Calcium Abstracts 3

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Ionized calcium.
            (Baird, 2011) Download
Calcium is the most abundant mineral in the human body. While most of the body's calcium is sequestered in the skeleton, the free, hydrated cation in solution is a key physiologic mediator in a host of metabolic and regulatory processes. The free cation concentration in the extracellular fluid, historically referred to as "ionized calcium" in clinical medicine, is frequently assayed in patients with suspected or known derangements of calcium metabolism. There is controversy in the literature as to whether direct measurement of ionized calcium, measurement of total (free plus chelated or protein-bound) calcium, or adjustment of total calcium for albumin concentration is the best or most practical clinical measure of calcium, as the three methods differ in costs and clinical sensitivities. This manuscript will review calcium biochemistry and homeostasis, compare the utilities of different methods of assessing calcium homeostasis, and discuss appropriate utilization of ionized calcium testing.

Vitamin D deficiency and low ionized calcium are linked with semen quality and sex steroid levels in infertile men.
            (Blomberg Jensen et al., 2016) Download
STUDY QUESTION:  Are low vitamin D levels linked with semen quality and sex steroids in infertile men? SUMMARY ANSWER:  Infertile men with vitamin D deficiency had lower sperm motility, total numbers of motile sperm, Inhibin B, sex-hormone-binding-globulin (SHBG) and testosterone/estradiol ratio, but higher levels of free sex steroids, than infertile men with normal vitamin D levels. WHAT IS KNOWN ALREADY:  Low vitamin D levels have been associated with decreased sperm motility in healthy men, but a relationship between vitamin D and calcium with semen quality and especially sex steroids has not been sufficiently described in infertile men. STUDY DESIGN, SIZE, DURATION:  This study comprises baseline characteristics of 1427 infertile men screened from 2011 to 2014 for inclusion in a randomized clinical trial, the Copenhagen-Bone-Gonadal Study. PARTICIPANTS/MATERIALS, SETTING, METHODS:  In total 1427 infertile men, consecutively referred to our tertiary andrological centre for fertility workup, underwent a physical examination and had semen quality assessed based on two samples and blood analysed for serum testosterone, SHBG, estradiol, inhibin B, luteinizing hormone, follicle-stimulating hormone (FSH), 25-hydroxyvitamin D (25-OHD), ionized calcium (Ca(2+)) and karyotype. There were 179 men excluded due to serious comorbidities or anabolic steroid usage, leaving 1248 patients for analyses. MAIN RESULTS AND THE ROLE OF CHANCE:  Men with 25-OHD >75 nmol/l had higher sperm motility and 66 and 111% higher total numbers of motile spermatozoa after 45 and 262 min, respectively, than men with 25-OHD <25 nmol/l (all P < 0.05). SHBG levels and testosterone/estradiol ratios were 15 and 14% lower, respectively, while free testosterone and estradiol ratios were 6 and 13% higher, respectively, in men with 25-OHD <25 nmol/l (all P < 0.05). Men with lower Ca(2+) levels had higher progressive sperm motility and inhibin B/FSH ratio but lower testosterone/estradiol ratio (all P < 0.05). LIMITATIONS, REASONS FOR CAUTION:  All outcomes presented are predefined end-points but inferral of causality is compromised by the descriptive study design. It remains to be shown whether the links between vitamin D, calcium, semen quality and sex steroids in infertile men are causal. WIDER IMPLICATIONS OF THE FINDINGS:  The associations between vitamin D deficiency and low calcium with semen quality and sex steroids support the existence of a cross-link between regulators of calcium homeostasis and gonadal function in infertile men. STUDY FUNDING/COMPETING INTERESTS:  This study was supported by the Danish Agency for Science, Technology and Innovation, Hørslev Fonden, Danish Cancer Society and Novo Nordisk Foundation. There are no conflicts of interest. TRIAL REGISTRATION NUMBER:  NCT01304927. DATE OF TRIAL REGISTRATION:  25 February 2011. DATE OF ENROLMENT OF FIRST PATIENT:  8 March 2011.

Accuracy of methods to estimate ionized and "corrected" serum calcium concentrations in critically ill multiple trauma patients receiving specialized nutrition support.
            (Dickerson et al., 2004) Download
BACKGROUND:  The purpose of this study was to determine the accuracy of 22 published methods to estimate serum ionized calcium (iCa) and "corrected" total serum calcium (totCa) concentrations in critically ill, multiple trauma patients. Seven of these formulas estimated iCa and 15 were directed toward predicting a "corrected" totCa. METHODS:  Adult patients admitted to the trauma intensive care unit who received specialized nutrition support were consecutively recruited for study. Patients who received blood products, i.v. calcium, or therapeutic doses of heparin within 24 hours before the laboratory measurements or had a history of cancer, bone disease, parathyroid disease, hyperphosphatemia (> or = 6 mg/dL), hyperbilirubinemia (> 3.5 mg/dL), or renal failure requiring dialysis were excluded. The 22 published methods were analyzed for sensitivity, specificity, percentage false negatives, and percentage false positives for predicting hypocalcemia or hypercalcemia. RESULTS:  One hundred patients were studied 4.9 +/- 3.3 days postinjury and were receiving enteral nutrition (n = 81), parenteral nutrition (n = 18), or both (n = 1) at the time of study. Twenty-one patients were hypocalcemic (iCa < or = 1.12 mmol/L) and 6 were hypercalcemic (iCa > or = 1.32 mmol/L). The mean sensitivity of the 22 methods for assessing hypocalcemia was 25% +/- 32% and the specificity was 90% +/- 18%. Although the average percentage of false positives for assessing hypocalcemia was 10% +/- 18%, the mean percentage of false negatives was inordinately high at 75% +/- 32%. The most common method for determination of "corrected" totCa concentration ["corrected" calcium = totCa + (0.8 x (4-serum albumin concentration))] had a sensitivity of only 5%. The McLean-Hastings nomogram method, the most common method for estimating serum iCa concentration, had a sensitivity of 67% but unfortunately also had a significant false-positive rate of 27%. Serum totCa correlated modestly with iCa (r2 = .334, p < .001). Those patients with a serum albumin < or = 2 g/dL (n = 43) had a significantly higher prevalence of hypocalcemia than those with a higher serum albumin concentration (37% incidence of hypocalcemia vs 10%, respectively, p < .002). CONCLUSIONS:  Aberrations in calcium homeostasis are frequent (27%) in postresuscitative critically ill multiple trauma patients. Methods for predicting hypocalcemia lack sensitivity and are often associated with an unacceptable rate of false negatives. Predictive methods for estimating ionized or corrected serum concentrations should not be used. Direct measurement of serum iCa concentration is indicated for assessing calcium status for this population.

Significance of the ionized calcium measurement to assess calcium status in osteopenic/osteoporosis postmenopausal outpatients.
            (Guiducci et al., 2017) Download
INTRODUCTION:  Evaluation of calcium status is important in the osteoporotic risk assessment. Although guidelines indicate total calcium (tCa) as first-line measurement, directly measured ionized calcium (m-iCa), considered as the gold standard, is more and more often required. Aim of this study is to evaluate the agreement between m-iCa, tCa and iCa calculated from a formula based on total calcium and albumin (c-iCa) in osteopenic/osteoporotic postmenopausal outpatients. METHODS:  A total of 140 postmenopausal outpatients, 41 osteopenic (OPN) and 99 osteoporotic (OP) were enrolled. Levels of tCa, m-iCa, c-iCa, total protein and albumin, vitamin D (25-OHD), parathyroid hormone 1-84 (PTH), bone alkaline phosphatase, osteocalcin and serum collagen type 1 cross-linked C-telopeptide (CTX) were also measured. RESULTS:  There were no statistically significant differences between OPN and OP groups regarding values of tCa, m-iCa, and c-iCa, 25-OHD and PTH. However, OP women had lower levels of CTX (p < 0.05). A significant direct correlation between m-iCa and tCa (r = 0.60, p < 0.001) and c-iCa (r = 0.61, p < 0.001) was found. Women with isolated hyper-m-iCa had similar DEXA parameter levels respect to the other patients. However, one patient with confirmed primary hyperparathyroidism presented hyper-m-iCa versus normal tCa and c-iCa values. CONCLUSIONS:  The use of tCa could be sufficient to characterize the calcium status in postmenopausal outpatients, but reflexive calcium testing strategy for m-iCa test is necessary to women presenting the low or high extremes of tCa levels, or in women with suspected PHPT.


Comparison Between Measured and Calculated Free Calcium Values at Different Serum Albumin Concentrations.
            (Mir et al., 2016) Download
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.

Is the calcium correct? Measuring serum calcium in dialysis patients.
            (Morton et al., 2010) Download
Abnormalities in calcium concentration are frequent in patients receiving dialysis therapy. Most cases of both hypo- and hypercalcemia are mild and asymptomatic. There is concern, however, that, on the one hand, hypocalcemia can drive hyperparathyroidism and eventually lead to gland hypertrophy and autonomous function. Hypercalcemia, on the other hand, can be associated with increased extraosseous calcium and phosphate deposition leading to vascular calcification with an attendant mortality and morbidity. Calcium exists in three main forms in the blood: the physiologically active free or ionized fraction (terms often used interchangeably), a protein bound fraction, and a fraction complexed to other anions. Although the ionized calcium can readily be measured using ion-specific electrodes, it is the total calcium that is most commonly measured because of sample handling and cost concerns. As it is the free or ionized form that is biologically active (and therefore of most relevance), a number of adjustment formulae have been derived to "correct" the total calcium for changes in albumin, protein, and complexing ion concentrations. These formulae show good statistical correlation with measured ionized calcium in populations studied as a whole, but are generally poor predictors of true ionized hypo- or hypercalcemia in individual patients. International guideline committees in nephrology recommend frequent assessment of calcium levels in dialysis patients and recommend that these levels be kept within the normal reference range. These guidelines are less clear on which measurement of calcium should be used to guide clinical decision making. This review examines the merits of making any adjustment to the total calcium measurement, and suggests when it is appropriate to measure the ionized or free calcium.

Calcium: total or ionized
            (Schenck and Chew, 2008) Download
Measurement of serum total calcium (tCa) has been relied on for assessment of calcium status, despite the fact that it is the ionized calcium (iCa) fraction that has biologic activity. Serum tCa does not accurately predict iCa status in many clinical conditions. For accurate assessment of iCa status, iCa should be directly measured. Anaerobic measurement of serum iCa under controlled conditions provides the most reliable assessment of calcium status; aerobic measurement of iCa with species-specific pH correction is highly correlated with anaerobic measurements.

The relation of serum parathyroid hormone and serum calcium to serum levels of prostate-specific antigen: a population-based study.
            (Skinner and Schwartz, 2009b) Download
Experimental and clinical data implicate calcium and parathyroid hormone (PTH) in the development of prostate cancer. However, epidemiologic data on the role of these variables in prostate health are sparse. We examined the relationship between serum levels of calcium, PTH, and prostate-specific antigen (PSA), an established marker of prostate growth, in a large, population-based study using multivariate linear regression. We studied 1,273 men in National Health and Nutrition Survey 2005 to 2006 who were >or=40 years of age and who were without clinical prostate cancer. Adjusted for age, race, body mass index, and serum levels of 25-hydroxyvitamin D, serum levels of PTH were significantly positively correlated with serum PSA (P = 0.01). Serum levels of PTH and calcium each were correlated significantly with free PSA (P = 0.05 and 0.008, respectively). The percentage of men who had elevated serum levels of PTH (PTH, >or=66 pg/mL) was significantly greater among African American men (19.2 versus 9.6%, P = 0.04). Compared with men whose PTH was at the lower end of the reference range, the predicted PSA for men with a PTH of 66 pg/mL was increased 43%. These findings support the hypothesis that serum calcium and serum PTH stimulate prostate growth in men without clinical prostate cancer and have implications for the use of PSA as a screening tool for prostate cancer.


 

A prospective study of total and ionized serum calcium and fatal prostate cancer.
            (Skinner and Schwartz, 2009a) Download
We recently reported a significant positive association in the National Health and Nutrition Examination Survey between high levels of total calcium in serum, measured prospectively, and risk of fatal prostate cancer. To confirm this, we examined associations between total and ionized serum calcium and prostate cancer mortality in an independent cohort, the Third National Health and Nutrition Examination Survey. Twenty-five prostate cancer deaths occurred over 56,625 person-years of follow-up. Compared with men in the lowest tertile of total serum calcium, the multivariate-adjusted relative risk for death from prostate cancer for men in the highest tertile was 2.07 (95% confidence interval, 1.06-4.04). For ionized serum calcium, the physiologically active fraction of total serum calcium, the relative risk for men in the highest tertile was 3.18 (95% confidence interval, 1.09-9.28). These findings support the hypothesis that serum calcium is a prospective biomarker of fatal prostate cancer.
Comparison of serum total calcium, albumin-corrected total calcium, and ionized calcium in 1213 patients with suspected calcium disorders.
            (Thode et al., 1989) Download
The correlations between serum ionized calcium, serum total calcium, total calcium corrected for albumin and calculated ionized calcium were investigated in a prospective multicentre investigation of 1213 patients suspected of having calcium metabolic disease. Diagnostic discordance between serum total calcium and measured ionized calcium was found in 31% of the patients. With the calculation of albumin-corrected total calcium or calculated ionized calcium the discordance decreased to 17.9%. The diagnostic discordance which could be ascribed to the analytical imprecision (CV = 1.5%) amounted to only 6.7%. Although we found highly significant correlations between the parameters, a considerable scatter around the regression line made prediction of ionized calcium from albumin-corrected total calcium unreliable in many patients.

Parathyroid Hormone Secretion Is Controlled by Both Ionized Calcium and Phosphate During Exercise and Recovery in Men.
            (Townsend et al., 2016) Download
CONTEXT:  The mechanism by which PTH is controlled during and after exercise is poorly understood due to insufficient temporal frequency of measurements. OBJECTIVE:  The objective of the study was to examine the temporal pattern of PTH, PO4, albumin-adjusted calcium, and Ca(2+) during and after exercise. DESIGN AND SETTING:  This was a laboratory-based study with a crossover design, comparing 30 minutes of running at 55%, 65%, and 75% maximal oxygen consumption, followed by 2.5 hours of recovery. Blood was obtained at baseline, after 2.5, 5, 7.5, 10, 15, 20, 25, and 30 minutes of exercise, and after 2.5, 5, 7.5, 10, 15, 20, 25, 30, 60, 90, and 150 minutes of recovery. PARTICIPANTS:  Ten men (aged 23 ± 1 y, height 1.82 ± 0.07 m, body mass 77.0 ± 7.5 kg) participated. MAIN OUTCOME MEASURES:  PTH, PO4, albumin-adjusted calcium, and Ca(2+) were measured. RESULTS:  Independent of intensity, PTH concentrations decreased with the onset of exercise (-21% to -33%; P ≤ .001), increased thereafter, and were higher than baseline by the end of exercise at 75% maximal oxygen consumption (+52%; P ≤ .001). PTH peaked transiently after 5-7.5 minutes of recovery (+73% to +110%; P ≤ .001). PO4 followed a similar temporal pattern to PTH, and Ca(2+) followed a similar but inverse pattern to PTH. PTH was negatively correlated with Ca(2+) across all intensities (r = -0.739 to -0.790; P ≤ .001). When PTH was increasing, the strongest cross-correlation was with Ca(2+) at 0 lags (3.5 min) (r = -0.902 to -0.950); during recovery, the strongest cross-correlation was with PO4 at 0 lags (8 min) (r = 0.987-0.995). CONCLUSIONS:  PTH secretion during exercise and recovery is controlled by a combination of changes in Ca(2+) and PO4 in men.

 


References

Baird, GS (2011), ‘Ionized calcium.’, Clin Chim Acta, 412 (9-10), 696-701. PubMed: 21238441
Blomberg Jensen, M, et al. (2016), ‘Vitamin D deficiency and low ionized calcium are linked with semen quality and sex steroid levels in infertile men.’, Hum Reprod, 31 (8), 1875-85. PubMed: 27496946
Dickerson, RN, et al. (2004), ‘Accuracy of methods to estimate ionized and “corrected” serum calcium concentrations in critically ill multiple trauma patients receiving specialized nutrition support.’, JPEN J Parenter Enteral Nutr, 28 (3), 133-41. PubMed: 15141404
Guiducci, L, et al. (2017), ‘Significance of the ionized calcium measurement to assess calcium status in osteopenic/osteoporosis postmenopausal outpatients.’, Gynecol Endocrinol, 33 (5), 383-88. PubMed: 28102095
Mir, AA, et al. (2016), ‘Comparison Between Measured and Calculated Free Calcium Values at Different Serum Albumin Concentrations.’, J Lab Physicians, 8 (2), 71-76. PubMed: 27365914
Morton, AR, JS Garland, and RM Holden (2010), ‘Is the calcium correct? Measuring serum calcium in dialysis patients.’, Semin Dial, 23 (3), 283-89. PubMed: 20492582
Schenck, PA and DJ Chew (2008), ‘Calcium: total or ionized’, Vet Clin North Am Small Anim Pract, 38 (3), 497-502, ix. PubMed: 18402876
Skinner, HG and GG Schwartz (2009a), ‘A prospective study of total and ionized serum calcium and fatal prostate cancer.’, Cancer Epidemiol Biomarkers Prev, 18 (2), 575-78. PubMed: 19190170
——— (2009b), ‘The relation of serum parathyroid hormone and serum calcium to serum levels of prostate-specific antigen: a population-based study.’, Cancer Epidemiol Biomarkers Prev, 18 (11), 2869-73. PubMed: 19861512
Thode, J, et al. (1989), ‘Comparison of serum total calcium, albumin-corrected total calcium, and ionized calcium in 1213 patients with suspected calcium disorders.’, Scand J Clin Lab Invest, 49 (3), 217-23. PubMed: 2662382
Townsend, R, et al. (2016), ‘Parathyroid Hormone Secretion Is Controlled by Both Ionized Calcium and Phosphate During Exercise and Recovery in Men.’, J Clin Endocrinol Metab, 101 (8), 3231-39. PubMed: 27294328