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Calcium, Urine

Analyte: Calcium

Specimen Type: Urine (24h w preservative) * Please contact PBI for collection instructions.

Optimum Volume: 1 mL

Stability:

2-8°C -20°C -70°C
2 weeks 3 weeks 6.5 years

Reporting units: mg/24 hr

Method: Colorimetric

Biological or Clinical Significance:

The calcium content of an adult is somewhat over 1 kg (25,000 mmol), i.e. about 2% of the body weight. Of this, 99% is present as calcium hydroxyapatite in bones and less than 1% is present in the extraosseous ICS (intracellular space) or ECS (extracellular space). The calcium level in the ECS (approx.100 mmol) is in dynamic equilibrium with the rapidly exchangeable fraction of bone calcium. Calcium ions affect the contractility of the heart and the skeletal musculature and are essential for the function of the nervous system. In addition, calcium ions play an important role in blood clotting and bone mineralization. In plasma, calcium is bound to a considerable extent to proteins (approx.40%), 10% is in the form of inorganic complexes and 50% is present as free (ionized) calcium. The body’s calcium balance is regulated by 1, 25-dihydroxyvitamin D, parathyroid hormone (PTH), calcitriol (CT) and calcitonin.

The complexometric test is used for determining calcium in both serum and urine. Serum testing is used for the diagnosis and monitoring of hypocalcemia (calcium deficiency) and hypercalcemia (excess calcium). Hypocalcaemia is generally due to the absence or impaired function of the parathyroid or impaired vitamin D-synthesis. Hypocalcaemia is primarily brought about by increased mobilization of calcium from the skeletal system (osteoporosis) or increased intestinal absorption. The majority of cases are due to primary hyperparathyroidism or bone metastasis of carcinoma of the breast, prostate or thyroid and bronchial carcinoma.

The main significance of determining urinary calcium lies in the differentiation between hypercalciuria and hypocalciuria and the differential diagnosis of nephrolithiasis. However, calcium excretion may be monitored during treatment to determine the effective of a therapy on calcium excretion (for example, reference 1). This may be done by monitoring either 24h urine excretion or the urine calcium/creatinine ratio. The reference range data provided in the test demographic section of this SOP is for 24h urine collections.

Complexometric methods are used in addition to atomic absorption spectrometry (AAS) for determining calcium. The following calcium determination is based on the reaction of calcium with o-cresolphthalein complexone in alkaline solution. The elimination of interference from Magnesium is accomplished with 8-hydroxyquinoline.

Principle of Test Method:

The urine calcium assay is an automated colorimetric assay.

References:

1. TsukamotoY, Watanabe, T, Nakagami T, Morishita K. Effect of treatment with oral calcitriol on calcium metabolism and fasting serum 25(OH)- or 1,25(OH)2-vitamin D level in Japanese postmenopausal women. Endocr J 2003; 50:681-687.

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