 2.41 mmol/l (1996).
In the face of all these clinical data the endocrinologist was asked whether the patient had undergone parathyroid autograft since the last parathyroid surgery. This was confirmed and we requested an interview with the patient, during which we asked for permission to take new blood specimens. Scars were observed on the left forearm corresponding to the site of the autogenous parathyroid graft. Blood specimens were taken from the engrafted forearm and from the contralateral forearm, in both cases from antecubital fossa veins, above the graft in the left forearm. Serum PTH concentrations in these samples measured by the same immunoassay gave the following results: 250 pmol/l from the left engrafted forearm (serum calcium 2.46 mmol/l), and 6.3 pmol/l from the right forearm (serum calcium 2,44 mmol/l).
This case report illustrates a patient with coexisting exaggerately high serum PTH concentration and normocalcemia which promoted unnecessary laboratory investigations. The analytical pattern observed in this patient could not be explained on a pathological, iatrogenic or methodologic basis. Simple information about the patient's clinical antecedents alone provided the clue to the analytical abnormality. The patient had undergone parathyroid autograft in his left forearm and knowledge of this preanalytical condition would have avoided the concern or at least allowed correct interpretation of the analytical results obtained.
Autogenous parathyroid grafts are used in the treatment of primary and secondary parathyroid hyperplasia and for salvaging normal parathyroid glands removed during thyroid surgery9. There is general agreement that when a first operation for primary hyperparathyroidism due to parathyroid hyperplasia fails, a portion of any gland resected during reoperation should either be transplanted or cryopreserved for future transplantation in the event of postoperative hypoparathyroidism10. Placing parathyroid grafts heterotopically in the forearm in order to facilitate documentation of graft function by measurement of PTH concentration in serum from blood obtained from each antecubital fossa has been advocated11. The demonstration of a significant gradient for PTH would confirm that the parathyroid graft is secreting. Demonstration of graft function in this manner can be obtained in approximately 80% of cases. The majority of the remaining 20% are normocalcemic and have PTH concentrations which indicate that functioning parathyroid tissue is present, despite the inability to demonstrate a gradient for this hormone11.
In this patient, serum PTH measurement was not requested with the aim to investigate the functioning of the grafted parathyroid tissue, but only to evaluate global parathyroid function after parathyroidectomy. In this respect, it is also opportune to point out that measurement of serum calcium should be sufficient, with PTH determination only being necessary in case of significant changes in serum calcium concentrations. PTH assays are expensive and should be applied only in appropriate clinical circumstances3.
Nowadays, laboratory staff are well aware of the different factors (drugs, diseases, preanalytical and methodological variables) which may affect the concentration of a considerable number of analytes. However, clinicians are generally not only unaware of the limitations of the analytical methods used to measure the analytes that they require to be measured, but also undervalue the importance of the effects of such variables on clinical laboratory tests12. Comprehensive databases have been constructed and updated to allow widespread dissemination of this information13-15. More than 30 preanalytical variables with potential effects on plasma PTH concentrations have been compiled in one of these15, but the situation we have reported here is not included among them. The case presented here also emphasizes how important it is that clinicians provide laboratory staff with the appropriate information to enable adequate interpretation of results.
