Metabolic alkalosis is characterized by a primary increase in bicarbonate (HCO₃⁻) concentration, leading to elevated blood pH. This elevation occurs due to excessive bicarbonate retention or loss of hydrogen ions. The rise in bicarbonate is the defining biochemical hallmark distinguishing it from respiratory causes of alkalosis. Compensation mechanisms often increase pCO₂ to counterbalance but do not correct the primary bicarbonate increase. Hence, an increased bicarbonate level is the key diagnostic feature of metabolic alkalosis.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.