Learning Objectives

By the end of this chapter, learners will be able to:
– Understand how kidney disease alters pharmacokinetics and pharmacodynamics.
– Identify drugs that require dose adjustment or avoidance in renal impairment.
– Apply practical principles for dosing across CKD stages, AKI, and renal replacement therapies (HD, PD, CRRT).
– Anticipate common pitfalls and use strategies (including TDM) to optimize therapy.
– Utilize reliable resources for renal dosing.

29.1 Impact of Kidney Disease on Pharmacokinetics and Pharmacodynamics

Kidney disease alters drug handling and response, increasing the risk of under- or over-exposure. Dosing must be individualized based on renal function, drug properties, and patient factors.

• Absorption
• Often minimally affected, but uremia can change gastric pH, motility, and intestinal edema; interactions with phosphate binders and iron can reduce absorption of some drugs.
• Distribution
• Volume of distribution (Vd): May increase for hydrophilic agents (fluid overload) and decrease for lipophilic agents (low body fat/malnutrition).
• Protein binding: Uremia reduces albumin binding; highly protein-bound drugs (e.g., phenytoin, warfarin) may have higher unbound fractions. Total concentrations can mislead—free levels may be more informative.
• Metabolism
• Hepatic metabolism (Phase I/II) may be impaired by uremic toxins/inflammation; non-renal clearance can be reduced. Active metabolites may accumulate if renally cleared.
• Elimination
• Reduced GFR and tubular secretion slow clearance of parent drugs and metabolites, prolonging half-life and raising toxicity risk.

Pharmacodynamics
– Altered receptor sensitivity: Increased CNS depressant effects; relative diuretic resistance; altered platelet function with uremia.
– Electrolyte/acid–base: Hyperkalemia and hypomagnesemia potentiate QT prolongation and digoxin toxicity; metabolic acidosis can affect ionization and distribution.
– Disease- and treatment-specific: Anemia, hypoalbuminemia, and dialysis can modify responses.

29.2 Identifying Drugs Requiring Dose Adjustment or Avoidance

Drugs likely to need adjustment in CKD:
– High renal clearance of parent drug (e.g., many beta-lactams, fluoroquinolones, aminoglycosides, acyclovir, gabapentin/pregabalin, digoxin, lithium).
– Renally cleared active/toxic metabolites (e.g., morphine-6-glucuronide; meperidine to normeperidine; allopurinol to oxypurinol).
– Narrow therapeutic index (e.g., vancomycin, aminoglycosides, digoxin, lithium, phenytoin).
– Nephrotoxic potential (e.g., NSAIDs, aminoglycosides, amphotericin B, tenofovir disoproxil fumarate, IV contrast). Avoid or use with great caution and monitoring.

Selected drug-class considerations (non-exhaustive; verify label/local guidance):
– Antimicrobials: Most beta-lactams, fluoroquinolones, TMP-SMX, vancomycin, aminoglycosides, acyclovir, ganciclovir, fluconazole require adjustment. Azithromycin, doxycycline often do not.
– Cardiovascular: Digoxin (reduce dose/monitor), sotalol/dofetilide (strict renal dosing), atenolol (renally cleared), most ACEi/ARBs do not need “renal dosing” but require monitoring for hyperkalemia/creatinine rise.
– Anticoagulants: Enoxaparin requires adjustment at low eGFR; unfractionated heparin preferred in advanced CKD when rapid reversal/monitoring desired. Direct oral anticoagulants (DOACs) have agent-, indication-, and region-specific renal dosing; some are not recommended in severe CKD/dialysis—follow product label/local policy.
– Endocrine: Metformin dosing guided by eGFR; generally avoid in eGFR <30 mL/min/1.73 m² and during AKI or severe hypoxic states; consider dose reduction and reassessment for eGFR 30–44 based on label/local guidance. SGLT2 inhibitors have eGFR thresholds for initiation/continuation; glycemic effect declines with lower eGFR.
– Neurologic pain/spasticity: Gabapentin/pregabalin require substantial reduction; baclofen can cause profound neurotoxicity in CKD—avoid or use very low doses with close monitoring; tramadol requires adjustment.
– Opioids: Avoid codeine and meperidine; use morphine/hydromorphone cautiously (active renally cleared metabolites); fentanyl, methadone, and buprenorphine are preferred options in advanced CKD (specialist oversight recommended).
– Gout: Allopurinol—start low and titrate; oxypurinol accumulates. Febuxostat requires caution with comorbidities; follow labeling.
– GI: H2 blockers (e.g., famotidine) require adjustment; PPIs generally do not but monitor for adverse effects with chronic use.
– Psychotropics: Lithium is renally cleared with a narrow therapeutic index—dose carefully and monitor levels; many SSRIs/SNRIs/antipsychotics require individualized consideration.

29.3 Principles of Drug Dosing in Renal Impairment

Estimating renal function for drug dosing
– Use the estimating method recommended in the product label or local policy. Many legacy labels use Cockcroft–Gault (CG) creatinine clearance; others accept eGFR (MDRD or CKD-EPI).
– Consider body size:
– Underweight: Actual body weight in CG is commonly used.
– Normal weight: Ideal body weight (IBW) often used.
– Obesity: Adjusted body weight may better reflect clearance when using CG.
– Serum creatinine alone is unreliable in extremes of age, muscle mass, or acute illness. In unstable kidney function (e.g., AKI), timed urine CrCl or TDM may be more informative.

Adjustment methods
– Reduce dose, keep interval (preferred for time-dependent antibiotics).
– Extend interval, keep dose (often used for concentration-dependent drugs).
– Combine both (for very low clearance or toxicity risk).
– Loading doses: Typically based on Vd and often unchanged in CKD; adjust only if Vd is clearly altered (e.g., fluid overload for hydrophilic drugs).

Special scenarios
– AKI: Renal function may change rapidly; re-estimate frequently; prioritize TDM where available; consider measured CrCl if feasible.
– CKD: Dose by CKD stage/eGFR; reassess regularly, especially with intercurrent illness or diuretic changes.
– Therapeutic Drug Monitoring (TDM): Use for narrow-therapeutic-index agents (e.g., vancomycin, aminoglycosides, digoxin, lithium, some anticonvulsants). For vancomycin, contemporary practice increasingly targets AUC-based exposure where feasible.

29.4 Dosing in Renal Replacement Therapy (RRT)

Drug removal depends on drug and RRT characteristics.

Drug factors favoring dialyzability
– Low molecular weight, low protein binding, small Vd, high water solubility.

Hemodialysis (HD)
– High-flux membranes and higher blood/dialysate flows increase clearance of small, unbound drugs (e.g., aminoglycosides, vancomycin, many beta-lactams).
– Practical points:
– Give many renally cleared drugs after HD on dialysis days.
– Supplemental post-HD doses are often required for highly dialyzable drugs.
– Drugs with large Vd and/or high protein binding (e.g., digoxin, warfarin) are minimally dialyzed.

Peritoneal dialysis (PD)
– Slower clearance than HD; fewer drugs require large adjustments relative to anuric HD.
– Intraperitoneal administration is preferred for PD peritonitis.
– Systemic dosing adjustments still required for renally cleared drugs.

Continuous renal replacement therapy (CRRT)
– For small, unbound drugs, extracorporeal clearance often approximates the effluent flow rate (dialysate + replacement fluids).
– Doses are frequently higher than in intermittent HD and may approach normal renal dosing depending on CRRT intensity.
– Variability between modalities (CVVH, CVVHD, CVVHDF), filters, and flow rates necessitates close monitoring and, where possible, TDM.

29.5 Common Pitfalls and Optimization Strategies

Common pitfalls
– Relying on serum creatinine alone.
– Ignoring active or toxic metabolites.
– Not accounting for dynamic changes (AKI recovery or progression).
– Overlooking drug–drug interactions that increase nephrotoxicity (e.g., NSAIDs with ACEi/ARB/diuretics; TMP-SMX with potassium-sparing agents).
– Failing to re-dose after HD when required or inadvertently re-dosing when drug is minimally dialyzed.

Optimization strategies
– Use an appropriate renal function estimator consistent with the product label/local policy; consider body size and acute changes.
– Consult reliable drug references for renal dosing and RRT adjustments.
– Employ TDM for narrow-therapeutic-index agents; interpret free (unbound) levels when protein binding is altered (e.g., phenytoin in uremia).
– Simplify regimens; deprescribe nephrotoxins where feasible.
– Educate patients about adherence, over-the-counter NSAID avoidance, and when to report symptoms (e.g., dizziness, confusion, palpitations).

29.6 Practical Approach: A Stepwise Dosing Workflow

1) Define the clinical goal and urgency (e.g., empirical sepsis therapy vs. chronic symptom control).
2) Determine renal function (estimator per label/policy) and stability (AKI vs. CKD).
3) Identify drug characteristics:
– Route of elimination, protein binding, Vd.
– Dialyzability (if on RRT) and need for post-dialysis dosing.
– Narrow therapeutic index?
4) Choose initial strategy:
– Loading dose (if indicated) based on Vd.
– Maintenance dose/interval tailored to clearance.
5) Plan monitoring:
– TDM timing and targets (if applicable).
– Clinical response and adverse effects.
– Renal function and electrolytes.
6) Reassess frequently and adjust:
– After dialysis sessions, changes in CRRT settings, or renal recovery/decline.
– With interacting medications or changes in volume status.

29.7 Selected Drugs: Quick Renal Dosing/Use Notes

• Aminoglycosides: Concentration-dependent killing; extended-interval dosing with careful TDM; significantly removed by HD—post-HD dosing often required.
• Vancomycin: Adjust to exposure targets; significantly cleared by high-flux HD; re-dose post-HD per levels/expected removal.
• Beta-lactams: Time-dependent; prefer dose reduction with standard intervals, or consider prolonged/continuous infusions for severe infections with TDM where available.
• TMP-SMX: Reduce dose in CKD; may raise serum creatinine (tubular secretion) without changing GFR; risk of hyperkalemia.
• Acyclovir/Ganciclovir: Substantial dose reductions required; crystalluria risk with dehydration.
• Fluconazole: Reduce maintenance dose in CKD; loading dose typically unchanged.
• Digoxin: Reduce dose and extend interval; monitor levels and electrolytes; toxicity risk increased with hypokalemia/hypomagnesemia.
• Lithium: Avoid nephrotoxins (NSAIDs); adjust with close trough monitoring; toxicity risk with volume depletion.
• Metformin: Dose and continuation depend on eGFR and clinical status; avoid in eGFR <30 or during AKI/hypoxic states; follow label/local policy for 30–44 and around iodinated contrast.
• Opioids: Prefer fentanyl, methadone, or buprenorphine in advanced CKD; avoid codeine/meperidine; use morphine/hydromorphone cautiously.
• Baclofen: Avoid or use with extreme caution in CKD due to risk of encephalopathy.

29.8 Resources for Drug Dosing in Renal Impairment

• Online calculators: eGFR/CrCl calculators (e.g., MDCalc).
• Drug databases: UpToDate, Lexicomp, Micromedex, Epocrates.
• Specialized handbooks and labeling:
• The Renal Drug Handbook (renal dosing compendium).
• Product labeling (SmPC/PI) for agent-specific renal dosing and RRT guidance.
• Local hospital formularies and antimicrobial stewardship resources.

SUMMARY

• Kidney disease alters absorption, distribution, metabolism, and elimination, with elimination changes most clinically impactful. Pharmacodynamic responses often change as well.
• Dose adjustments should follow an appropriate renal function estimator consistent with product labeling/local policy, with special attention to body size and dynamic renal changes.
• Choose between dose reduction, interval extension, or both; loading doses generally depend on Vd rather than clearance.
• RRT substantially affects drug clearance; know drug dialyzability and dialysis modality specifics. Post-HD supplemental dosing is common for highly dialyzable drugs; CRRT dosing often scales with effluent flow.
• Avoid or adjust nephrotoxins; employ TDM for narrow-therapeutic-index drugs; reassess frequently.

CLINICAL PEARLS

• In obesity, Cockcroft–Gault with adjusted body weight is often preferred for drug dosing when CG is required by labeling.
• A normal serum creatinine can mask severe GFR reduction in sarcopenic or elderly patients—use eGFR/CrCl, not creatinine alone.
• For hydrophilic antibiotics, give the full loading dose even in CKD; adjust maintenance thereafter.
• Digoxin toxicity risk rises with hypokalemia and hypomagnesemia—correct electrolytes and monitor levels.
• Baclofen can precipitate profound encephalopathy in advanced CKD—avoid when possible.
• Gentamicin is highly dialyzable; plan post-HD supplementation. Digoxin is minimally dialyzed.
• Free (unbound) phenytoin levels are more reliable than total levels in uremia.
• In CRRT, small unbound drug clearance often approximates effluent rate—dose higher than in anuric ESRD.

VISUAL MATERIALS

• Diagram: Pharmacokinetic changes in CKD (absorption, distribution, metabolism, elimination) with key examples.
• Table: Common drugs requiring adjustment, suggested strategies, and dialysis considerations (e.g., aminoglycosides, vancomycin, digoxin, gabapentinoids, metformin, DOACs).
• Flowchart: Stepwise dosing workflow from estimating renal function to monitoring and reassessment.
• Schematic: Factors influencing dialyzability (protein binding, Vd, MW) across HD, PD, CRRT.
• Table: Comparison of dosing approaches by RRT modality with typical need for post-dialysis supplementation.

MULTIPLE CHOICE QUESTIONS

1) Which drug property most strongly predicts significant removal during high-flux hemodialysis?
– A. High protein binding and large Vd
– B. Low molecular weight, low protein binding, small Vd
– C. Lipophilicity and hepatic metabolism
– D. Enterohepatic recirculation
Answer: B
Explanation: Small, unbound, hydrophilic drugs with small Vd are efficiently removed by HD.

2) When Cockcroft–Gault creatinine clearance is required by a drug label in a patient with obesity, which weight is most appropriate?
– A. Ideal body weight
– B. Actual body weight in all cases
– C. Adjusted body weight
– D. Lean body mass by DEXA
Answer: C
Explanation: In obesity, adjusted body weight often best approximates renal drug clearance when using CG.

3) Which statement about loading doses in CKD is most accurate?
– A. Loading doses should always be halved
– B. Loading doses are based on Vd and often unchanged
– C. Loading doses depend on creatinine clearance only
– D. Loading doses are unnecessary for hydrophilic antibiotics
Answer: B
Explanation: Loading dose depends on Vd; CKD mainly affects maintenance dosing.

4) Which medication is generally preferred for analgesia in advanced CKD due to favorable pharmacokinetics?
– A. Codeine
– B. Meperidine
– C. Fentanyl
– D. Morphine
Answer: C
Explanation: Fentanyl lacks active renally cleared metabolites and is minimally dialyzed; codeine/meperidine are unsafe, morphine metabolites accumulate.

5) Which agent typically requires a supplemental dose after high-flux hemodialysis due to significant removal?
– A. Digoxin
– B. Warfarin
– C. Gentamicin
– D. Fentanyl
Answer: C
Explanation: Aminoglycosides are small, unbound, hydrophilic, and highly dialyzable; digoxin/warfarin are minimally dialyzed.

6) Regarding metformin use in CKD, which is the best statement?
– A. Safe at any eGFR with dose reduction
– B. Generally avoid if eGFR <30 mL/min/1.73 m² or during AKI; follow label/local policy for 30–44
– C. Contraindicated if age >65 years regardless of eGFR
– D. Only contraindicated during hemodialysis sessions
Answer: B
Explanation: Metformin use is guided by eGFR and clinical status; avoid in advanced CKD/AKI and follow labeling/local policy.

7) A 68-year-old with CKD stage 4 develops severe confusion after starting a muscle relaxant. Which agent is the most likely cause?
– A. Tizanidine
– B. Baclofen
– C. Cyclobenzaprine
– D. Methocarbamol
Answer: B
Explanation: Baclofen accumulates in CKD and can cause profound neurotoxicity.

8) Which drug most clearly benefits from routine therapeutic drug monitoring in CKD?
– A. Amlodipine
– B. Vancomycin
– C. Acetaminophen
– D. Atorvastatin
Answer: B
Explanation: Vancomycin has a narrow therapeutic index; TDM guides safe and effective exposure.

9) Which statement about digoxin in CKD is correct?
– A. No dose adjustment is needed
– B. It is efficiently removed by hemodialysis
– C. Dose reduction and level monitoring are required; toxicity risk increases with hypokalemia
– D. Total levels are sufficient even with uremia
Answer: C
Explanation: Digoxin is renally cleared; minimal dialyzability and electrolyte disturbances heighten toxicity risk.

10) In CRRT, clearance of small, unbound antibiotics most closely correlates with:
– A. Hemoglobin concentration
– B. Effluent flow rate (dialysate + replacement)
– C. Serum albumin
– D. Residual urine output only
Answer: B
Explanation: For small unbound drugs, extracorporeal clearance often approximates the CRRT effluent rate.

POWERPOINT PRESENTATION (Slide Outline)

Slide 1: Title – Drug Dosing in Kidney Disease
– Why renal dosing matters
– Goals: efficacy, safety, stewardship

Slide 2: CKD and Pharmacokinetics
– Absorption/distribution changes
– Reduced protein binding; altered Vd

Slide 3: Metabolism and Elimination
– Uremia impacts hepatic metabolism
– Elimination and half-life prolongation

Slide 4: Identifying High-Risk Drugs
– Renal clearance, active metabolites
– Narrow TI and nephrotoxins

Slide 5: Estimating Renal Function
– CG vs eGFR per label/policy
– Body size considerations; unstable function

Slide 6: Adjustment Strategies
– Reduce dose vs extend interval vs both
– Loading dose based on Vd

Slide 7: RRT Principles
– Dialyzability factors
– HD: post-dialysis dosing; PD: slower; CRRT: effluent-based

Slide 8: TDM and Monitoring
– Vancomycin/aminoglycosides/digoxin/lithium
– Free levels when binding altered

Slide 9: Case-Based Pearls
– Metformin thresholds, baclofen risk
– Opioid selection; digoxin/electrolytes

Slide 10: Antimicrobial Highlights
– Beta-lactams, aminoglycosides, vancomycin
– Post-HD supplementation; CRRT dosing

Slide 11: Avoiding Pitfalls
– Don’t use creatinine alone
– Reassess with AKI/RRT changes; interactions

Slide 12: Resources and Take-Home
– Reliable references and local policies
– Stepwise workflow and team approach

Educational disclaimer: This chapter provides general educational information for clinicians and is not a substitute for professional judgment. Dosing should follow current product labeling, institutional policies, and specialist guidance, taking into account individual patient factors.

Visual learning: Drug Dosing in Kidney Disease

Presentation resource: The Kidney Hub clinical-series PowerPoint for Chapters 22–30 accompanies these chapters for teaching use.