Acid-base interpretation requires arterial blood gas (ABG) and chemistry panel (chem 7). Venous blood gas (VBG) can be used as surrogate, but accurate interpretation requires ABG whenever patients are sick. There are three different methods to interpret acid-base at the bedside. The physiologic approach which employs anion gap (AG) is most widely used and discussed in this chapter. You need to refer normal values to interpret acid-base derangement (Table 1.1). First step is to figure out the primary process by looking at pH and PaCO2 (Table 1.2), then figure out appropriate compensation by the Table 1.3. Deviation from expected compensation would indicate additional acid-base processes. Figure 1.1 is the steps for interpretating acid-base status. Next follow individual flowchart based on the primary process. Follow Fig. 1.2 if primary metabolic acidosis and Fig. 1.3 for metabolic alkalosis. Table 1.4 shows acronym for metabolic acidosis so that you can quickly work to find out the causes of acid-base derangement. Refer Table 1.5 to interpret urine anion gap. Table 1.6 for use of urine pH and serum K to delineate different types of RTA. Respiratory acidosis caused by hypoventilation may result from chronic lung disease or neuromuscular dysfunction. Respiratory alkalosis, due to hyperventilation is commonly observed in sepsis, chronic liver disease and acute pulmonary embolism. Table 1.7 for how to start and titrate bicarb for advanced CKD patients. Just be aware that debates on the reliability of urine anion gap continues. Finally, keep it in mind, acid-base status is a condition, not a diagnosis.