Determination of ionised calcium by ion selective electrode is not independent of albumin concentration

Effects of pH and the plasma or serum concentrations of total calcium, chloride, magnesium, l-lactate, and albumin on the plasma ionized calcium concentration in calves

BACKGROUND

The plasma ionized calcium concentration (cCa2+ ) represents the biologically active form of calcium and is the preferred method for evaluating calcium status in animals. Different pH-corrective equations have been developed for human plasma, but the validity of the equations for bovine plasma is unknown.

HYPOTHESIS

We hypothesized that pH-corrective equations for bovine plasma would be similar to those used for human plasma; cCa2+ was dependent on the plasma concentrations of total calcium (cTCa), chloride (cCl), L-lactate (cLactate), and albumin (cAlbumin); and the in vitro and in vivo cCa2+ -pH relationships would differ.

ANIMALS

Ten healthy calves (in vitro study), 1426 critically ill calves.

METHODS

The in vitro plasma log10 (cCa2+ )-pH relationship was determined by CO2 tonometry of 465 plasma samples. Plasma cCl was altered by equivolume dilution of plasma with 3 electrolyte solutions of different cCl. The in vivo plasma cCa2+ -pH relationship was investigated and validated using clinicopathologic data extracted from the medical records of 950 (model development) and 476 (model validation) critically ill calves.

RESULTS

pH-corrective equations for bovine plasma were similar to those used for human plasma. Plasma cCa2+ increased in vitro with increases in plasma cCl. Plasma cCa2+ in critically ill calves was associated with plasma cTCa, blood pH, plasma cCl, serum cMg, and cL-lactate (R2  = 0.69) but not plasma cAlbumin.

CONCLUSIONS AND CLINICAL IMPORTANCE

Calculation of cCa2+ from cTCa in calf plasma or serum requires adjustment for at least pH and cCl when 1 or both are outside the reference range.

Sodium ion specifically modifies plasma ionized calcium concentration

BACKGROUND/AIM

Recently, we observed a progressive decline in the plasma ionized calcium (iCa) concentration after ingestion of Na-free beverage. Although both plasma pH and Na significantly correlated with iCa, the correlation of the latter was stronger than that of the former. Therefore, we explored the effect of Na on plasma iCa.

METHODS

Pooled human plasma was diluted with normal saline, 5% glucose, 0.9% KCl, or distilled water to examine the effect of plasma dilution on iCa. Also, to evaluate the effects of an isovolemic increase in ion concentration and osmolarity on plasma iCa, NaCl, KCl, and D-glucose were added to plasma. Electrolytes (Na(+), K(+), Cl(-), iCa, HCO(-)(3)), pH, and osmolarity were measured by using ion-selective electrodes.

RESULTS

In the plasma dilution study, although all solutions significantly decreased the iCa concentration, the decrease in iCa concentration in the normal saline treated solution was significantly lower than in others. When the plasma osmolarity was isovolemically increased, the iCa concentration significantly increased in parallel with the increase in osmolarity in the NaCl group (r = 0.604, p < 0.01, n = 50). In contrast, no significant change in the iCa concentration was seen in the other solutions. NaCl did not increase, but rather decreased the iCa concentration when added to a protein-free solution.

CONCLUSION

The present study demonstrated that Na specifically affects plasma iCa independently of pH, presumably via modulating Ca binding to plasma proteins.

Dependence of measured ionized calcium on protein concentration as measured by three ion-selective electrodes

A positive effect of protein on the measurement of ionized calcium in serum by ion-selective electrodes (ISEs) has been previously reported and the present study confirms this finding. Ionized calcium in serum was measured in the presence of increasing protein concentrations induced by venous stasis in 17 healthy volunteer subjects. Ionized calcium was measured using two commercial analysers, a Radiometer ICA2 analyser and a Baker Analyte+2 analyser, and a calcium cell devised by Covington for the calcium reference method (CRM). Both commercial analysers used charged ionophores and the CRM used a neutral carrier ionophore in the selective membrane. A small but significant rise in ionized calcium with increasing protein was measured on all analysers. Substitution of isotonic KCl for saturated KCl in the reference electrode of the CRM resulted in significantly reduced values for ionized calcium in paired serum samples when measured using the isotonic salt bridge. This study supports the premise that the positive effect of protein is related to the salt bridge concentration of the reference electrode rather than the ISE membrane composition.

Effects of ionic strength on ionized calcium measured using reference electrode liquid junctions of varying design and composition

Five commercial analysers were used to measure ionized calcium in aqueous and protein solutions which contained the same amount of calcium but had sodium chloride concentrations ranging from 100 to 150 mmol/L. In aqueous solutions measured activity fell significantly with increasing ionic strength while in protein solutions it increased. When isotonic sodium chloride was substituted for the hypertonic potassium chloride reference electrode liquid junction of an analyser with an open junction, there was a marked positive change in the effect of ionic strength in both aqueous and protein solutions. In contrast, when either isotonic sodium chloride or potassium chloride was substituted for the hypertonic potassium chloride of an instrument with a membrane-restricted junction, there was no effect on the change of measured values with ionic strength. Increasing the protein concentration by ultrafiltration did not change ionized calcium values when isotonic reference solutions were used with either open or membrane-restricted junctions. Because membrane-restricted isotonic junctions respond like hypertonic junctions to changes in sample ionic strength but do not exhibit protein interference, they may prove to have advantages over both open isotonic junctions and all configurations of hypertonic junction for measurements in patients.

Protein interference with ion-selective electrode measurement depends on reference electrode composition and design

There is controversy about whether protein interferes with ion measurements using ion-selective electrodes. We have investigated the effects of changes in the salt-bridge composition of five commercially available analysers with open, membrane-restricted or porous frit-restricted reference electrode junctions on measurements of an albumin solution prepared by gel filtration. When the manufacturers' salt bridges were used, instruments with open or membrane-restricted junctions showed apparent increases in the activity of ionized calcium, sodium and potassium in the presence of protein. When the hypertonic bridge solutions were replaced with 150 mmol/L potassium chloride this increase disappeared. The instrument with a porous frit-restricted junction showed no protein effect, but its response to changes in sample sodium chloride concentration in protein-free solution suggested that its junction was functionally equivalent to that formed with an isotonic sodium chloride bridge. Our results emphasize that liquid junction design and composition affect ion measurements in protein-containing solutions and suggest that the use of hypertonic bridge solutions for biological samples needs to be re-examined.

Ionized calcium: pediatric perspective

Ionized calcium is a physiologically critical calcium pool. It is easily determined, although accuracy depends on sample handling. As a clinical parameter, directly measured ionized calcium has particular import in the care of neonates, patients with sepsis or other cardiovascular instability, massively transfused patients, and those undergoing cardiopulmonary bypass or liver transplantation. Disturbances of calcium occur in many other settings, however, and accurate diagnosis and research conclusions may depend on using the best measurement available. Clinical and investigational use of ionized calcium determinations represent appropriate applications of current proven technology. In the future, clinical calcium manipulation may include modifying specific transmembrane transport processes and intracellular calcium pools and movements. At the current time we are largely restricted to studies of extracellular calcium concentration and its interactions. Much is known, but Mother Nature still has too many secrets. The interested reader is referred to discussions of ionized calcium and hemodynamics, reviews of the endocrine disturbances of calcium and phosphorus, textbook discussions pertinent to general calcium disturbances, and critical care issues.

An isotonic potassium chloride liquid junction minimises the effects of ionic strength, protein and haematocrit on ionised calcium measurement

When the reference electrode liquid junction of a Nova 2 analyser was changed to isotonic potassium chloride, increasing the ionic strength of aqueous solutions containing a constant total calcium concentration had a negligible effect on measured ionised calcium. In contrast, measurements using hypertonic potassium chloride, hypertonic sodium formate and isotonic sodium chloride liquid junctions showed significant sample ionic strength effects. Interferences by sample protein concentration and haematocrit were marked with hypertonic, but negligible with isotonic junctions. Ionised calcium values in samples containing 25 mmol/L acetate, bicarbonate, beta-hydroxybutyrate, lactate or pyruvate were all lower by 6-7% with an isotonic than a hypertonic potassium chloride junction. Thus, anions that replace bicarbonate during metabolic acidosis have a similar effect on residual liquid junction potential. The clinical usefulness of an isotonic potassium chloride liquid junction needs to be evaluated.

Protein does not interfere with the ion-selective electrode determination of calcium, sodium or potassium ions

The protein interference with the determination of ionised Ca2+, Na+ and K+ reported in several studies could be due to effects on liquid/liquid junction potentials or on the ion-selective electrodes, but could also be due to Donnan effects during sample preparation, e.g. ultrafiltration or dialysis. The possible interference of protein was studied by subjecting a bovine serum albumin solution, 100 g/l, to gel-filtration in Sephadex G-25 columns equilibrated with 150 mmol/l NaCl, 5 mmol/l KCl, 20 mmol/l MOPS, pH 7.4 and varying concentrations of Ca2+; 0.75 and 1.25 mmol/l. The albumin was dissolved in the basic electrolyte solution and pH adjusted before the gel-filtration. Samples were taken for measurements before, during and after the elution of the protein peak. In this way it was ensured that the activities were not changed by the presence of albumin. Also the temperature, the ionic strength of the electrolyte and the bridge solution were varied. Ca2+, Na+ and K+ activities were measured with the aid of a Kone Microlyte instrument and Ca2+ in addition with the Radiometer ICA-1 instrument or a measuring system consisting of a Philips IS 561-Ca electrode, a Beckman KCl reference electrode connected to the sample chamber via a 2% agarose bridge containing either 0.15 or 2 mol/l KCl, a pH/voltmeter and a voltage-bucking device. No interference by protein was found.

Protein interferes with ionised calcium measurement at the reference electrode liquid junction

The ionised calcium concentration of sequential retentates prepared by ultrafiltration of a human control serum increased with increasing protein concentration when measured with both a Nova 2 and a Radiometer ICA1 analyser using their standard reference electrodes. In contrast, the ionised calcium in the same retentates fell slightly with increasing protein when the reference electrode liquid junctions of the instruments were changed from hypertonic to isotonic solutions, the values then paralleling those in the filtrates. Thus, the clinically significant positive relationship between ionised calcium and protein that has been reported with the Nova 2 and ICA1 analysers is almost certainly an effect of protein on the reference electrode liquid junction potential rather than a consequence of a Donnan effect on true ionised calcium distribution.