Context: The “free hormone” hypothesis suggests that the free 25-hydroxyvitamin D (25OHDFree) level may usefully indicate bone health. Objective: To determine which vitamin D measure is optimally correlated with clinical and bone parameters in healthy children. Design and Participants: A  cross-sectional study including 146 healthy children (71 boys, 9.5±1.9 years) at a tertiary medical center. Main Outcome Measures: We used a multiplex liquid chromatography-tandem mass spectrometry-based assay to simultaneously measure vitamin D metabolites. The 25OHDFree level was directly measured (m-25OHDFree) or calculated using genotype-constant or genotypespecific affinity coefficients of vitamin D-binding proteins (con-25OHDFree or spe-25OHDFree). Bone mineral content (BMC) and density (BMD) were assessed via dual-energy X-ray absorptiometry. Results: The concentrations of total 25OHD (25OHDTotal), the three forms of 25OHDFree, and 24,25-dihydroxyvitamin D3 correlated with parathyroid hormone levels (all p<0.01). Serum 25OHDTotal and m-25OHDFree levels reflected age, puberty, season, body mass index (BMI), daylight hours, and vitamin D intake (all p<0.05). The con-25OHDFree level better reflected puberty and daylight hours than did the spe-25OHDFree level (both p<0.01). The association between the 25OHDTotal level and bone parameters varied according to the BMI (interaction p<0.05). In 109 normal-weight children, the con-25OHDFree level correlated with BMC and BMD (both p<0.05), but the 25OHDTotal and 24,25-dihydroxyvitamin D3 levels were associated with BMC (both p<0.05). No association was found in overweight or obese children. Conclusions: In healthy children, total and free 25OHD levels comparably reflected lifestyle factors. In normal-weight children, the con-25OHDFree level reflected BMC and BMD, whereas the 25OHDTotal level was associated with BMC. Bone and Mineral Metabolism VITAMIN D, DIABETES AND ENERGY METABOLISM Association Between Population Vitamin D Status and SARS-CoV-2 Related Serious-Critical Illness and Deaths Dimitrios T. Papadimitriou, MD, MSc(2), PhD, Alexandros K. Vassaras, MD, Michael F. Holick, MD, PhD. Pediatric Endocrine Clinics, Athens, Greece, Aristotle University of Thessaloniki, Thessaloniki, Greece, Boston University School of Medicine, Boston, MA, USA. Background: Vitamin-D population status may have possible unappreciated consequences to the COVID-19 pandemic. Α significant association between vitamin-D sufficiency and reduction in clinical severity and inpatient

Bone and Mineral Metabolism

Serum calcium status in health and disease: A comparison of measured and derived parameters☆

Calcium in plasma is distributed between ionised and bound fractions (Fig. I). (Throughout this paper. the term 'plasma' is synonymous with the plasma phase of whole blood. undiluted separated blood plasma. or undiluted serum). In normal subjects about 50'}';. is present as free hydrated calcium ions (hereafter called ionised calcium; Ca 1 ) . Bound calcium comprises several species which arc usually distinguished on the basis of molecular size as protein-bound and complex-bound calcium; albumin accounts for about 75% of the calcium bound to protein. I ..' Calcium can bind to albumin at about 30 binding sites which have various association constants;' although the overall effect can be described by a more simplistic model with 12 sites having an owmil apparent binding constant at I'll 7·4 of 95 lJmol. -l. 5 The association of ('a~+ ions is pl l-dcpendent and the presence of fatty acids bound to albumin enhances Ca.!t association.v The binding of calcium to protein is also temperature dependent. being greater as temperature incrcascs.v 7 As I'll is also influenced by temperature. the overall effect is that plasma ('a~ I concentration at 37°(' is about 6% higher than at 200C and that of whole blood. about 3% higher. 2. -l. 7 Bicarbonate and. to a lesser extent. lactate and phosphate arc the most important of the low molecular weight calcium ligands ill vivo" 4. s, "and in total they bind about 10% of all calcium in normal plasma. However. in some circumstances

The measurement of ionised calcium in blood plasma.

SUMMARY
Background: Approximately 12% of bound blood calcium is linked to various anions including phosphate. In patients with end-stage kidney disease (ESKD), serum phosphate is highly variable. We propose that establishing a formula to calculate albumin- and phosphate-corrected total calcium would be more appropriate to estimate free calcium in ESKD patients.

Methods:  In 82 haemodialysis patients, serum ionized calcium (Caion) and pH were measured by blood gas analyser with ion-selective electrodes at the point-of-care, while bicarbonate, phosphate, albumin, magnesium and total calcium (Catot) were measured at the central laboratory. Linear regression analysis of measured variables was used to best fit adjusted calcium versus Caion.

Results:  The most parsimonious multiple linear regression model (r2 = 0.81) of variables associated with Caion included Catot (coeff 0.820, P < 0.0001), albumin (coeff −0.016, P < 0.0001) and phosphate (coeff −0.063, P < 0.002). Modelling of available variables yielded the following equation to adjust calcium for albumin and phosphate: CaalbPh = Catot + (0.015 × (40 − [albumin]) + 0.07 × (1.5 − [phosphate])). At an ambient albumin of 40 g/L, CaalbPh would be 0.07 mmol/L lower than Catot for every mmol/L of phosphate. In vitro data using three different albumin levels and increasing phosphate concentrations demonstrated this relationship, with the slope of the phosphate effect being stronger at lower albumin concentrations.

Conclusion:  Because guidelines recommendations indicate that corrected serum calcium should be maintained within the normal range in ESKD patients, inclusion of phosphate to correct Catot in these patients may have clinical implications on the choice of phosphate binders and the prescription of vitamin D or calcimimetic agents.

Serum phosphate is an important determinant of corrected serum calcium in end‐stage kidney disease

Theory and applications of ion-selective electrodes: Part 7

We studied, in 70 acidotic and non-acidotic uremic patients, the analytical variance in serum ionized calcium as related to duration and temperature of storage. Storage of serum or whole blood at 4 degrees C for as long as 6 h did not significantly alter the measured concentration of ionized calcium in the serum. Storage at room temperature for 6 h, or longer at 4 degrees C or -20 degrees C, resulted in inaccuracies in 39 to 79% of the samples of serum and in 38 to 92% of the samples of whole blood. These errors were not negated by correcting the values for ionized calcium to a pH of 7.40. Indeed, corrected values for calcium were even more unreliable in acidotic patients. We conclude that samples from uremic patients should be analyzed for ionized calcium within 2 h, or within 6 h if stored at 4 degrees C.

E. Gaiger

Papers

Effect of storage on measurement of ionized calcium in serum of uremic patients.