Advanced Pharmacology · Topic 1 of 13

Pharmacokinetics & Dosing in Organ Failure and Aging

Pharmacology · dosing principles

Pharmacokinetics & Dosing in Organ Failure and Aging

Volume of distribution sets the loading dose, clearance sets the maintenance dose — give the full load, then adjust maintenance for kidney and liver function.

Key numbers

Steady state takes 4–5 half-lives4–5 half-lives

Cockcroft-Gault creatinine clearance× 0.85 if female

Augmented renal clearance underdoses antibioticsCrCl > 130 mL/min

01 · The ruleLoading dose depends on Vd only

The loading dose depends only on volume of distribution, not on clearance, so give it in full even in renal or hepatic failure; organ failure changes the maintenance dose, which depends on clearance.

02 · The ruleSteady state takes 4–5 half-lives

With regular dosing a drug reaches steady state after about 4–5 half-lives; a long half-life therefore delays target levels unless a loading dose is given, and trough levels are drawn only at steady state.

03 · CalculateCockcroft-Gault creatinine clearance

Cockcroft-Gault CrCl (mL/min) = (140 − age) × weight (kg) ÷ (72 × serum creatinine, mg/dL), × 0.85 for women; valid only with stable kidney function, it still underlies many label dose tables.

The rules

Know these

In AKI, creatinine-based estimates mislead dosing must knowAll eGFR and CrCl equations assume steady-state kidney function; in AKI use a non–steady-state estimate or a timed urine creatinine clearance, dose narrow-therapeutic-index drugs conservatively, and use drug levels where available.

No liver marker equals creatinine clearance must knowNo endogenous marker predicts hepatic drug clearance; Child-Pugh class gives only rough guidance. Use extreme caution with narrow-therapeutic-index drugs in liver disease and with any drug in Child-Pugh class C.

Critical illness expands hydrophilic Vd: load fullyCapillary leak, IV fluids, hypoalbuminemia, ventilation and extracorporeal circuits raise the Vd of hydrophilic drugs (beta-lactams, aminoglycosides, vancomycin): give full or higher loading doses and delay renal dose reduction of beta-lactams for about 48 hours.

Dose by eGFR not indexed to BSAKDIGO 2024 favors dosing by eGFR in mL/min, not indexed to body surface area (eGFR × BSA ÷ 1.73), adding cystatin C when creatinine is unreliable — frailty, low muscle mass, liver failure.

Dose lipophilic sedatives on ideal or adjusted weightIn severe obesity, dose opioids, propofol, dexmedetomidine, ketamine and midazolam infusions on ideal or adjusted body weight and titrate to effect; actual-weight dosing overdoses (see Severe Obesity in Acute Care).

Cirrhosis raises oral bioavailability of high-extraction drugsPortosystemic shunting bypasses first-pass metabolism, so oral bioavailability of high-extraction drugs (morphine, midazolam, metoprolol, carvedilol, labetalol) rises about 1.7- to 4.4-fold in cirrhosis — reduce oral doses.

What it is

Core idea

Volume of distribution: dose ÷ plasma concentrationApparent Vd = amount of drug in the body ÷ plasma concentration. Tissue-bound or lipophilic drugs have a large Vd and low plasma levels; protein-bound or hydrophilic drugs stay in plasma (small Vd).

Causes & risk factors

Why it happens

Augmented renal clearance underdoses antibioticsAugmented renal clearance (GFR above 130 mL/min), seen in younger critically ill patients with intact kidneys, causes subtherapeutic levels of renally cleared drugs: raise doses or extend beta-lactam infusions.

Low albumin raises free drug levelsWith low albumin, highly protein-bound drugs (phenytoin, warfarin) have a larger unbound, active fraction — toxicity can occur while the total level looks normal, so interpret total levels cautiously.

Older adults: normal creatinine hides low GFRAfter 40, GFR falls about 8 mL/min/1.73 m² per decade, yet serum creatinine often stays normal because muscle mass falls — estimate clearance before dosing renally cleared drugs in every older adult.

Aging prolongs lipophilic drug half-livesWith age, body fat rises and water falls: lipophilic drugs (diazepam) gain a larger Vd and longer half-life, digoxin and aminoglycosides reach higher levels, and clearance of many CYP450-metabolized drugs falls 30–40%.

Diagnosis & monitoring

Work-up

Half-life = 0.693 × Vd ÷ clearanceElimination half-life = 0.693 × Vd ÷ clearance: half-life lengthens when clearance falls (renal or hepatic failure) or when volume of distribution rises (fluid overload, obesity for lipophilic drugs).

Ideal and adjusted body weight formulasIdeal body weight = 50 kg (men) or 45.5 kg (women) + 2.3 kg per inch over 60 inches; adjusted weight = IBW + 0.4 × (actual − IBW), used in Cockcroft-Gault for obesity.

Loading dose or maintenance dose?

Commonly confused

Set by	Volume of distribution
In kidney failure	Give in full
Purpose	Reach target concentration immediately
Critical illness or ascites	Often larger — hydrophilic Vd expands

More detail

Reference

Show 7 reference facts

Edema or ascites in chronic liver disease raises the Vd of hydrophilic drugs, so a larger loading dose may be needed when a rapid effect is required.

In severe liver disease, creatinine-based estimates such as Cockcroft-Gault often overestimate GFR (low muscle mass, increased tubular creatinine secretion); cystatin C may reflect GFR more accurately.

Sedatives, opioids and anxiolytics need particular caution in cirrhosis because they can precipitate hepatic encephalopathy; prefer short-acting agents and reduced doses.

Renal replacement therapy removes drugs with low protein binding and small molecular size (for example levetiracetam), which then need a supplemental dose during or after dialysis.

Most drugs follow first-order kinetics (a constant fraction eliminated per unit time); when metabolizing enzymes saturate, elimination becomes zero-order (a fixed amount per hour), so small dose increases cause large level rises.

Measured creatinine clearance overestimates GFR by about 10–20% with normal kidney function and by up to 50% in advanced kidney failure, because creatinine is also secreted by the tubules.

Common label cutoffs for renal dose adjustment are kidney function below 60, 45 and 30 mL/min, with separate dosing for dialysis.

Pearl

Load for the volume, maintain for the clearance: organ failure changes the second dose, not the first.

Sources · Merck Manual Professional — Overview of Pharmacokinetics (table: formulas defining basic pharmacokinetic parameters) · Merck Manual Professional — Drug Distribution to Tissues (Aug 2026) · Merck Manual Professional — Drug Excretion (augmented renal clearance; age-related decline) · Merck Manual Professional — Drug Metabolism (first- vs zero-order kinetics; CYP450) · Merck Manual Professional — Pharmacokinetics in Older Adults · Fleuren LM et al. Clinically relevant pharmacokinetic knowledge on antibiotic dosing among intensive care professionals is insufficient. Crit Care 2019;23:185 (model answers: steady state after 4–5 half-lives; loading dose depends on Vd only) · Gorham J, Taccone FS, Hites M. Ensuring target concentrations of antibiotics in critically ill patients through dose adjustment. Expert Opin Drug Metab Toxicol 2022;18(3):177–187 (open copy: ULB repository) · Verbeeck RK. Pharmacokinetics and dosage adjustment in patients with hepatic dysfunction. Eur J Clin Pharmacol 2008;64:1147–1161 (doi 10.1007/s00228-008-0553-z; open copy UCLouvain) · Awdishu L et al. KDIGO 2024 clinical practice guideline on evaluation and management of CKD: a primer on what pharmacists need to know. Am J Health Syst Pharm 2025 · MDCalc — Updated review of Creatinine Clearance (Cockcroft-Gault Equation): formula, IBW/adjusted weight and weight selection · Erstad BL, Barletta JF. Drug dosing in the critically ill obese patient — a focus on sedation, analgesia, and delirium. Crit Care 2020;24:315 · Saran S, Rao NS, Azim A. Drug dosing in critically ill patients with acute kidney injury and on renal replacement therapy. Indian J Crit Care Med 2020;24(Suppl 3):S129–S134 · Merck Manual Professional — Evaluation of the Patient With Renal Issues (creatinine clearance overestimates GFR) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).