Turnaround Time: 1-2 days
CPT Code:


Test Type: 1 mL Serum
Stability Time:



Room temperature

14 days


14 days

Reference Range:

4.5-5.6 mg/dL


Evaluate nonbound calcium, calcium metabolism, physiologically active calcium fraction, hyperparathyroidism, ectopic hyperparathyroidism. Occasionally useful when hypercalcemia coexists with abnormal protein state such as myeloma. Useful in assessing active calcium fraction in hypoproteinemia and acidosis when calcium is low.

Total calcium remains the first line test for evaluation of calcium abnormality.

Calcium in serum exists ionized, bound to organic anions such as phosphate and citrate, and bound to proteins (mainly albumin). Of these, ionized calcium is the physiologically important form. Measurement of serum ionized calcium provides insight into the effect of total protein and albumin on serum calcium levels. A patient can have high total calcium, with normal ionized calcium and increased total protein and/or albumin, as in dehydration or in myeloma. Women have greater circadian variation of ionized calcium and intact PTH than men.1 There is an inverse relationship between ionized calcium and phosphate concentration.2

1. Calvo MS, Eastell R, Offord KP, Bergstralh EJ, Burritt MF. Circadian variation in ionized calcium and intact parathyroid hormone: Evidence for sex differences in calcium homeostasis. J Clin Endocrinol Metab. 1991 Jan; 72(1):69-76. PubMed 1986029

2. Lehmann M, Mimouni F. Serum phosphate concentration. Effect on serum ionized calcium concentration in vitro. Am J Dis Child. 1989 Nov; 143(11):1340-1341. PubMed 2816862

Cooper RS, Shamsi N. Ionized serum calcium in black hypertensives: Absence of a relationship with blood pressure. J Clin Hypertens. 1987 Dec; 3(4):514-519. PubMed 3453386

Forman DT, Lorenzo L. Ionized calcium: Its significance and clinical usefulness. Ann Clin Lab Sci. 1991 Sep-Oct; 21(5):297-304. PubMed 1952778

Ionized Calcium Workshop N° 1. Scand J Clin Lab Invest Suppl. 1983; 165:1-126. PubMed 6578561

Loughead JL, Mimouni F, Tsang RC. Serum ionized calcium concentrations in normal neonates. Am J Dis Child. 1988 May; 142(5):516-518. PubMed 3358391

Rasmussen N1, Frølich A, Hornnes PJ, Hegedüs L. Serum ionized calcium and intact parathyroid hormone levels during pregnancy and postpartum. Br J Obstet Gynaecol. 1990 Sep; 97(9):857-859. PubMed 2242375

Roelofsen JM1, Berkel GM, Uttendorfsky OT, Slegers JF. Urinary excretion rates of calcium and magnesium in normal and complicated pregnancies. Eur J Obstet Gynecol Reprod Biol. 1988 Mar; 27(3):227-236. PubMed 3350196

Thode J, Holmegaard SN, Transbøl I, Fogh-Andersen N, Siggaard-Andersen O. Adjusted ionized calcium (at pH 7.4) and actual ionized calcium (at actual pH) in capillary blood compared for clinical evaluation of patients with disorders of calcium metabolism. Clin Chem. 1990 Mar; 36(3):541-544. PubMed 2311230

Wu AH, Bracey A, Bryan-Brown CW, Harper JV, Burritt MF. Ionized calcium monitoring during liver transplantation. Arch Pathol Lab Med. 1987 Oct; 111(10):935-938. PubMed 3307683

Collection Details:

Collection Instructions:

Gel-barrier tube.

Centrifuge and send unopened gel-barrier tube with vacuum intact. It is critical that the vacuum not be disrupted. Exposure of the serum to room air causes a loss of carbon dioxide to the atmosphere. This changes the sample pH and can prevent the lab from measuring the ionized calcium. To ensure that testing can be performed, send a separate tube for ionized calcium and adhere a piece of labeling tape over the top of the tube with the following written on it “Ionized Calcium − DO NOT OPEN.”

Room temperature. Do not freeze.