Calcium, Ionized (Serum)

CPT Code(s):



Ion-selective electrode (ISE)


Monday - Friday




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 the evaluation of calcium abnormality.

Specimen Requirements:

Type: Serum

Note: No isotopes should be administered to the patient within 24 hours prior to the venipuncture.

Container/Tube: Gel-barrier tube

Sample Volume: 1 mL

Storage: Do not freeze specimens.

Stability (collection to time of analysis/testing):

  • Ambient: 14 days
  • Refrigerated: 14 days
Rejection Criteria:

  • Hemolysis
  • Opened gel-barrier tube
  • Gel-barrier not completely sealing the clot
  • Plasma specimen

Collection Procedure:


Reference Values:

Reference Interval: 4.5 - 5.6 mg/dL

Interpretation Data:



UFHPL Test #: 28030

UFHPL Epic order code: LAB5227

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; 111:935-938. PubMed 3307683

Reported Notes: