Synonyms: Blood calcium, total calcium, urinary calcium
Related tests: parathyroid hormone, Vitamin D
Clinical significance
Almost all the body’s calcium is stored in the bones; only 1% of calcium is present in the blood, in three forms: ionised, bound to salts (citrate, bicarbonate, etc.) and bound to proteins.
The calcium ion regulates neuromuscular activity and excitability, cell and vascular permeability, the coagulation cascade and the activity of many enzymes; because of this essential role, the body tends to keep the calcium concentration constant (calcium homeostasis).
Absorbed in the small intestine thanks to the active form of vitamin D (1,25), calcium is eliminated in the stools and urine; in the kidneys it is first filtered by the glomeruli and then almost entirely reabsorbed in the tubules under the influence of parathyroid hormone (which increases reabsorption).
A rise in plasma calcium is mainly caused by hyperparathyroidism.
A fall in plasma calcium can be caused by hypoparathyroidism, reduced absorption from the gut or increased excretion in the urine.
Increased urinary calcium excretion can be caused by increased absorption from the gut, increased bone turnover or reduced reabsorption by the kidneys.
Reduced urinary calcium excretion is mainly caused by insufficient intake.
Clinical indications
Suspected disorder of calcium metabolism.
Sample type
The patient must have a blood sample taken and, if requested, collect a 24-hour urine sample.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.
Notes
Urinary calcium excretion varies widely both in perfect health and in disease: excretion varies with the calcium and protein content of the diet and with urinary phosphate excretion.
The reference range shown is indicative and refers to a diet with an average calcium content.