In blood, approximately 50% of the plasma calcium is free, 40 to 45% is protein bound mainly associated with albumin and between 5 to 10% is complexed. Calcemia measures total calcium but only free calcium is biologically active. Calcium plays an active physiological role in bone mineralisation, neuromuscular excitability, muscle contraction, and blood coagulation.
Serum protein levels must be considered for the proper interpretation of total serum calcium levels. Hypocalcemia can result from chronic renal failure with hypercreatinemia and hyperphosphatemia, or hypoparathyroidism, or vitamin D deficiency (osteomalacia, rickets…). The most common cases of hypercalcemia are associated with hyperparathyroidism, tumours and bone metastases, hyperthyroidism or vitamin D overdose… Calciuria has little practical value in the various diagnoses except for the lithiases and renal tubulopathies.
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