DrugTheGuide
Pharmacokinetics

Renal Clearance CLR

The volume of plasma completely cleared of a drug by the kidneys per unit time — encompassing glomerular filtration, active tubular secretion, and passive tubular reabsorption. Reduced GFR in kidney disease requires proportional dose adjustments for renally eliminated drugs.

Definition and Components

Renal clearance (CLR) is determined by three simultaneous processes:

CLR = GFR × fu + CLsecretion − CLreabsorption
Glomerular filtration

Passive ultrafiltration of unbound drug at the glomerulus. Rate = GFR × fu. Normal GFR ≈ 90–120 mL/min. Only unbound drug is filtered (protein-bound drug cannot pass the glomerular basement membrane).

Active tubular secretion

Active transport of drug from peritubular capillaries into the tubular lumen by OAT, OCT, P-gp, and MRP transporters. Can allow CLR > GFR × fu. Subject to saturation (Michaelis-Menten kinetics) and drug interactions. See drug transporters guide.

Tubular reabsorption

Passive back-diffusion of lipophilic, non-ionised drug from tubular lumen into blood. Reduces effective CLR. Magnitude depends on drug lipophilicity, pKa, and urine pH. Alkaline urine promotes reabsorption of basic drugs; acidic urine promotes reabsorption of acidic drugs.

GFR, CKD Stages, and Dose Adjustment

For drugs predominantly eliminated renally, CLR falls proportionally with GFR. The kidney accumulates drug relative to the degree of GFR impairment.

CKD stage eGFR (mL/min/1.73m²) Dose adjustment needed? Key drug examples
Normal ≥90 No All standard doses
G2 mild 60–89 Rarely Metformin: continue; some antibiotics: minor
G3a mild-moderate 45–59 For some drugs Metformin: reduce to 1 g/day max; dose-adjust apixaban
G3b moderate 30–44 Yes, many drugs Metformin: contraindicated; DOAC dose reductions; enoxaparin
G4 severe 15–29 Yes, most renally cleared drugs Gabapentin, digoxin, aminoglycosides, penicillins
G5 kidney failure <15 / dialysis Major adjustment or avoid Consult renal dosing guidelines for all drugs

For detailed dose-adjustment guidance see our renal dosing adjustments guide, clearance glossary, and FDA guidance on renal impairment PK studies ↗.

Cockcroft-Gault Formula and Creatinine Clearance

Creatinine clearance (CrCl) is the most widely used clinical surrogate for glomerular filtration rate (GFR) and is essential for renal dosing adjustments. The Cockcroft-Gault formula estimates CrCl from serum creatinine, age, weight, and sex:

CrCl (mL/min) = [(140 - age) x weight (kg)] / [72 x serum creatinine (mg/dL)] x 0.85 if female

The Cockcroft-Gault equation remains the reference standard for most drug labelling dose adjustments, even though newer equations (CKD-EPI, MDRD) are preferred for CKD staging. Most DailyMed prescribing information for renally cleared drugs specifies dose adjustments based on Cockcroft-Gault-estimated CrCl rather than CKD-EPI eGFR. A creatinine clearance calculator using the Cockcroft-Gault formula is an essential bedside tool — see our creatinine clearance calculator.

Important limitations of the Cockcroft-Gault formula include: overestimation in obesity (use ideal or adjusted body weight), inaccuracy at extremes of age or muscle mass, and unreliability with rapidly changing creatinine (acute kidney injury). The National Kidney Foundation GFR calculator provides both Cockcroft-Gault and CKD-EPI estimates. The FDA Guidance for Industry on Pharmacokinetics in Patients with Impaired Renal Function discusses when each equation is appropriate for regulatory submissions.

FAQ

What is renal clearance?
Renal clearance (CLR) is the volume of plasma completely cleared of a drug by the kidneys per unit time (mL/min or L/h). It is the sum of three processes: glomerular filtration of unbound drug (GFR × fu), active tubular secretion (mediated by OAT, OCT, P-gp, and MRP transporters), and minus passive tubular reabsorption. CLR = GFR × fu + CL_secretion − CL_reabsorption.
How does GFR affect drug dosing?
For drugs primarily eliminated by renal filtration, clearance is proportional to GFR (corrected for protein binding: CLR ≈ GFR × fu). As GFR falls in chronic kidney disease, clearance decreases and the drug accumulates — t½ increases. Dose reduction (or interval extension) is required proportional to the GFR reduction. GFR is estimated clinically using CKD-EPI or Cockcroft-Gault equations. For the Cockcroft-Gault equation, creatinine clearance (CrCl) = [(140 − age) × weight (kg) × (0.85 if female)] / (72 × serum creatinine mg/dL).
What is the Cockcroft-Gault formula for creatinine clearance?
The Cockcroft-Gault formula estimates creatinine clearance (CrCl) as: CrCl (mL/min) = [(140 - age) x weight (kg)] / [72 x serum creatinine (mg/dL)], multiplied by 0.85 for females. This equation accounts for the age-related decline in GFR and the lower muscle mass (and therefore creatinine production) in women. The Cockcroft-Gault formula remains the standard for drug dosing adjustments in most FDA-approved labelling, even though CKD-EPI is preferred for CKD staging. Limitations include inaccuracy in extremes of body weight (use ideal or adjusted body weight in obesity), unstable renal function (acute kidney injury), and very elderly or cachectic patients where muscle mass does not reflect renal function. A creatinine clearance calculator using this formula should be used at the bedside whenever prescribing renally cleared drugs such as aminoglycosides, vancomycin, metformin, enoxaparin, and digoxin.
What drugs are actively secreted by the kidneys, and why does this matter?
Active tubular secretion adds to glomerular filtration for some drugs, allowing CLR to exceed GFR × fu. OAT1/OAT3 transporters secrete organic anions (methotrexate, penicillins, NSAIDs). OCT2 secretes organic cations (metformin, procainamide). P-gp and MRP2 contribute to cationic drug secretion. Drug interactions at these transporters can reduce secretion: probenecid inhibits OAT secretion of methotrexate (toxicity risk) and penicillins (historically used to prolong penicillin action). Trimethoprim and cimetidine inhibit OCT2, falsely raising serum creatinine without affecting GFR.
What is tubular reabsorption and how can it be manipulated?
Passive tubular reabsorption occurs when filtered drug diffuses back from the tubular lumen into blood down its concentration gradient — particularly for lipophilic, non-ionised drug forms. Urine pH manipulation can alter the ionisation state of weak acids and bases, affecting reabsorption. In salicylate or methotrexate overdose, urinary alkalinisation (sodium bicarbonate) increases ionisation of the acidic drug in tubular urine, reducing reabsorption and increasing elimination. In amphetamine overdose, urinary acidification similarly traps the basic drug in the tubule.