The expected compensation for chronic respiratory alkalosis is a decrease in the serum bicarbonate of 4 to 5 mEq/L (4-5 mmol/L) for each 10 mm Hg (1.3 kPa) decrease in the PCO2. The expected serum bicarbonate concentration in this patient is calculated to be 14 to 16 mEq/L (14-16 mmol/L), and the measured serum bicarbonate of 20 mEq/L (20 mmol/L) suggests coexistence of a metabolic alkalosis.