Repeatedly low plasma alkaline phosphatase in a 56-year-old woman. A case of hypophosphatasia diagnosed in adulthood
Abstract
A 56-year-old woman had blood samples taken regularly to monitor lithium therapy for bipolar affective disorder. Plasma alkaline phosphatase (P-ALP) was repeatedly< 10 IU/L and the consultant psychiatrist (OBS) contacted the laboratory to exclude erroneous measurements. P-ALP was analyzed on a Dimension Vista® (Siemens Healthineers, Ballerup, Denmark) routinely calibrated and quality assured with certified internal and external quality control material. Over the previous five years, only this patient had more than one measurement of P-ALP<10 IU/L and faulty results were not suspected. Medical causes of hypophosphatasemia were excluded and possible explanations according to the algorithm suggested by Fraser were considered [2]. The patient was not anemic, did not take estrogen supplements, bisphosphonate, clofibrate, omeprazole, or lansoprazole. Nor did she follow any specific diet. She was previously diagnosed with hypothyroidism, and was well-supplemented with levothyroxine. There was no history of cardiac surgery or cardiopulmonary bypass [1]. Repeat laboratory tests on plasma revealed consistently low P-ALP activity, increased pyridoxal 5 ́-phosphate (PLP) (the major form of vitamin B6 [3]) to more than ten times the upper reference level, and low phosphate concentration. Thyrotropin (TSH) and zinc were within the respective reference ranges (Table 1). Vitamin D was not measured, though it would have been relevant. The patient had a history of bilateral metatarsal…
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