Official reprint from UpToDate®
www.uptodate.com ©2017 UpToDate, Inc. and/or its affiliates. All Rights Reserved.

Relation between total and ionized serum calcium concentrations

Alan S L Yu, MB, BChir
Jason R Stubbs, MD
Section Editors
Stanley Goldfarb, MD
Mitchell E Geffner, MD
Deputy Editor
Albert Q Lam, MD


The plasma (or serum) calcium concentration is usually reported in units of mg/dL in the United States, in mmol/L in many other countries, and in meq/L by some laboratories. The relationship between these units is defined by the following equations:

 mmol/L  =  [mg/dL  x  10]  ÷  mol wt

 meq/L  =  mmol/L  x  valence

Since the molecular weight of calcium is 40 and the valence is +2, 1 mg/dL is equivalent to 0.25 mmol/L and 0.5 meq/L. Thus, the normal range of total serum calcium concentration of 8.8 to 10.3 mg/dL is equivalent to 2.2 to 2.6 mmol/L and 4.4 to 5.2 meq/L.


The total serum calcium concentration consists of three fractions [1,2]:

To continue reading this article, you must log in with your personal, hospital, or group practice subscription. For more information on subscription options, click below on the option that best describes you:

Subscribers log in here

Literature review current through: Nov 2017. | This topic last updated: Mar 11, 2016.
The content on the UpToDate website is not intended nor recommended as a substitute for medical advice, diagnosis, or treatment. Always seek the advice of your own physician or other qualified health care professional regarding any medical questions or conditions. The use of this website is governed by the UpToDate Terms of Use ©2017 UpToDate, Inc.
  1. Moore EW. Ionized calcium in normal serum, ultrafiltrates, and whole blood determined by ion-exchange electrodes. J Clin Invest 1970; 49:318.
  2. Bushinsky DA, Monk RD. Electrolyte quintet: Calcium. Lancet 1998; 352:306.
  3. Ladenson JH, Lewis JW, Boyd JC. Failure of total calcium corrected for protein, albumin, and pH to correctly assess free calcium status. J Clin Endocrinol Metab 1978; 46:986.
  4. DENT CE. Some problems of hyperparathyroidism. Br Med J 1962; 2:1419.
  5. Mutlu EA, Keshavarzian A, Mutlu GM. Hyperalbuminemia and elevated transaminases associated with high-protein diet. Scand J Gastroenterol 2006; 41:759.
  6. Lindgärde F, Zettervall O. Hypercalcemia and normal ionized serum calcium in a case of myelomatosis. Ann Intern Med 1973; 78:396.
  7. Merlini G, Fitzpatrick LA, Siris ES, et al. A human myeloma immunoglobulin G binding four moles of calcium associated with asymptomatic hypercalcemia. J Clin Immunol 1984; 4:185.
  8. Pearce CJ, Hine TJ, Peek K. Hypercalcaemia due to calcium binding by a polymeric IgA kappa-paraprotein. Ann Clin Biochem 1991; 28 ( Pt 3):229.
  9. McCloskey EV, Galloway J, Morgan MA, Kanis JA. Pseudohyperphosphataemia in multiple myeloma. BMJ 1989; 299:1381.
  10. Oberleithner H, Greger R, Lang F. The effect of respiratory and metabolic acid-base changes on ionized calcium concentration: in vivo and in vitro experiments in man and rat. Eur J Clin Invest 1982; 12:451.
  11. Wang S, McDonnell EH, Sedor FA, Toffaletti JG. pH effects on measurements of ionized calcium and ionized magnesium in blood. Arch Pathol Lab Med 2002; 126:947.
  12. Krapf R, Jaeger P, Hulter HN. Chronic respiratory alkalosis induces renal PTH-resistance, hyperphosphatemia and hypocalcemia in humans. Kidney Int 1992; 42:727.
  13. Gauci C, Moranne O, Fouqueray B, et al. Pitfalls of measuring total blood calcium in patients with CKD. J Am Soc Nephrol 2008; 19:1592.
  14. Evenepoel P, Bammens B, Claes K, et al. Measuring total blood calcium displays a low sensitivity for the diagnosis of hypercalcemia in incident renal transplant recipients. Clin J Am Soc Nephrol 2010; 5:2085.
  15. Movilli E, Zani R, Carli O, et al. Direct effect of the correction of acidosis on plasma parathyroid hormone concentrations, calcium and phosphate in hemodialysis patients: a prospective study. Nephron 2001; 87:257.
  16. Kaye M, Somerville PJ, Lowe G, et al. Hypocalcemic tetany and metabolic alkalosis in a dialysis patient: an unusual event. Am J Kidney Dis 1997; 30:440.
  17. Ladenson JH, Lewis JW, McDonald JM, et al. Relationship of free and total calcium in hypercalcemic conditions. J Clin Endocrinol Metab 1979; 48:393.
  18. Nordenström E, Katzman P, Bergenfelz A. Biochemical diagnosis of primary hyperparathyroidism: Analysis of the sensitivity of total and ionized calcium in combination with PTH. Clin Biochem 2011; 44:849.
  19. Morton AR, Garland JS, Holden RM. Is the calcium correct? Measuring serum calcium in dialysis patients. Semin Dial 2010; 23:283.