BACKGROUND: We investigated the relationship between serum total carbon dioxide (COâ‚‚) and bicarbonate ion (HCO₃â») concentrations in pre-dialysis chronic kidney disease (CKD) patients and devised a formula for predicting low bicarbonate (HCO₃â»< 24 mmol/L) and high bicarbonate (HCO₃â»â‰¥ 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 COâ‚‚ and HCO₃⻠concentrations was analyzed using Pearson’s correlation coefficient. An approximation formula was developed using clinical parameters correlated independently with HCO₃⻠concentration. Diagnostic accuracy of serum total COâ‚‚ and the approximation formula was evaluated by receiver operating characteristic curve analysis and a 2 × 2 table. RESULTS: Serum total COâ‚‚ correlated strongly with HCO₃⻠concentration (r = 0.91; P < 0.001). The following approximation formula was obtained by a multiple linear regression analysis: HCO₃⻠(mmol/L) = total CO₂− 0.5 × albumin − 0.1 × chloride − 0.01 × (estimated glomerular filtration rate + blood glucose) + 15. The areas under the curves of serum total COâ‚‚ 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 COâ‚‚ (86.6% vs. 81.3%). CONCLUSION: Serum total COâ‚‚ correlated strongly with HCO₃⻠concentration in pre-dialysis CKD patients. An approximation formula including serum total COâ‚‚ showed superior diagnostic accuracy for low and high bicarbonate compared with serum total COâ‚‚.