Acid-base disorders can usually be approached by following the steps outlined in the text and doing the calculations shown in the box on page 257. Clues about the underlying disorder can be obtained from history taking and physical examination. Assessment of pH, PaCO2, and HCO3- allows determination of whether a primary metabolic or respiratory disorder is present. Calculation of the predicted compensatory response for simple acid-base disorders might suggest the presence of an additional disease process if compensation is not appropriate. Calculation of the various gaps can be helpful in differential diagnosis (i.e., anion gap for diagnosis of metabolic acidosis, delta anion gap for diagnosis of high-anion-gap metabolic acidosis, and urine anion gap for diagnosis of a non-anion gap metabolic acidosis). Most acid-base problems can be solved with use of the stepwise approach described.