Serum Calcium

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Latest Edit: Iva Lloyd, ND 2021-08-24 (EDT)

See Also Lab Tests

Calcium reservoirs or major functional stores in the body are primarily found in the bone and teeth (98-99%). These reservoirs are used to maintain the calcium blood levels, which are tightly regulated. The absorption of calcium is primarily dependent on an optimal level of acidity of the stomach, as well as the amount of phosphate and magnesium present. Absorption occurs in the upper part of the small intestine, affecting protein absorption and helping fats move through the intestinal wall.[1],[2]


  • Ionized calcium is required for biochemical functions such as:
  • inflammation and tissue repair in leukocytes for phagocytosis
  • facilitating cell to cell adhesion, communication, and providing cell membrane stability, as a vital component of the interstitial matrix
  • providing vascular integrity, facilitating clotting and bone metabolism

Serum calcium is found in three different forms in the plasma:

1. 40% is combined with plasma proteins such as albumin. This is non-diffusible through the cell membrane. Hence calcium levels can be interpreted by knowing the serum albumin level.

2. 10% of calcium in the plasma is found combined to other substances, such as phosphate and citrate. This is freely diffusible through the cell membrane.

3. The remaining 50% of plasma calcium is in the ionized form and is freely diffusible through the cellular membrane.

The ionized form of calcium is the only form of calcium that can be used by the body for its physiological and metabolic functions. These functions include: muscle contraction, blood clotting, cardiac function, and transmission of nerve impulses. Ionized calcium levels are regulated by parathyroid hormone (PTH) and vitamin D.

Patient Preparation

  • Although no fasting is required, serum calcium may be part of a multichemical analysis in which fasting is required for the other studies.
  • Factors which can cause increased levels
  • Calcium supplements taken prior to blood draw, excess Vitamin D supplementation, excessive ingestion of milk, a decrease in pH, diurnal variation around 9pm
  • Drugs: calcium salts, hydralazine, lithium, thiazide diuretics, PTH, thyroid hormone, alkaline antacids, calcium salts, ergocalciferol, and androgens
  • Factors which can cause decreased levels
  • Laxative use (increased intestinal Ca+ loss), hypoalbuminemia
  • Drugs: acetazolamide, anticonvulsants, asparaginase, aspirin, calcitonin, cisplatin, corticosteroids, heparin, laxatives, loop diuretics, magnesium salts, thiazide diuretic, estrogens, albuterol, and oral contraceptives

Clinical Implications

Ranges: The following are the reference ranges for this lab. However, lab ranges can vary by laboratory and country. [2]

Standard U.S. Units Standard International Units
Conventional Laboratory Range 8.5-10.8 mg/dL 2.13-2.70 mmol/L
Optimal Range 9.2-10.0 mg/dL 2.30-2.50 mmol/L
Alarm Ranges < 7.0 or > 12.0 mg/dL < 1.75 or > 3.00 mmol/L

High levels indicate:

Low levels indicate:

Associated Tests


  1. Pagana Kathleen D, Pagana Timothy J (1998) Mosby's Manual of Diagnostic and Laboratory Tests, Mosby, Inc
  2. 2.0 2.1 Weatherby Dicken, Ferguson Scott (2002) Blood Chemistry and CBC Analysis: Clinical Laboratory Testing from a Functional Perspective, Bear Mountain