02.04.2.1 Introduction

The kidney is the most important site for excreting drugs. Drugs are eliminated from the body either after conversion to metabolites that are excreted, or they are excreted unchanged.

Kidney function and drug excretion varies with the life cycle. When renal function is low or impaired, the excretion of drugs may be reduced, leading to a build up of drug and toxicity. For instance, renal function is low in the neonate but matures rapidly in the first few months. This means that neonates may need very small doses, even measured as per kilogram, compared to older people. After maturation, kidney function declines about 1% per year in adulthood. Thus, older-adults (≥ 65 years) may have functional kidney impairment, and require small doses of drugs that undergo excretion from the kidney than the doses used in younger adults.

The first step in excretion from the kidney is glomerular filtration. Drugs are delivered to the kidney in the blood stream, and free drug (water soluble or free fraction of lipid soluble drugs), with the exception of macromolecular substances (heparin), freely diffuse into glomerular filtrate. Drugs bound to albumin (e.g. warfarin) are held back. Free warfarin and most other drugs/drug metabolites are freely filterable. Some drugs are actively secreted into the kidney tubules.