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J Bone Miner Res. 2017 Mar;32(3):549-559. doi: 10.1002/jbmr.3013. Epub 2016 Nov 1.

Development and Validation of a Novel Laboratory-Specific Correction Equation for Total Serum Calcium and Its Association With Mortality Among Hemodialysis Patients.

Author information

1
Harold Simmons Center for Kidney Disease Research and Epidemiology, Division of Nephrology and Hypertension, University of California Irvine, School of Medicine, Orange, CA, USA.
2
Biostatistics, Epidemiology, and Research Design Unit, Institute for Clinical and Translational Science, University of California Irvine, Orange, CA, USA.
3
Department of Medicine, University of California Irvine, School of Medicine, Orange, CA, USA.
4
Kidney Research Institute and Harborview Medical Center, Division of Nephrology, University of Washington, Seattle, WA, USA.
5
Division of Nephrology, University of Tennessee Health Science Center, Memphis, TN, USA.
6
Nephrology Section, Memphis VA Medical Center, Memphis, TN, USA.
7
Fielding School of Public Health at UCLA, Los Angeles, CA, USA.
8
Los Angeles Biomedical Research Institute at Harbor-UCLA, Torrance, CA, USA.

Abstract

Conventional albumin-corrected calcium is inaccurate in predicting ionized calcium, and hidden hypercalcemia, characterized as high ionized calcium with normal total calcium, is associated with higher mortality in hemodialysis patients. By using a national cohort of hemodialysis patients in the Unites States, a novel laboratory-specific prediction equation composed of total calcium, albumin, and phosphorus was derived from 242 patients in the South Atlantic division (adjusted R2  = 0.80 versus 0.71 for the conventional equation) and then validated among 566 patients in the other divisions (adjusted R2  = 0.79 versus 0.68 for the conventional equation). Compared with the conventional equation, the novel equation showed a greater correlation with intact parathyroid hormone. Its relative performance against the conventional equation was consistent across subgroups based on medications related to calcium metabolism. The novel equation also had a higher sensitivity (57% versus 34%) and an equivalent specificity (99% versus 100%) against ionized hypercalcemia at a cut-off value of 10.2 mg/dL. Sensitivity and specificity at 9.4 mg/dL was 94% and 76% (versus 87% and 82% for the conventional equation), respectively. A survival analysis in 87,779 incident hemodialysis patients showed that among patients who were categorized as having a high-normal calcium status (ie, >9.4 to 10.2 mg/dL) by the conventional equation, there appeared a trend toward higher adjusted mortality risk across higher calcium status defined according to the novel equation. Meanwhile, the mortality risk was consistent across calcium strata defined according to the conventional equation within the categories defined by the novel equation. In conclusion, in comparison to the conventional equation, a novel laboratory-specific correction equation derived for correction of total calcium performs significantly better in ascertaining hidden hypercalcemia in hemodialysis patients, and aids in identifying patients at higher risk for mortality.

KEYWORDS:

DISORDERS OF CALCIUM/PHOSPHATE METABOLISM; EPIDEMIOLOGY; STATISTICAL METHODS

PMID:
27714897
PMCID:
PMC5947953
DOI:
10.1002/jbmr.3013
[Indexed for MEDLINE]
Free PMC Article

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