## Renal Clearance and Dose Adjustment in Renal Impairment **Key Point:** Renal clearance depends on three mechanisms: **glomerular filtration, active tubular secretion, and active tubular reabsorption**. Only filtration and secretion contribute to drug elimination; reabsorption **reduces** clearance. ### Mechanisms of Renal Drug Elimination ```mermaid flowchart TD A[Drug in plasma]:::outcome --> B{Renal handling}:::decision B -->|Glomerular filtration| C[Drug in filtrate]:::action B -->|Active secretion| C C --> D{Tubular fate}:::decision D -->|Reabsorption| E[Back to plasma]:::action D -->|No reabsorption| F[Urine excretion]:::outcome E --> G[Reduced renal clearance]:::urgent F --> H[Increased renal clearance]:::outcome ``` ### Analysis of Each Statement | Statement | Correctness | Rationale | |-----------|-------------|----------| | Renal clearance ∝ GFR (passive filtration) | ✓ Correct | For drugs eliminated by filtration alone, Cl~renal~ = GFR × f~u~ (fraction unbound) | | Active secretion increases total renal clearance | ✓ Correct | Penicillin, probenecid, cimetidine are actively secreted; Cl~renal~ > GFR | | 90% renal elimination → proportional dose reduction | ✓ Correct | With GFR 25 (≈25% of normal), dose should be reduced to ~25% of normal | | Active reabsorption → unchanged renal clearance | ✗ **INCORRECT** | Active reabsorption **decreases** renal clearance by returning drug to plasma | ### Why Option 4 is Wrong **High-Yield:** Active tubular reabsorption is a **salvage mechanism** that removes drug from the filtrate and returns it to the bloodstream. This **reduces** renal clearance, not leaves it unchanged. **Clinical Pearl:** Glucose, amino acids, and uric acid undergo active reabsorption. In renal impairment, if a drug undergoes significant reabsorption, its renal clearance is **preserved better** than expected from GFR alone — but it is still reduced compared to normal kidney function. ### Dose Adjustment in This Patient **Mnemonic: FILTER-SECRETE-REABSORB** — Only the first two increase clearance; reabsorption decreases it. With GFR 25 mL/min (≈25% of normal 100 mL/min): - Expected renal clearance ≈ 25% of normal - Dose should be reduced to **25% of normal** or given at **normal dose every 4 days** (if normal interval is 24 hours) - Therapeutic drug monitoring is essential [cite:KD Tripathi 8e Ch 4; Harrison 21e Ch 297]
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