Low Plasma Ionized Calcium Is Associated With Increased Mortality: A Population-based Study of 106 768 Individuals

Derivation and comparison of formulae for the adjustment of total calcium

Background

Free ionized calcium (Ca²⁺) is the biologically active component of total calcium (TCa) and hence responsible for its biological action. TCa is routinely adjusted for albumin using several formulae (e.g. James, Orell, Payne and Berry) to more closely reflect Ca²⁺. Here, we derive a novel formula to estimate Ca²⁺ and compare its performance to established formulae.

Methods

Cohort for prediction of Ca²⁺: 2806 serum samples (TCa) taken contemporaneously with blood gas samples (Ca²⁺) at Imperial College Healthcare NHS Trust were used to derive formulae to estimate Ca²⁺ using multivariable linear regression. Cohort for prediction of PTH: Performance of novel and existing formulae to predict PTH in 5510 patients was determined by Spearman correlation.

Results

Ca²⁺ prediction Cohort: Adjusted calcium (r² = 0.269) was less strongly associated with Ca²⁺, than TCa (r² = 0.314). Prediction of Ca²⁺ from a newly derived formula incorporating TCa, potassium, albumin, and hematocrit had an improved r² of 0.327, whereas inclusion of all available parameters increased the r² further to 0.364. Of the established formulae, James performed best in predicting Ca²⁺ (r² = 0.27). PTH prediction cohort: Berry resulted in higher whereas Orell in lower adjusted calcium levels. Prediction of PTH was strongest in the setting of hypercalcemia, with James having the highest Spearman correlation coefficient (+0.496) similar to including all parameters (+0.499).

Conclusion

Adjustment of calcium for albumin using established formulae does not always outperform unadjusted TCa in the reflection of Ca²⁺. Further prospective studies are needed to optimise adjustment of TCa and to establish bounds for validity.

Serum calcium concentrations and risk of all-cause and cause-specific mortality: results from two prospective cohorts

CONTEXT

Calcium plays a critical role in various physiological activities. However, the association between circulating calcium concentrations and mortality in a general healthy population remains undetermined.

OBJECTIVE

To examine the association of serum calcium concentrations with all-cause and cause-specific mortality.

METHODS

Leveraging data from the UK Biobank (n=361 662) and the US National Health and Nutrition Examination Survey (NHANES, n=36 985), we prospectively examined the association of serum calcium concentrations with all-cause and cause-specific mortality using Cox proportional hazard and restricted-cubic spline models.

RESULTS

During a median follow-up of 12.0 years, UK Biobank documented 18 327 deaths, including 3119 (17.0%) from cardiovascular disease (CVD) and 9599 (52.4%) from cancer. We found a U-shape relationship of albumin-adjusted calcium concentrations with all-cause and CVD mortality. Compared to participants with moderate calcium levels (the third quintile, Q3), those with low and high levels had an increased risk of all-cause (hazard ratio [HR]=1.02 for Q1 vs. Q3; 1.10 for Q5 vs. Q3) and CVD mortality (HR=1.11 for Q1 vs. Q3; 1.25 for Q5 vs. Q3). In contrast, there was a linear positive relationship with cancer mortality (HR=1.09 for Q5 vs. Q1). Similar results for all-cause, CVD, and cancer mortality were observed in US NHANES.

CONCLUSIONS

Our findings provide novel insights into the association between serum calcium concentrations and mortality in the general healthy population.

L-shaped association of serum calcium with all-cause and CVD mortality in the US adults: A population-based prospective cohort study

Background

Calcium is involved in many biological processes, but the impact of serum calcium levels on long-term mortality in general populations has been rarely investigated.

Methods

This prospective cohort study analyzed data from the National Health and Nutrition Examination Survey (1999-2018). All-cause mortality, cardiovascular disease (CVD) mortality, and cancer mortality were obtained through linkage to the National Death Index. Survey-weighted multivariate Cox regression was performed to compute hazard ratios (HRs) and 95% confidential intervals (CIs) for the associations of calcium levels with risks of mortality. Restricted cubic spline analyses were performed to examine the non-linear association of calcium levels with all-cause and disease-specific mortality.

Results

A total of 51,042 individuals were included in the current study. During an average of 9.7 years of follow-up, 7,592 all-cause deaths were identified, including 2,391 CVD deaths and 1,641 cancer deaths. Compared with participants in the first quartile (Q1) of serum calcium level [≤2.299 mmol/L], the risk of all-cause mortality was lower for participants in the second quartile (Q2) [2.300-2.349 mmol/L], the third quartile (Q3) [2.350-2.424 mmol/L] and the fourth quartile (Q4) [≥2.425 mmol/L] with multivariable-adjusted HRs of 0.81 (95% CI, 0.74-0.88), 0.78 (95% CI, 0.71-0.86), and 0.80 (95% CI, 0.73, 0.88). Similar associations were observed for CVD mortality, with HRs of 0.82 (95% CI, 0.71-0.95), 0.87 (95% CI, 0.74-1.02), and 0.83 (95% CI, 0.72, 0.97) in Q2-Q4 quartile. Furthermore, the L-shaped non-linear associations were detected for serum calcium with the risk of all-cause mortality. Below the median of 2.350 mmol/L, per 0.1 mmol/L higher serum calcium was associated with a 24% lower risk of all-cause mortality (HR: 0.76, 95% CI, 0.70-0.83), however, no significant changes were observed when serum calcium was above the median. Similar L-shaped associations were detected for serum calcium with the risk of CVD mortality with a 25% reduction in the risk of CVD death per 0.1 mmol/L higher serum calcium below the median (HR: 0.75, 95% CI, 0.65-0.86).

Conclusion

L-shaped associations of serum calcium with all-cause and CVD mortality were observed in US adults, and hypocalcemia was associated with a higher risk of all-cause mortality and CVD mortality.