Approximation of Corrected Calcium Concentrations in Advanced Chronic Kidney Disease Patients with or without Dialysis Therapy

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Background: The following calcium (Ca) correction formula (Payne) is conventionally used for serum Ca estimation: corrected total Ca (TCa) (mg/dl) = TCa (mg/dl) + [4 - albumin (g/dl)]; however, it is inapplicable to advanced chronic kidney disease (CKD) patients. Methods: 1,922 samples in CKD G4 + G5 patients and 341 samples in CKD G5D patients were collected. Levels of TCa (mg/day), ionized Ca²⁺ (iCa²⁺) (mmol/l) and other clinical parameters were measured. We assumed the corrected TCa to be equal to eight times the iCa²⁺ value (measured corrected TCa). We subsequently performed stepwise multiple linear regression analysis using the clinical parameters. Results: The following formula was devised from multiple linear regression analysis. For CKD G4 + G5 patients: approximated corrected TCa (mg/dl) = TCa + 0.25 × (4 - albumin) + 4 × (7.4 - pH) + 0.1 × (6 - P) + 0.22. For CKD G5D patients: approximated corrected TCa (mg/dl) = TCa + 0.25 × (4 - albumin) + 0.1 × (6 - P) + 0.05 × (24 - HCO₃^-) + 0.35. Receiver operating characteristic analysis showed the high values of the area under the curve of approximated corrected TCa for the detection of measured corrected TCa ≥8.4 mg/dl and ≤10.4 mg/dl for each CKD sample. Both intraclass correlation coefficients for each CKD sample demonstrated superior agreement using the new formula compared to the previously reported formulas. Conclusion: Compared to other formulas, the approximated corrected TCa values calculated from the new formula for patients with CKD G4 + G5 and CKD G5D demonstrates superior agreement with the measured corrected TCa.