A 52-year-old man with chronic kidney disease (eGFR 25 mL/min/1.73m²) is started on a new drug. The clinician suspects altered drug clearance due to renal impairment. Which investigation is most appropriate to assess the pharmacokinetic profile and guide dosing adjustments?

## Investigation of Choice for Assessing Drug Clearance in Renal Impairment ### Why Plasma Drug Concentration (TDM) at Steady State is Correct **Key Point:** Therapeutic drug monitoring (TDM) measures the actual plasma concentration of the drug, which directly reflects the balance between absorption, distribution, metabolism, and elimination. This is the gold standard for assessing whether a drug is accumulating or being cleared appropriately in a patient with renal impairment. **High-Yield:** At steady state (typically 3–5 half-lives), the plasma concentration reflects the net effect of renal and hepatic clearance. If the concentration is elevated despite dose adjustment, it confirms reduced clearance and guides further dose reduction. **Clinical Pearl:** TDM is particularly valuable for drugs with: - Narrow therapeutic index (e.g., digoxin, gentamicin, vancomycin, lithium) - Significant renal elimination - Non-linear pharmacokinetics ### Why Other Investigations Are Suboptimal | Investigation | Limitation | |---|---| | **Serum creatinine & BUN** | Indirect markers of renal function; do not measure drug concentration or clearance directly. Used to estimate GFR, not to assess actual drug levels. | | **24-hour urinary creatinine clearance** | Measures renal function, not drug clearance. Cumbersome, time-consuming, and less accurate than eGFR in modern practice. | | **GFR by cystatin C** | Better marker of renal function than creatinine alone, but still only estimates kidney function—does not measure actual drug concentration or clearance. | **Mnemonic:** **TDM = Therapeutic Drug Monitoring** — the direct measurement of drug concentration in plasma to confirm adequate clearance and prevent toxicity or subtherapeutic levels. ### Clinical Context In CKD, drugs that are renally eliminated accumulate. TDM confirms whether the dose adjustment was sufficient by measuring the actual plasma level. If the level is in the therapeutic range, dosing is appropriate; if elevated, further reduction is needed. [cite:KD Tripathi 8e Ch 5]