Aminohippuric Acid (DB00345): A Comprehensive Pharmacological and Clinical Monograph

Executive Summary

Aminohippuric acid, also known as para-aminohippuric acid (PAH), is a small molecule diagnostic agent that has served for decades as the clinical and physiological gold standard for the measurement of renal hemodynamics.[1] As a synthetic derivative of hippuric acid, formed from the amino acid glycine and para-aminobenzoic acid, it is not found endogenously in humans and must be administered intravenously for its diagnostic purposes.[1] The foundational principle of its utility lies in its unique and remarkably efficient handling by the kidneys. In a healthy individual, aminohippuric acid is almost completely extracted from the blood in a single transit through the renal circulation, a process achieved through a dual mechanism of glomerular filtration and highly active tubular secretion.[3]

This high renal extraction ratio, approximately 92%, allows its plasma clearance to be used as a precise and reliable measure of Effective Renal Plasma Flow (ERPF).[1] Furthermore, by infusing the drug to concentrations that saturate the renal transport machinery, it is possible to determine the maximal functional capacity of the tubular secretory mechanism (TmPAH), providing a quantitative index of functional proximal tubular mass.[3] Historically, its properties as a substrate for renal transporters were leveraged in a therapeutic context; during World War II, it was co-administered with penicillin to competitively inhibit the antibiotic's renal clearance, thereby prolonging its therapeutic effect.[1]