PK in Renal Impairment
How kidney disease affects drug elimination and dosing, and practical approaches to dose adjustment in renal impairment.
Renal Impairment and Drug Therapy
Chronic kidney disease (CKD) affects approximately 15% of adults worldwide. Reduced renal function directly impairs excretion of renally cleared drugs and their metabolites, but also alters absorption, protein binding, distribution, and even hepatic metabolism through uremic toxins.
Effects on Each PK Parameter
Absorption
- Uremia raises gastric pH (urease-producing bacteria generate ammonia), reducing absorption of drugs requiring acid
- Edema of the GI tract may impair absorption
- Gastroparesis from diabetic nephropathy slows gastric emptying
Distribution
- Hypoalbuminemia (nephrotic syndrome, advanced CKD) increases free drug fraction. Phenytoin free fraction may double, making total levels misleading — free phenytoin levels should be monitored.
- Uremic toxins displace drugs from albumin binding sites, further increasing free fractions
- Fluid overload increases Vd for hydrophilic drugs (vancomycin, aminoglycosides)
Metabolism
CKD reduces hepatic CYP activity even in the absence of liver disease. Uremic toxins inhibit CYP3A4, CYP2C9, and CYP2D6. Non-renal clearance of drugs like erythromycin and codeine is reduced by 30-50% in severe CKD. This is often underappreciated.
Excretion
The primary concern. As GFR declines:
| CKD Stage | eGFR (mL/min) | Clinical Action |
|---|---|---|
| 1 | > 90 | Usually no adjustment needed |
| 2 | 60-89 | Monitor; adjust selected drugs |
| 3a/3b | 30-59 | Adjust most renally cleared drugs |
| 4 | 15-29 | Significant adjustments; avoid nephrotoxins |
| 5 | < 15 | Dialysis considerations; major adjustments |
Dose Adjustment Methods
Dose Reduction
Reduce each dose while maintaining the same dosing interval. This lowers Cmax and average Css while maintaining time-dependent activity. Preferred for concentration-dependent toxicity.
Adjusted dose = Normal dose x (Patient CrCl / Normal CrCl)
Interval Extension
Keep the same dose but extend the interval between doses. This maintains Cmax for concentration-dependent drugs (aminoglycosides) while allowing more elimination time.
Adjusted interval = Normal interval x (Normal CrCl / Patient CrCl)
Combination
Many drugs require both dose reduction and interval extension. Drug-specific guidelines from package inserts or renal dosing references should be followed.
Dialysis Considerations
Hemodialysis removes drugs with:
- Low molecular weight (< 500 Da for conventional dialysis, < 20 kDa for high-flux)
- Low protein binding (free drug is dialyzable)
- Small Vd (drug is in the blood compartment)
- Water solubility
Post-dialysis supplemental doses are needed for aminoglycosides, vancomycin, and many beta-lactams. Drugs with large Vd (digoxin, amiodarone) are minimally affected by dialysis.
Key Takeaways
- CKD affects all PK parameters, not just renal excretion
- Uremic toxins reduce hepatic CYP activity, affecting non-renal clearance
- Hypoalbuminemia increases free drug fractions — monitor free levels when applicable
- Choose dose reduction, interval extension, or both based on the drug's PK profile
- Dialyzability depends on molecular weight, protein binding, Vd, and water solubility