Approximation of bicarbonate concentration using serum total carbon dioxide concentration in patients with non-dialysis chronic kidney disease

Background: We investigated the relationship between serum total carbon dioxide (CO2) and bicarbonate ion (HCO3-) concentrations in pre-dialysis chronic kidney disease (CKD) patients and devised a formula for predicting low bicarbonate (HCO3- < 24 mmol/L) and high bicarbonate (HCO3- >= 24 mmol/L) using clinical parameters. Methods: In total, 305 samples of venous blood collected from 207 pre-dialysis patients assessed by CKD stage (G1 + G2, 46; G3, 50; G4, 51; G5, 60) were investigated. The relationship between serum total CO2 and HCO3- concentrations was analyzed using Pearson's correlation coefficient. An approximation formula was developed using clinical parameters correlated independently with HCO3- concentration. Diagnostic accuracy of serum total CO2 and the approximation formula was evaluated by receiver operating characteristic curve analysis and a 2 x 2 table. Results: Serum total CO2 correlated strongly with HCO3- concentration (r = 0.91; P < 0.001). The following approximation formula was obtained by a multiple linear regression analysis: HCO3- (mmol/L) = total CO2- 0.5 x albumin - 0.1 x chloride - 0.01 x (estimated glomerular filtration rate + blood glucose) + 15. The areas under the curves of serum total CO2 and the approximation formula for detection of low bicarbonate and high bicarbonate were 0.981, 0.996, 0.993, and 1.000, respectively. This formula had superior diagnostic accuracy compared with that of serum total CO2 (86.6% vs. 81.3%). Conclusion: Serum total CO2 correlated strongly with HCO3- concentration in pre-dialysis CKD patients. An approximation formula including serum total CO2 showed superior diagnostic accuracy for low and high bicarbonate compared with serum total CO2.