Maximal Drug Elimination Through GFR Alone
If a drug is eliminated solely by glomerular filtration (no tubular secretion or reabsorption), the maximal elimination rate is approximately 120-130 ml/min in healthy adults, which represents the normal GFR. 1
Physiological Basis
- Normal GFR is approximately 130 ml/min per 1.73 m² for men and 120 ml/min per 1.73 m² for women 1
- For drugs cleared exclusively by glomerular filtration, renal clearance cannot exceed the GFR because filtration is the only mechanism removing the drug from plasma 2
- The filtration process has inherent limitations: fractional extraction by glomerular filtration is defined by the product of the unbound fraction of the drug and the filtration fraction, making this a relatively inefficient extraction process compared to tubular secretion 2
Key Distinction from Secreted Drugs
- Drugs that undergo tubular secretion in addition to filtration can achieve renal clearances substantially higher than GFR (often 150-200+ ml/min), as demonstrated by furosemide and penciclovir clearances in clinical studies 3
- The parallel tube model shows that when tubular secretion is highly effective, renal clearance becomes dependent on kidney plasma flow (approximately 600-700 ml/min) rather than just GFR 2
- However, your question specifically asks about drugs cleared only by GFR, which limits the maximal clearance to the filtration rate itself 2
Clinical Context
- The model predicts proportionality of renal drug clearance to GFR only for compounds exclusively excreted by filtration; compounds with tubular secretion exhibit a curvilinear relationship 2
- For filtration-only drugs, dosage adjustment in renal insufficiency follows direct proportionality to measured or estimated GFR 1, 2