Calcium, Total

Calcium is an essential ion within the human body. The maintenance of a constant free ionised calcium concentration is biologically important for the function of excitable tissues. Abnormalities in serum calcium values may have profound effects on neurological, gastrointestinal and renal function. Normal calcium concentrations are maintained as a result of tightly regulated ion transport by the kidneys, intestinal tract and bone. This is mediated by calcaemic hormones, in particular parathyroid hormone and the active form of Vitamin D.

The calcium content of an adult is somewhat over 1 kg (about 2%of the body weight). Of this, 99% is present as calcium hydroxyapatitein bones and less <1% is present in the extra-osseous intracellularspace or extracellular space (ECS). The calcium level in the ECS isin dynamic equilibrium with the rapidly exchangeable fraction of bonecalcium.

Plasma calcium exists in the blood in three forms; 50% is ionized, (2) 40-45% is protein bound,and (3) 5-10% is complexed to anions such as bicarbonate, citrate, sulfate, phosphate, lactate.

Plasma ionized calcium is the biologically active moiety.

Total calcium levels are maintained between 8.8 and 10.2 mg/dL. (2.25-2.55mmol/L)

Multiple biological functions of calcium

  • Cell signalling
  • Neural transmission
  • Muscle function
  • Blood coagulation
  • Enzymatic co-factor
  • Membrane and cytoskeletal functions
  • Secretion
  • Biomineralization

Calcium homeostasis

Calcium homeostasis is regulated by three hormones, parathyroid hormone, vitamin D and calcitonin. The free, ionised calcium concentration is physiologically important for the functions of excitable tissues such as nerve and muscle.

1. Vitamin D:Vitamin D3 (cholecalciferol) is produced by the action of sunlight and is converted to 25-hydroxycholecalciferol in the liver. The 25-hydroxy-cholaecalciferol is converted in the proximal tubules of the kidneys to the more active metabolite 1,25-hydroxy-cholaecalciferol. 1,25-hydroxychlecalceriferol synthesis is regulated in a feedback fashion by serum calcium and phosphate. Its formation is facilitated by parathyroid hormone.

1. Enhances calcium absorption from the intestine

2. Facilitates calcium absorption in the kidney

3. Increases bone calcification and mineralization

4. In excess, mobilises bone calcium and phosphate

2. Parathyroid hormone (PTH)

Parathyroid hormone is secreted by the chief cells in the parathyroid glands. Plasma ionized calcium acts directly on the parathyroid glands in a feedback manner to regulate the secretion of PTH. In hypercalcaemia, secretion is inhibited, and the calcium is deposited in the bones. In hypocalcaemia, parathyroid hormone secretion is stimulated. The actions of PTH are aimed at raising serum calcium.

1. Increases bone resorption by activating osteoclastic activity

2. Increases renal calcium reabsorption by the distal renal tubules

3. Increases renal phosphate excretion by decreasing tubule phosphate reabsorption

4. Increases the formation of 1,25-dihydrocholecalciferol by increasing the activity of alpha-hydroxyls in the kidney

3. Calcitonin

Calcitonin is secreted by the parafollicular cells in the thyroid gland. It tends to decrease serum calcium concentration and, in general, has effects opposite to those of PTH. The actions of calcitonin are as follows:

1. Inhibits bone resorption

2. Increases renal calcium excretion

The exact physiological role of calcitonin in calcium homeostasis is uncertain.

A large amount of calcium is filtered in the kidneys, but 99% of the filtered calcium is reabsorbed. About 60% is reabsorbed in the proximal tubules and the remainder in the ascending limb of the loop of Henle and the distal tubule. Distal tubule absorption is regulated by parathyroid hormone.

Calcium-sensing receptor (CASR) It is a G protein-coupled receptor that plays an essential part in regulation of extracellular calcium homeostasis. This receptor is expressed in all tissues related to calcium control, i.e. parathyroid glands, thyroid C-cells, kidneys, intestines and bones. By virtue of its ability to sense small changes in plasma calcium concentration and to couple this information to intracellular signalling pathways that modify PTH secretion or renal calcium handling, the CASR plays an essential role in maintaining calcium ion homeostasis.

The main causes of hypercalcemia are

Parathyroid hormone-related /
  • Primary hyperparathyroidism*
  • Sporadic, familial, associated with multiple endocrine neoplasia I or II
  • Tertiary hyperparathyroidism
  • Associated with chronic renal failure or vitamin D deficiency

Vitamin D related /
  • Vitamin D intoxication
  • Usually 25-hydroxyvitamin D2in over-the-counter supplements
  • Granulomatous disease sarcoidosis, berylliosis, tuberculosis
  • Hodgkin's lymphoma

Malignancy related /
  • Humoral hypercalcemia of malignancy* (mediated by PTHrP)
  • Solid tumors, especially lung, head, and neck squamous cancers, renal cell tumors
  • Local osteolysis (mediated by cytokines) multiple myeloma, breast cancer

Medications /
  • Thiazide diuretics (usually mild)*
  • Lithium
  • Milk-alkali syndrome (from calcium antacids)
  • Vitamin A intoxication (including analogs used to treat acne)

Other endocrine disorders /
  • Hyperthyroidism
  • Adrenal insufficiency
  • Acromegaly
  • Pheochromocytoma

Genetic disorders /
  • Familial hypocalciuric hypercalcemia: mutated calcium-sensing receptor

Other /
  • Immobilization, with high bone turnover (e.g., Paget's disease, bedridden child)
  • Recovery phase of rhabdomyolysis

Parathyroid related

  • Primary
  • Sporadic, familial, associated with multiple endocrine neoplasia I or II
  • Tertiary hyperparathyroidism
  • Associated with chronic renal failure or vitamin D deficiency

Increased screening of calcium levels and wide availability of reliable assays for intact PTH levels have led to more frequent and earlier diagnoses of primary hyperparathyroidism. In 80 percent of cases, a single parathyroid adenoma is responsible. However, hyperparathyroidism also can result from hyperplasia of the parathyroid glands or, rarely, parathyroid carcinoma. In primary or tertiary hyperparathyroidism, PTH levels are normal or high in the setting of hypercalcemia

Parathyroid carcinoma occurs in 1 to 3% of cases. Hyperparathyroidism also occurs in multiple endocrine neoplasia type 1 and 2A.
Evaluation of hypercalcemia usually begins with measurement of total calcium. If total calcium is markedly elevated, an ionized calcium level is usually not needed. Slight to moderately elevated total calcium should be confirmed by measurement of ionized calcium.

Hypercalcemia of malignancy

  • Malignancies can raise serum calcium levels by either direct bone destruction or secretion of calcemic factors. Patients with squamous cell carcinoma of the lung, metastatic breast cancer, multiple myeloma, and renal cell carcinoma are most prone to hypercalcemia. These tumors may produce PTH related protein (PTH-rp) which binds to PTH receptors, but is not detected by standard intact PTH immunoassays. Specific assays for PTH-rp are available.

Vitamin D-related

  • Vitamin D intoxication
  • Usually 25-hydroxyvitamin D2in over-the-counter supplements
  • Granulomatous disease sarcoidosis, berylliosis, tuberculosis
  • Hodgkin's lymphoma

In suspected overdose of over-the-counter vitamin D, the level of 25-hydroxyvitamin D3should be measured. Macrophages can cause granuloma-forming (i.e., sarcoidosis, tuberculosis, hodgkin's lymphoma) increased extra-renal conversion of 25-hydroxyvitamin D3to calcitriol. PTH levels are suppressed, and levels of 1,25-dihydroxyvitamin D3are elevated.

Medications

  • Thiazide diuretics (usually mild)*
  • Lithium
  • Milk-alkali syndrome (from calcium antacids)
  • Vitamin A intoxication (including analogs used to treat acne)

Thiazide diuretics increase renal calcium resorption and cause mild hypercalcemia that should resolve when the medication is discontinued. Thiazide diuretic therapy can unmask many cases of primary hyperparathyroidism. Consumption of large amounts of calcium carbonate via calcium-containing antacids can lead to hypercalcemia, alkalosis, and renal insufficiency—an uncommon disorder termed milk-alkali syndrome.14Lithium use can cause hypercalcemia by increasing the set point of PTH,15requiring a higher serum calcium level to switch off PTH secretion. Large doses of vitamin A and its analogs can cause hypercalcemia, which appears to be mediated through increased bone resorption.

If time permits, total calcium levels should be repeated two more times to rule out a transient cause of hypercalcemia before undertaking a complete work-up.

If hypercalcemia is still evident, serum albumin and total protein should be determined. Calcium levels should be corrected for elevated albumin levels (see below). If total protein is high, but albumin is normal or low, a monoclonal gammopathy should be ruled out by serum protein electrophoresis.

Serum chloride, phosphorus and intact PTH are also useful in diagnosing the most frequent causes of hypercalcemia; malignancy and hyperparathyroidism. Serum chloride is mildly elevated in primary hyperparathyroidism.

Test / Hyperparathyroidism / Malignancy
Total calcium (mg/dL) / <12.4 / >12.4
Chloride (meq/L) / >103 / <103
Phosphorus / normal to low / normal
Chloride : phosphorus ratio / 29 or greater / <29
Intact PTH / elevated / suppressed
PTH-rp / normal / elevated
Calcitriol / elevated / low

Hypocalcemia most commonly results from PTH deficiency or failure to produce 1,25 dihydroxy vitamin D.

Most common causes of hypoparathyroidism are parathyroid or thyroid surgery and parathyroid infiltration by cancer, sarcoid, amyloid or hemochromatosis. Acute illnesses such as pancreatitis, hepatic failure, sepsis, and various medications can also cause hypocalcemia.
Drugs associated with hypocalcemia. Gentamicin and cisplatin cause renal magnesium loss, which leads to hypocalcemia. Heparin therapy releases fatty acids that bind calcium ions and cause transient hypocalcemia. Anticonvulsants such as dilantin and phenobarbital induce the microsomal oxidase pathway which accelerates inactivation of vitamin D. Loop diuretics such as furosemide enhance renal calcium excretion. Phosphate salts bind up calcium ions causing hypocalcemia.
The laboratory evaluation of a low total plasma calcium level should include measurement of ionized calcium, magnesium, and phosphorus levels.

  • Low ionized calcium rules out artefactual causes of hypocalcemia, such as hypoalbuminemia.
  • Abnormally high or low magnesium levels should be excluded because they can inhibit PTH secretion.
  • A low serum phosphorus level is consistent with vitamin D deficiency, while a high level suggests chronic renal failure or pseudohypoparathyroidism.
  • Measurement of intact PTH levels helps to differentiate between conditions caused by PTH and vitamin D defects.
  • The demonstration of an inappropriately low intact PTH level in the presence of hypocalcemia is consistent with the diagnosis of hypoparathyroidism. Serum 25-hydroxyvitamin D levels can be measured to confirm vitamin D deficiency

Normal range

The serum level of calcium is closely regulated with a normaltotal calciumof 2.2-2.6mmol/L (9-10.5mg/dL) and a normalionized calciumof 1.1-1.4mmol/L (4.5-5.6mg/dL). The amount of total calcium varies with the level ofserum albumin, a protein to which calcium is bound. The biologic effect of calcium is determined by the amount ofionized calcium, rather than the total calcium.

Ionizedcalcium does not vary with the albumin level, and therefore it is useful to measure the ionized calcium level when the serum albumin is not within normal ranges, or when a calcium disorder is suspected despite a normal total calcium level.

Corrected calcium level

Total calcium levels are affected by changes in plasma protein concentrations. Most of the protein bound fraction of calcium is bound to albumin; each 1 g/dL of albumin binds 0.8 mg/dL of calcium. One can derive acorrected calcium levelwhen the albumin is abnormal. This is to make up for the change in total calcium due to the change in albumin-bound calcium, and gives an estimate of what the calcium level would be if the albumin were within normal ranges.

Three formulas have been used to correct calcium for decreased serum albumin levels:
Corrected calcium (mg/dL) = measured total Ca (mg/dL) + 0.8 (4.0 - serum albumin [g/dL]), where 4.0 represents the average albumin level in g/dL.

In other words, each 1 g/dL decrease of albumin will decrease 0.8mg/dL in measured serum Ca and thus 0.8 must be added to the measured Calcium to get a corrected Calcium value.

Or: Corrected calcium (mmol/L) = measured total Ca (mmol/L) + 0.02 (40 - serum albumin [g/L]), where 40 represents the average albumin level in g/L

In other words, each 1 g/L decrease of albumin, will decrease 0.02mmol/L in measured serum Ca and thus 0.02 must be added to the measured value to take this into account and get a corrected calcium.

When there is hypoalbuminemia, the corrected calcium level is higher than the total calcium.

Each formula will give a slightly different value for corrected calcium. A better approach is to directly measure ionized calcium levels.
Two of the four approved gadolinium based magnetic resonance (MR) imaging contrast agents, gadodiamide (Omniscan) and gadoversetamide (OptiMARK), have recently been shown to interfere with calcium measurements on some chemistry analyzers, resulting in falsely low values. Patients with normal renal function may have spuriously low calcium measurements up to 24 hours after administration of these contrast agents, but patients with renal insufficiency may be affected for up to 4.5 days. (Am J Clin Pathol 2004; 121:282-92).
Reference range is 8.8 - 10.2 mg/dL. Calcium levels less than 6.0 mg/dL or greater than 13.0 mg/dL are considered critical values.
Prolonged venous stasis should be avoided because it can produce artefactual hypercalcemia.

Reference Values

Males

0-11 months: not established

1-14 years: 9.6-10.6 mg/dL

15-16 years: 9.5-10.5 mg/dL

17-18 years: 9.5-10.4 mg/dL

19-21 years: 9.3-10.3 mg/dL

> or =22 years: 8.9-10.1 mg/dL

Females

0-11 months: not established

1-11 years: 9.6-10.6 mg/dL

12-14 years: 9.5-10.4 mg/dL

5-18 years: 9.1-10.3 mg/dL

> or =19 years: 8.9-10.1 mg/dL

Cautions

Gadolinium is known to interfere with most metals tests. If gadolinium-containing contrast media has been administered a specimen cannot be collected for 48 hours.

Gadolinium from MRI contrast media may decrease results significantly. Consider testing specimen by spectroscopic (ICP or AA) methodologies for calcium if applicable.

Gadolinium from magnetic resonance imaging (MRI) contrast media may decrease results significantly when testing is performed using this method. If gadolinium-containing contrast media has been administered, a specimen should not be collected for 48 hours. Alternately, other suitable testing options include ion-selective electrode-based assays (Calcium, Ionized, Serum) or spectroscopic methods (eg, inductively couple plasma and atomic absorption) if available.

Specimen TypeSerum

Container/Tube:Plain, red top or serum gel

Reject Due To

Specimens other thanSerum

HemolysisMild OK; Gross reject

References

Causes of Hypocalcemia

Decreased Entry of Calcium into the Circulation

  • Hypoparathyroidism (absence of PTH secretion)
  • Postoperative
  • Autoimmune (isolated or part of polyglandular autoimmune syndrome)
  • Congenital (mutations of CaSR, PTH, and parathyroid aplasia)
  • Pseudohypoparathyroidism, types 1a, 1b and 2
  • Magnesium depletion
  • Severe hypermagnesemia
  • Deficiency of vitamin D

Increased Loss of Calcium from the Circulation

  • Hyperphosphatemia
  • Renal failure
  • Rhabdomyolysis
  • Tumor lysis
  • Phosphate administration
  • Acute pancreatitis
  • Hungry bone syndrome
  • Chelation
  • Citrate
  • EDTA
  • Lactate
  • Foscarnet
  • Widespread osteoblastic metastases
  • Prostate cancer
  • Breast cancer

Other Causes

  • Sepsis
  • Fluoride administration
  • Surgery
  • Chemotherapy
  • Cisplatin
  • 5-Fluorouracil
  • Leucovorin

Reference

  • Tietz Textbook of Clinical Chemistry, Edited by CA Burtis, CR Ashwood. WB Saunders Company, Philadelphia, 1994
  • Baldwin TE, Chernow B: Hypocalcemia in the ICU. J Crit Illness 1987;2:9-16
  • Peacock M.. Calciummetabolism in health and disease. Clin J Am Soc Nephrol.2010 Jan;5 Suppl 1:S23-30
  • Polyzois Makras, Socrates E. Papapoulos Medical treatment of hypercalcaemia HORMONES 2009, 8(2):83-95
  • Guyton AC. Parathyroid Hormone, Calcitonin, Calcium and phosphate metabolism, Vitamin D, Bone and Teeth. Text Book of Medical Physiology;2001:899-915.
  • Bushinsky DA, Monk RD. Calcium. Lancet 1998:352:306-311
  • National Kidney Foundation. K/DOQI clinical practice guidelines for bone metabolism and disease in chronic kidney disease. Am J Kidney Dis. 2003;42(4 suppl 3):S1-S201.

Makras P,Papapoulos SE. Medical treatment of hypercalcaemia. Hormones (Athens).2009 Apr-Jun;8(2):83-95.