Serum free calcium

ExtraceHuiar calcium provides calcium ion for the maintenance of intracellular calcium, bone mineralization, blood coagulation, and plasma membrane potential. Calcium stabilizes the plasma membranes and influences permeability and excitability. A decrease in the serum free calcium concentration causes increased neuromuscular excitability and tetany an increased concentration reduces neuromuscular excitability. 

Most other preanalytical factors are less lilcely to lead to confusion. Prolonged immobilization and bed rest can decrease bone density and increase total and free calcium. Hyperventilation and exercise decrease and increase the concentration of free calcium, respectively, because of changes in serum Both, serum free calcium and calcium... 

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... 

Serum calcium or serum ionized calcium concentrations (ionized calcium concentrations are preferable to determine free and bound calcium, especially with concurrent low serum albumin)... 

The Ca(II) concentration in blood is closely controlled normal values He between 2.1 and 2.6 mmol/L (8.5—10.4 mg/dL) of semm (21). The free calcium ion concentration is near 1.2 mmol/L the rest is chelated with blood proteins or, to a lesser extent, with citrate. It is the free Ca(II) in the serum that determines the calcium balance with the tissues. The mineral phase of bone is essentially in chemical equilibrium with calcium and phosphate ions present in blood semm, and bone cells can easily promote either the deposition or dissolution of the mineral phase by localized changes in pH or chelating... 

A number of observations both in experimental animals and in humans are suggestive of a crucial role for dietary calcium in regulating blood pressure (Villar et al. 1986 Karanja and McCarron 1986 McCarron et al. 1984, 1982 McCarron 1985, 1983, 1982 Belizan et al. 1983A,B). For example, laboratory rats fed insufficient calcium experience a rise in blood pressure. Increasing the calcium intake either decreases blood pressure or reduces the development of high blood pressure in animals predisposed to hypertension (Karanja and McCarron 1986). Furthermore, disturbances in calcium metabolism, including a depression in serum ionized or free calcium, have been reported in spontaneously hypertensive rats, as well as in untreated patients with hypertension. 

Boisseau, A.M. Donatien, P. Surleve-Bazeille, J.E. Amedee, J. Harmand, M.F. Bezian J.H. Maleville, J. Taieb, A., Production of epidermal sheets in a serum free culture system a further appraisal of the role of extracellular calcium. J. Dermatol. Sci. 3, 111-120, 1992. 

CFDA (Sigma) is used at 40 )Ug/ml final concentration in serum free medium, 0.1% BSA, pH 6.0, for 30 min at 37°C. At the end of the assay, adherent labeled tumor cells are placed in 0.2% SDS for 30 min at 37°C to release the fluorescent marker. Three volumes of calcium-magnesium-free-PBS are added, and the fluorescence of the cell lysate is measured with a Perkin-Elmer LS-5 luminescence spectrophotometer (excitation maximum 485 nm, emission maximum 538 nm) (Price et al., 1995). 

About 40% of serum calcium is protein bound, with most of it 80%) being bound to albumin. Generally, one or two calcium ions are associated with serum albumin. Albumin serves as a calcium buffer. Jt can bind more calcium ions when excessive concentrations of calcium appear in the bloodstream. About 13% of the calcium in scrum is weakly complexed with phosphate, citrate, and sulfate. About half (47%) of serum calcium occurs as the free calcium ion. The level of free scrum Ca is maintained within narrow limits, 1.0 to 1.25 mM (40 to 50 pg/mJ). The normal concentration of total serum calcium (bound plus free) is 85 to 105 pg/ml. Conditions in which the level of free serum calcium fails below and rises above the normal range are called hypocalcemia and hypercalcemia, respectively. The term ionized calcium is often used to refer to the concentrations of free calcium. This term is not scientifically accurate, because all of the calcium in the body is ionized. Calcium does not engage in the formation of covalent bonds (Cotton and Wilkinson, 1966). 

Many sick persons have low serum albumin levels. The hypocalcemia occurring with low serum albumin results in a decrease in the total concentration of plasma calcium, but not in a decrease in that of free calcium ions. This type of hypocalcemia, which can occur with cirrhosis of the liver, does not result in the clinical signs of hypocalcemia, because free plasma Ca levels are maintained. 

With prolonged bed rest, fluid retention occurs and plasma protein and albumin concentrations may be decreased by an average of 0.5 and 0.3g/dL, respectively. The concentrations of protein-bound constituents are also reduced, although mobilization of calcium from bones with an increased free ionized fraction compensates for the reduced protein-bound calcium, so serum total calcium is less affected. Serum aspartate aminotransferase activity is usually slightly less in individuals confined to bed than in those undertaking normal physical activity. Initially and paradoxically, creatine kinase (CK) activity is increased as a result of its release from skeletal muscles, but ultimately, CK activity may be less than in active, healthy individuals. Serum potassium may be reduced by up to 0.5mmol/L because of reduction of skeletal muscle mass. 

Calcium can be redistributed among the three plasma pools, acutely or chronically, by alterations in the concentration of protein and small anions, changes in pH, or changes in the quantities of free calcium and total calcium in the serum (Figure 49-2). 

Low total serum calcium (hypocalcemia) may be due to either a reduction in the albumin-bound calcium, the free firaction of calcium, or both (Box 49-Hypoalbu-minemia is the most common cause of pseudohypocalcemia (decreased total and norma free calcium) because 1 g/dL of albumin binds approximately 0.8 mg/dL of calcium. Common clinical conditions associated with low serum... 

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.

An unusual and treatment-resistant form of symptomatic hypocalcemia is seen in patients treated with large amounts of burn cream that contains polyethylene glycols. The glycols are absorbed and metabolized to dicarboxylic adds that bind calcium. Patients develop markedly increased total calcium and decreased free calcium, along with a metabolic acidosis and increased serum osmolality from the glycols. 

Primary hyperparathyroidism is diagnosed by laboratory studies. Hypercalcemia should be documented by measuring total calcium and serum albumin, or ideally free calcium, on more than one occasion before initiating further testing. Measurement of intact PTH (with concomitant measurement of calcium) is the most sensitive and specific test for parathyroid function and is central to the differential diagnosis of hypercalcemia. Serum l,25(OH)2D is usually in the upper half of the reference interval or increased in primary hyperparathyroidism, as PTH stimulates its production. By contrast, l,25(OH)2D (lilte PTH) is. low-normal or suppressed in nonparathyroid hypercalcemia, except in sarcoidosis, other granulomatous diseases, and certain... 

If analysis cannot be completed within 30 to 60 minutes at room temperature, tlien serum may be the optimal specimen because of elimination of the anticoagulant and microclots and its greater stability. Serum specimens should be collected in evacuated gel tubes. The tubes should be filled completely and centrifuged to form an effective barrier between the serum and the clot with its cellular elements. Once centrifuged, specimens are stable for hours at 25 C and for days at 4 °C, provided the tube remains sealed. Free calcium has been reported to be less stable in specimens from both acidotic and nonacidotic patients with uremia. ... 

Aqueous quality control materials are commercially avail-able for free calcium. Because simple aqueous controls may not reliably detect changes in performance with patient specimens, serum-based quality control materials should be used daily. Serum-based controls may be prepared by acidifying serum with 10 pL of 1 mol/L HCl and leaving it exposed in the refrigerator for 1 week to remove carbon dioxide. The pH is then adjusted to 7.4, and the serum is ahquoted and frozen. Alternatively, serum-based controls can be equilibrated with carbon dioxide before their analysis. 

Reference intervals for Total and Free Calcium IN Serum and Plasma Total Calcium... 

Whole blood specimens develop a fiquid-junction potential different from that of serum or plasma because of the presence of erythrocytes. A positive bias that is directly proportional to the hematocrit has been reported. In addition, free calcium values have been reported to differ among capillary blood, venous blood, and serum samples because of differences in pH. Therefore, reference intervals... 

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. 

Instruments for the measurement of free magnesium in whole blood, plasma, or serum are available commercially. These instruments use ISEs with neutral carrier ionophores, including ETH 5220, ETH 7025, or a proprietary ionophore. Current ionophores or electrodes have insufficient selectivity for magnesium over calcium. Free calcium is simultaneously determined and used chemometrically with the signal from the magnesium electrode to calculate free magnesium concentrations. 

In secondary hyperparathyroidism, PTH is increased before total or free calcium becomes abnormally low, a consequence of homeostatic mechanisms for maintenance of serum calcium. Consequently, PTH is more sensitive than calcium for identifying secondary hyperparathyroidism. 

The metabolism of calcium is linked intimately with that of phosphate (Figure 49-23). " The homeostatic mechanisms are directed principally toward the maintenance of normal extracellular calcium and phosphate concentrations, which sustain the extracellular and intracellular processes and provide substrate for skeletal mineralization. The parathyroid gland responds to a decrease in free calcium concentration within seconds. During a time of calcium deprivation, the increase in serum PTH rapidly alters both renal and skeletal metabolism. 

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