Three distinct approaches are currently used in assessing acid-base disorders: the traditional - physiological or bicarbonate-centered approach, the base-excess approach, and the "modern" physicochemical approach proposed by Peter Stewart, which uses the strong ion difference (particularly the sodium chloride difference) and the concentration of nonvolatile weak acids (particularly albumin) and partial pressure of carbon dioxide (pCO(2)) as independent variables in the assessment of acid-base status. The traditional approach developed from the pioneering work of Henderson and Hasselbalch and the base-excess are still most widely used in clinical practice, even though there are a number of problems identified with this approach. The approach works well clinically and is recommended for use whenever serum total protein, albumin and phosphate concentrations are normal. Although Stewart's approach has been largely ignored by physiologists, it is increasingly used by anesthesiologists and intensive care specialists, and is recommended for use whenever serum's total protein, albumin or phosphate concentrations are markedly abnormal, as in critically ill patients. Although different in their concepts, the traditional and modern approaches can be seen as complementary, giving in principle, the same information about the acid-base status.