Calculation of plasma ionised calcium from total calcium, proteins and pH: comparison with measured values

The influence of calcium-phosphate metabolism abnormalities on the quality of life in patients with hemodynamically significant mitral regurgitation

Background

In recent years, studies have indicated that vitamin D [25(OH)D₃] and other calcium-phosphate (Ca-P) metabolism parameters and their disturbances might be potential new factors that may influence health-related quality of life (HRQoL). The aim of our study was to assess the extent of Ca-P metabolism abnormalities in patients with significant mitral regurgitation (MR) and their effect on patients’ HRQoL.

Methods

We included 99 patients with significant MR (median age, 75 years [Q1–Q3, 66.0–81.5], 35.4% females). Hemodynamically significant MR was assessed using transthoracic echocardiography (vena contracta > 3 mm, effective orifice area > 0.2 cm², and MR volume > 30 mL/s). HRQoL was evaluated using a cardiac-specific (MacNew) tool.

Results

A significant negative correlation between parathormone (PTH) levels and HRQoL was demonstrated (r = − 0.242, − 0.243, and − 0.255; p = 0.018, 0.018, and 0.013 for Global Scores, and physical and social domains, respectively). Additionally, we confirmed that patients with higher NT-proBNP levels, NYHA heart failure (HF) class, and larger left ventricles had poorer HRQoL. Moreover, patients with poorer HRQoL walked a shorter distance in a 6-min walking test.

Conclusions

To the best of our knowledge, this report is the first to show that Ca-P abnormalities resulted in significantly worse HRQoL, especially in the physical domain, in a population of patients with hemodynamically significant MR.

Comparison Between Measured and Calculated Free Calcium Values at Different Serum Albumin Concentrations

INTRODUCTION

Free ionic calcium is the metabolically active component of total calcium (TCa) in blood. However, most laboratories report TCa levels that are dependent on serum albumin concentration. Hence, several formulae have evolved to calculate free calcium levels from TCa after adjustment for albumin. However, free calcium can directly be measured using direction selective electrodes rather than spectrophotometric methods used in autoanalyzers.

OBJECTIVES

This study compares the levels of free calcium obtained by measurement by direct ion selective electrode (ISE) and the one calculated as a function of TCa by formulae.

MATERIALS AND METHODS

A total of 254 serum samples submitted to clinical biochemistry laboratory of a tertiary care hospital were analyzed for total protein, albumin, and TCa by standard spectrophotometric methods and for free calcium by direct ISE. Three commonly used formulae viz. Orrell, Berry et al. and Payne et al. were used to calculate adjusted TCa. Calculated free calcium was obtained by taking 50% of these values.

RESULTS

A significant difference (P < 0.05) was observed between calculated free calcium by all the three formulae and measured free calcium estimated by direct ISE using paired t-test and Bland-Altman plots.

CONCLUSION

Formulae for predicting free calcium by estimating TCa and albumin lacks consistency in prediction and free calcium should be evaluated by direct measurement.

A formula to predict corrected calcium in haemodialysis patients

BACKGROUND

The conventional calcium correction formula (corrected total calcium (mmol/L) = TCa (mmol/L) + 0.02 [40 (g/L) - albumin (g/L)]) is broadly applied for the estimation of serum calcium in haemodialysis (HD) patients, despite the fact that it was not derived or validated in a HD population. A novel formula was derived and validated for corrected serum calcium in HD patients.

METHODS

Total calcium (TCa), ionized calcium (iCa(2+)), magnesium, phosphate, albumin and bicarbonate were collected from 60 HD patients to derive the formula. A validation set of 237 stable HD patients was then examined, and subjects were classified as hyper-, hypo- and normocalcaemic based on the iCa(2+). Agreement of the new formula was calculated with iCa(2+) as the gold standard, using the intraclass correlation coefficient (ICC). This was compared to the agreement between iCa(2+) and the following: uncorrected total serum calcium (TCa), the conventional correction formula, the Orrell formula and the Clase formula.

RESULTS

Using multiple linear regression the following formula was derived: corrected total calcium (mmol/L) = TCa (mmol/L) + 0.01 [30 (g/L) - albumin (g/L)]. The new formula had superior agreement compared to all of the other formulae. There was a statistically significant greater agreement between the new formula and the iCa(2+) as compared to the conventional formula (P < 0.01). However, the new formula did not significantly outperform the Orrell formula, the Clase formula or Total calcium.

CONCLUSIONS

The use of our simple new formula should enable more appropriate decision making compared to the conventional formula in the highly complex HD population.

Hypercalcemia associated with extracorporeal life support in neonates

BACKGROUND/PURPOSE

Disturbances in calcium homeostasis are common at initiation of extracorporeal life support (ECLS). At the authors' institution many neonates undergoing ECLS have developed hypercalcemia. To determine the frequency of hypercalcemia in neonates during ECLS we performed retrospective chart review of neonates that required ECLS at our neonatal intensive care unit.

METHODS

The authors identified 76 consecutive neonates who underwent ECLS before 10 days of age at Cincinnati Children's Hospital Medical Center from July 1, 1991 to June 30, 1996. The hospital charts and ELSO forms were reviewed. Demographic, clinical, and laboratory data for each of the patients were reviewed, both before initiation and during ECLS. Hypercalcemia was defined as total serum calcium concentration of greater than 11 mg/dL (2.74 mmol/L) on at least one occasion beyond the first 24 hours of ECLS.

RESULTS

The hospital charts and ELSO forms from 70 patients were available for review. One patient was excluded because he was only on ECLS for 33 hours, and there were no calcium levels obtained after 24 hours of ECLS and until death. Twenty-five (36%) neonates undergoing ECLS had hypercalcemia (serum Ca > 11 mg/dL [2.74 mmol/L]). Hypercalcemia was associated with longer duration of ECLS (hypercalcemia group, 243 +/- 115 hours and normocalcemia group, 139 +/- 64 hours) and greater requirements for platelet transfusions (hypercalcemia group, 538 +/- 282 mL and normocalcemia group, 372 +/- 233 mL). This could not be explained by differences in primary diagnosis, amounts of calcium administered, and acid-base status.

CONCLUSIONS

Hypercalcemia was found to be common in neonates that require ECLS and is associated with longer duration of ECLS support. Conservative calcium administration for neonates while on ECLS may be warranted.

Albumin-corrected calcium and ionized calcium in stable haemodialysis patients

BACKGROUND

It is ionized calcium that is physiologically active and under homeostatic control; however, total calcium is more conveniently measured. Formulae for correction of calcium to account for albumin binding have not been validated in a dialysis setting.

METHODS

We measured ionized calcium simultaneously with total calcium (t[Ca]), albumin, total protein and pH before dialysis in 50 stable outpatients and convalescent inpatients.

RESULTS

Although 92% of patients were taking calcium supplements and 70% taking alphacalcidol, 11 patients (22%) had ionized hypocalcaemia. To facilitate comparison of calculated ionized calcium, measured total calcium (t[Ca]), and 'corrected' calcium (c[Ca]), with the criterion measure of ionized calcium, all measurements were converted to z scores, standardized on the normal range for each variable. Results are expressed as intraclass correlation coefficients (ICC: 0, all differences are due to error; 1, all differences are due to between patient variation).

CONCLUSIONS

None of the published formulae greatly improved the test characteristics beyond simply using the total calcium. A correction formula in widespread use (Payne), quoted in reference texts, agreed less well with ionized calcium than did the unadjusted measured calcium. Correction formulae should be abandoned in favour of the use of uncorrected calcium. In cases of doubt, ionized calcium should be directly measured.

Is the routine measurement of ionized calcium worthwhile in patients with cancer?

This study was undertaken to analyse the relationship between total calcium (TCa) and ionized calcium (ICa) in patients with cancer, and to assess the clinical value of routine measurements of ICa in these patients. Serum TCa, ICa, albumin, proteins and creatinine were measured in 188 adult patients with solid malignant tumours. Most of them were out-patients, the Karnofsky score being 80 or above in 67%. The correlation coefficient between ICa and TCa was 0.85 (P less than 0.001) and did not improve after correcting TCa for protein concentration with several published formulae. Although TCa measurements had a global diagnostic accuracy (percent of patients correctly classified) of 90%, they failed to identify a substantial proportion of patients with increased levels of ICa (57% for uncorrected TCa, and 27-57% for protein-corrected TCa). However, the finding of slightly increased ICa levels did not seem to predict the development of frank hypercalcaemia and did not impair the prognosis. According to these results, the routine measurement of ICa in unselected patients with cancer has no clinical usefulness.

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.

Serum calcium and parathyroid function linked to control of respiration

It is suggested that the plasma concentration of ionized calcium in a complex way is connected to control or respiration in mammals. The level of ionized calcium in plasma is suggested to correlate with the arterial carbon dioxide tension as well as with the arterial oxygen tension. The primary function of the parathyroid glands, which are absent in fish, but develop in land vertebrates, is suggested to be control of plasma calcium and pH in respiratory alkalosis due to hyperventilation.

Raised parathyroid hormone levels in the milk alkali syndrome: an appropriate response?

A case of the 'milk alkali syndrome' associated with grossly elevated levels of amino terminal parathyroid hormone is described. The hypercalcaemia (calcium 4.09 mmol/l) and hyperparathyroidism settled on conservative measures. Factors in the milk alkali syndrome which might stimulate the release of parathyroid hormone include parathyroid gland hyperplasia secondary to suppression of ionized calcium, alteration in sensitivity of calcium receptors on the cells of the parathyroid glands, the stimulation of an intermittent alkaline tide in the blood and the high intake of phosphate and bicarbonate. We suggest that high levels of parathyroid hormone in the milk alkali syndrome may be appropriate rather than paradoxical.

Effect of change in growth environment on cultured myocardial cells investigated in a standardized medium

Neonatal rat heart cells cultivated in either of two different media which varied only in their serum supplements were transferred to chemically defined medium (Ham's F10) for 24 h before measuring a variety of parameters. The 24-h period of exposure to chemically defined medium was not sufficient to reverse the effects imposed on the cells by the serum used in the first phase of growth. The cells differed in rate and duration of action potentials and contractions. The initial serum composition affected the response of the cells to calcium deficiency. Studies involving the effects of pharmaceutical reagents such as isoproterenol were also influenced by the serum. In attempting to determine the cause and possible mechanism, it was found that mitochondrial membrane permeability for nitroblue tetrazolium (NBT) was unchanged. Although the serum supplements differed in fatty acid composition, the fatty acid profiles of the cell phospholipids were relatively constant. We conclude that the function of the cells is affected by the growth environment, particularly serum; that a short exposure to a uniform chemically defined medium is not sufficient to reverse these effects; and that the differences in effects are not the result of changes in the fatty acid composition of the whole cell phospholipids nor in mitochondrial membrane permeability as measured by NBT.

Free ionised calcium--a critical survey

It is our aim to summarise and discuss procedures for the evaluation of the concentration of free ionised calcium in serum or plasma. Stress is laid upon the interrelations and relative validity of the most common algebraic expressions to appraise the calcium status. The multitude of formulae proposed in the literature are, by mathematical discussion, reduced to variations on a single theme. A second topic is the direct potentiometric measurement of free ionised calcium concentration. Finally we review the literature on the clinical utility of measuring or calculating the free ionised calcium concentration.