Calcium measurements

Calcium ion-selective electrodes have recently been commercialized for the measurement of either total or ionized calcium Approximately 45 % of the calcium present in serum is bound to proteins, 5% is complexed to simple anions and 50% exists as the free ion. Traditionally, total calcium measurements have been made by releasing the protein bound fraction. An ion-selective electrode has now allowed the free (ionized) calcium to be measured directly. There has been much debate on the clinical significance of these measurements. The dependence of ionized calcium on pH must be considered. Samples must be either treated anaerobically, tonometered to a constant pH or have a correction factor applied. 

For acute symptomatic hypocalcemia, 200 to 300 mg of elemental calcium is administered IV and repeated until symptoms are fully controlled. This is achieved by infusing 1 g of calcium chloride or 2 to 3 grams of calcium at a rate no faster than 30 to 60 mg of elemental calcium per minute. More rapid administration is associated with hypotension, bradycardia, or cardiac asystole. Total calcium concentration is commonly monitored in critically ill patients. Under normal circumstances, about half of calcium is loosely bound to serum proteins while the other half is free. Total calcium concentration measures bound and free calcium. Ionized calcium measures free calcium only. Under usual circumstances, a normal calcium level implies a normal free ionized calcium level. Ionized calcium should be obtained in patients with comorbid conditions that would lead to inconsistency between total calcium and free serum calcium (abnormal albumin, protein, or immunoglobulin concentrations). For chronic asymptomatic hypocalcemia, oral calcium supplements are given at doses of 2 to 4 g/day of elemental calcium. Many patients with calcium deficiency have concurrent vitamin D deficiency that must also be corrected in order to restore calcium homeostasis.2,37,38... 

For a typical limestone scrubber inlet liquor pH range of 5.2-6.0, and for liquors having a chloride-to-magnesium ratio of 0.2 mole/mole or less, the following simplified equation can be used to determine gypsum saturation from calcium measurements ... 

Use of dissolved calcium permits the use of simpler correlations, but the accuracy of calcium measurements in the presence of magnesium is dependent upon the analytical method used. 

Hormesis, in which compensatory adaptive changes precede and occur at lower doses than degenerative changes, was detected for half of the toxic drugs for cell proliferation, cell morphology and mitochondria [4, 33]. Hormesis could not be assessed for parameters that normally have low values, such as intracellular calcium measured by fluo4 or membrane permeability measured by toto-3, because assay methods were not sufficiently sensitive. However, for calcium, more sensitive dyes. 

G.A. Smith, R.T. Hesketh, J.C. Metcalfe, J. Feeney, P.G. Morris, Intracellular calcium measurements by F-19 Nmr of fluorine-labeled chelators, Proc. Natl. Acad. Sci. USA 80 (1983) 7178-7182. 

Vicat Kate of absorption of calcium measured indirectly front a saturated CatOHb solution in contact with excess C atOH), (solid) by determining residual Ca(OI I),. Residual free portlandite is measured by DTA. TG. or solvent extraction. Correlation with strength gain is poor... 

Basset, O., Boittin, F.X., Dorchies, O.M., Chatton, J.Y., van Breemen, C., and Ruegg, U.T., 2004, Involvement of inositol 1,4,5-trisphosphate in nicotinic calcium responses in dystrophic myotubes assessed by near-plasma membrane calcium measurement, J Biol Chem, 279, pp 47092 17100. Baumbach, L.L., Chamberlain, J.S., Ward, P.A., Farwell, N.J., and Caskey, C.T., 1989, Molecular and clinical correlations of deletions leading to Duchenne and Becker muscular dystrophies, Neurology, 39, pp 465 174. 

Fig. 13. Inhibition by PMA of calcium mobilization induced by glucagon, cAMP analogue or forskolin in isolated rat hepatocytes. Hepatocytes were incubated for 5 min and total cell calcium measured as described in Ref. 140. Abbreviations are those defined in the legend to Fig. 10. Reproduced from Ref. 5 by permission of the authors and publisher.

Lin J, Idee JM, Port M, Dial A, Berthommier C, Robert M, Raynal I, Devoldere L, Corot C Interference of magnetic resonance imaging contrast agents with the serum calcium measurement technique using colorimetric reagents. JPharm Biomed Anal21 931-943,1999. 

Kang HP, Scott MG, Joe BN, Narra V, Heiken J, Parvin CA Model for predicting the impact of gadolinium on plasma calcium measured by the o-cresolphthalein method. Clin Chem 50 741 -746,2004. 

Brett, E.M. and Hicks, J.M. (1981). Total-calcium measurement in serum from neonates Limitations of current methods. Clin. Chem. 27, 1733-1737. 

E265 Gosling, P. (1986). Analytical reviews in clinical biochemistry Calcium measurement. Ann. Clin. Biochem. 23, 146-156. 

Figure 49-2 Equilibria and determinations of calcium in serum. Calcium can move among three physiochemical pools (1) free calcium, (2) protein-bound calcium, and (3) calcium complexed with inorganic and organic anions. Methods for determining total calcium measure all three pools, whereas methods for determining free calcium measure only that pool.

The methods most widely used for quantifying calcium measure either free (ionized) calcium or total calcium. The term ionized calcium is a misnomer because all plasma or serum calcium is ionized whether or not it is associated with protein or small anions by ionic binding. Throughout this... 

How the patient is prepared and the specimen is obtained can have a significant effect on both free and total calcium measurements. For information on these preanalyticai effects, the reader is dfrected to a later section in this chapter on the subject and to Box 49-4. 

Patient Preparation and Sources of Preanalytical Error for Total and Free Calcium Measurements... 

Bone and mineral disorders are common in patients with renal disease. Therapy and calcium metabolism of these patients are best evaluated with the determination of free calcium because of alterations in protein, pH, protein binding of calcium, and calcium complexes with organic and inorganic anions. Total and adjusted calcium concentrations are often invalid when compared with free calcium measurements... 

Free calcium has proved to be more useful than total calcium in the diagnosis of hypercalcemia. Patients with subsequently surgically proven primary hyperparathyroidism more often have increases of free calcium than of total calcium (Figure 49-7). Free calcium is more sensitive than total calcium in detecting hypercalcemia associated with malignancy, as may be expected in patients who frequently have decreased serum albumin. Less commonly, paraproteins produced in myeloma may bind calcium, complicating the interpretation of total or corrected calcium measurements. 

Corns CM. Interference by haemoglobin with cresolphthalein complexone method for serum calcium measurement. Ann Clin Biochem 1990 27 152-5. 

Landt M, Hortin GL, Smith CH, McClellan A, Scott MG. Interference in ionized calcium measurements by heparin salts. Clin Chem 1994 40 565-70. 

Payne RB, Jones DP. Protein interferes with ionized calcium measurement at the reference electrode liquid junction, Ann Chn Biochem 1987 24 400-7. 

Robertson WG, Marshall RW. Calcium measurements in serum and plasma total and ionized. CRC Crit Rev Clin Lab Sci 1979 11 271-304. 

Sachs CH, Rabouine PH, Chaneac M, Kindermans C, Dechaux M, Falch-Christiansen T. Preanalytical errors in ionized calcium measurements induced by the use of liquid heparin. Ann Chn Biochem 1991 28 167-73. 

Toffaletti J. Use of novel preparations of heparin to eliminate interference in ionized calcium measurement have ah the problems been solved. Clin Chem 1994 40 508-9. 

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