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Calcium is present in the blood stream in the following distribution.

Form of Calcium Percentage
Ionized 53%
Complexed 13%
Phosphate 3%
Bicarbonate 7%
Citrate 3%
Globulins 7%
Albumin 27%

Calcium metabolism is controlled at four main points.

Parathyroid hormone (PTH)
Vitamin D


Reference Methods  
Clinical Utility  
Interfering Diseases or Substances that Alter Levels  
Commonly Used Terms  
Internet Links  


IFCC recommended reference method for the determination of the substance concentration of ionized calcium in undiluted serum, plasma or whole blood.

Burnett RW, Christiansen TF, Covington AK, Fogh-Andersen N, Kulpmann WR, Lewenstam A, Maas AHJ, Muller-Plathe O, Sachs C, Andersen OS, VanKessel AL, Zijlstra WG; International Federation of Clinical Chemistry and Laboratory Medicine. IFCC Scientific Division, Working Group on Selective Electrodes.

Hartford Hospital, Department of Pathology, CT, USA.

Clin Chem Lab Med 2000 Dec;38(12):1301-14 Abstract quote

A reference method is described for the determination of the substance concentration of ionized calcium in plasma by which ionized calcium (free or unbound) may be reliably determined on the basis of calibration with aqueous solutions with known concentration of ionized calcium.

The composition of the calibration solutions is chosen such that the activity coefficient of the calcium ion is assumed to be identical both in the calibration solutions and in "normal" plasma, i.e. by convention, the ionic strength (Im) is 0.160 mol/kg. The convention is adopted of reporting ionized calcium measurements as concentration expressed as mmol/l. The proposed reference method for ionized calcium measurement in plasma is based on the use of a cell consisting of an external reference electrode with a saturated potassium chloride liquid/liquid junction in combination with a calcium ion-selective membrane electrode of defined construction and performance. Procedures for using the reference cell and a protocol for sample measurement are described.

The preparation of the calibration solutions to be used are described in detail in Appendix A, secondary calibration solutions and check standards in Appendix B, and reference cell vessel design in Appendix C.



Hypercalcemia of malignancy--new insights into an old syndrome.

Esbrit P.

Bone and Mineral Metabolism Laboratory, Research Unit, Fundacion Jimenez Diaz, Madrid, Spain.

Clin Lab 2001;47(1-2):67-71 Abstract quote

Hypercalcemia is a common paraneoplastic syndrome. Tumors induce hypercalcemia by a local mechanism associated with the tumor's production of various cytokines increasing bone osteolysis. In addition, many tumors release humoral factors, mainly parathyroid hormone (PTH)-related protein (PTHrP), which stimulates bone resorption and/or tubular calcium reabsorption leading to hypercalcemia.

Interaction of PTHrP with other tumor-elaborated cytokines might explain some nonPTH-like features associated with the hypercalcemia of malignancy syndrome. Using assays recognizing various PTHrP epitopes, the majority of hypercalcemic cancer patients have higher immunoreactive PTHrP levels in either plasma or urine than normal subjects. Present data support the concept that PTHrP might also be a factor which promotes tumor growth and also the development of osteolytic metastasis. A variety of therapeutic approaches are available to lower serum calcium in hypercalcemic cancer patients.

The pathophysiological mechanisms of hypercalcemia appear to be a determinant of the efficacy of different antihypercalcemic treatments.


Neonatal hypocalcemic seizures: case report and literature review.

Kossoff EH, Silvia MT, Maret A, Carakushansky M, Vining EP.

Pediatric Epilepsy, The Johns Hopkins Hospital, Baltimore, Maryland 21287, USA.

J Child Neurol 2002 Mar;17(3):236-9 Abstract quote

Seizures during the neonatal period have a broad differential diagnosis, many with a specific treatment and prognosis. In the case reported, a combination of dietary and endocrinologic abnormalities resulted in hypocalcemic seizures, which continued despite aggressive correction of serum ionized calcium levels.

Serial electroencephalograms (EEG) performed during the hospitalization were markedly abnormal, and treatment with anticonvulsant drugs was considered given the persistence of seizures despite normalization of serum calcium levels. After 4 days of intravenous calcium administration, the seizure activity resolved, and the patient returned to his normal baseline level of functioning.

This case highlights the clinical course of neonatal hypocalcemic seizures, EEG findings in several cases, and possible mechanisms for both hypocalcemic precipitation of seizures and anticonvulsant ineffectiveness.

Transient pseudohypoparathyroidism and neonatal seizure.

Manzar S.

Department of Child Health, Sultan Qaboos University Hospital, Al-Khoud, Muscat, Sultanate of Oman

J Trop Pediatr 2001 Apr;47(2):113-4 Abstract quote

The case of a neonate is presented who had late onset seizure associated with hypocalcemia, hyperphosphatemia, and raised parathyroid hormone. The infant did not have any stigmata of pseudohypoparathyroidism. The hypocalcemia was initially resistant to calcium therapy, but responded to vitamin D analog therapy.

The diagnosis of 'transient neonatal pseudohypoparathyroidism' was entertained, as the infant remained stable and seizure-free with normal serum biochemistry during 8 months of follow-up.

Effects of single dose calcium gluconate infusion in hypocalcemic preterm infants.

Porcelli PJ Jr, Oh W.

Women & Infants' Hospital of Rhode Island, Department of Pediatrics, Brown University, Providence 02905-2409, USA.

Am J Perinatol 1995 Jan;12(1):18-21 Abstract quote

We conducted a prospective, double-blind study of 43 preterm infants to examine the effect of a single calcium gluconate infusion as therapy for neonatal hypocalcemia on serum calcium concentrations and hypocalcemic signs in preterm infants with low total serum calcium concentrations.

Total and ionized serum calcium was measured and signs of irritability, jitteriness, and twitching were scored (scale 0-9) by blinded observers before and after receiving one dose of either calcium gluconate (100 mg/kg) or placebo (normal saline). Total and ionized serum calcium increased 3 to 6 hours following the calcium, but not the placebo, infusion. Of the infants with hypocalcemic signs, the average score of hypocalcemic signs decreased in the 11 calcium-treated infants; the 12 infants with hypocalcemic signs showed no change of hypocalcemic signs following treatment with placebo.

We conclude from this study that a single dose of calcium gluconate (100 mg/kg) in hypocalcemic preterm infants raise total and ionized serum calcium and decrease clinical signs of hypocalcemia.


pH of Blood  

pH Effects on Measurements of Ionized Calcium and Ionized Magnesium in Blood.

Wang S, McDonnell EH, Sedor FA, Toffaletti JG.

Department of Pathology/Clinical Laboratories, Duke University Medical Center, Durham, NC. Dr Wang is currently with the Department of Medicine, Northwestern University Medical Center, Chicago, Ill.

Arch Pathol Lab Med 2002 Aug;126(8):947-50 Abstract quote

Context.-It is well known that the concentration of ionized calcium in blood is affected by the pH of the specimen, since hydrogen ions compete with calcium for binding sites on albumin and other proteins. However, the relationship between pH and ionized magnesium concentration is not as well characterized.

Objective.-To determine the effects of pH on ionized magnesium concentration over a wide range of pH values in serum or plasma.

Design.-Both ionized calcium and ionized magnesium concentrations were measured in 3 sets of samples. (1) Pools of serum or whole blood at different pH values (7.20-7.60) achieved by adding a constant volume of acid or base (diluted solutions of either hydrochloric acid or sodium hydroxide) plus saline. These pools consisted of 2 serum and 3 heparinized whole blood pools collected from leftover blood remaining in clinical specimens in the Clinical Chemistry and Blood Gas Laboratories, respectively, at Duke University Medical Center. (2) Five whole blood specimens obtained from apparently healthy individual donors. (3) Twenty-six whole blood specimens obtained from individual patients (leftover blood from the Blood Gas Laboratory) in which pH was varied by in vitro loss or gain of carbon dioxide.

Results.-Both ionized calcium and ionized magnesium concentrations decreased as the pH in the specimen increased, indicating the stronger binding of these ions with proteins in the more alkaline environment.

Conclusion.-We conclude that the rate of change of ionized magnesium concentration with pH change (0.12 mmol/L per pH unit) is significantly less than that of ionized calcium (0.36 mmol/L per pH unit). Furthermore, our findings indicate that if adjustment to pH 7.40 is necessary, the ionized magnesium test results need to be adjusted when pH is markedly abnormal, as is sometimes done for ionized calcium.


Serum phosphate concentration. Effect on serum ionized calcium concentration in vitro.

Lehmann M, Mimouni F.

Department of Pediatrics, University of Cincinnati College of Medicine, OH 45267-0541.

Am J Dis Child 1989 Nov;143(11):1340-1 Abstract quote

We examined the effects of variations of serum phosphate levels on serum ionized calcium concentrations in vitro. A single donor serum sample was divided into 25 aliquots stored in tubes sealed with carbon dioxide and divided into 5 subsets of tubes.

The pH was altered in 4 of the 5 subsets by adding various concentrations of hydrochloric acid or sodium hydroxide. The pH levels studied ranged from 7.09 to 7.63. The phosphate concentration was altered in each subset by adding various concentrations of a phosphate buffer. The phosphate concentrations studied ranged between 0.81 and 3.58 mmol/L. There was an inverse relationship between ionized calcium and phosphate at all pH levels studied. The ionized calcium concentration correlated inversely with pH.

We suggest that in addition to factors well known to influence serum ionized calcium concentration (such as protein, bicarbonate, and pH values), serum phosphate concentration also plays an important role.

Henry JB. Clinical Diagnosis and Management by Laboratory Methods. Twentieth Edition. WB Saunders. 2001.

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Last Updated 9/18/2003

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