Question 1

What is a trough level in therapeutic drug monitoring?

Accepted Answer

A trough level (also called a trough concentration or Cmin) is the lowest plasma drug concentration that occurs just before the next dose — typically measured at the end of a dosing interval, immediately before the next scheduled dose. Trough levels are used in therapeutic drug monitoring (TDM) to ensure that the drug concentration does not fall below the minimum effective concentration (MEC) and to detect drug accumulation. For drugs with narrow therapeutic indices — such as vancomycin, aminoglycosides, and cyclosporin — trough monitoring is essential for efficacy and safety.

Question 2

When should trough levels be drawn?

Accepted Answer

Trough levels must be drawn at a consistent and standardised time relative to the dose — ideally 0–30 minutes before the next scheduled dose. If drawn too early, the level will reflect distribution phase kinetics rather than true trough (for drugs with multi-compartment distribution). For once-daily aminoglycosides, the standard monitoring window is 6–14 hours post-dose (the Hartmann-Conn nomogram method). For vancomycin with AUC/MIC monitoring, a peak and trough are drawn around a steady-state dose (typically the third or fourth dose) to calculate AUC using a Bayesian model. Timing errors are a major cause of TDM result misinterpretation.

Question 3

What are the target trough levels for common drugs?

Accepted Answer

Target trough levels vary by drug and clinical indication: Vancomycin traditional trough target was 10–20 mg/L (now replaced by AUC/MIC target of 400–600 mg·h/L for serious infections); aminoglycosides (once-daily gentamicin/tobramycin): trough <1 mg/L; cyclosporin: 100–300 ng/mL (varies by indication and time post-transplant); tacrolimus: 5–15 ng/mL (early post-transplant), 5–10 ng/mL (maintenance); lithium: 0.6–1.0 mmol/L (maintenance), 0.8–1.2 mmol/L (acute mania); phenytoin: 10–20 mg/L total (or 1–2 mg/L free); digoxin: 0.5–0.9 ng/mL (heart failure), up to 2 ng/mL historically for AF rate control.

Question 4

Why is vancomycin trough monitoring being replaced by AUC/MIC monitoring?

Accepted Answer

Vancomycin efficacy is best predicted by the ratio of the AUC over 24 hours to the minimum inhibitory concentration (AUC/MIC). Traditional trough-only monitoring (targeting 15–20 mg/L) was associated with nephrotoxicity without consistently achieving target AUC/MIC. The 2020 ASHP/IDSA/SIDP vancomycin consensus guidelines recommended AUC/MIC monitoring (target AUC 400–600 mg·h/L for MRSA infections with MIC ≤1 mg/L) using Bayesian estimation software. AUC/MIC monitoring has been shown to reduce nephrotoxicity rates while maintaining clinical efficacy — though implementation requires Bayesian dosing tools and peak + trough sample pairs.

Question 5

Which drugs require therapeutic drug monitoring?

Accepted Answer

Drugs requiring therapeutic drug monitoring (TDM) include: vancomycin (AUC/MIC-guided dosing for MRSA), aminoglycosides (gentamicin, tobramycin — peak for efficacy, trough for toxicity), tacrolimus and cyclosporine (transplant immunosuppressants with narrow ranges), phenytoin (nonlinear kinetics), carbamazepine, valproate, lithium (narrow therapeutic range 0.6-1.2 mmol/L), digoxin (0.5-0.9 ng/mL for heart failure), theophylline, and methotrexate (high-dose oncology use). TDM is indicated when a drug has a narrow therapeutic index, variable pharmacokinetics, serious concentration-dependent toxicity, and when a measurable plasma concentration correlates with efficacy.

Question 6

What is the difference between peak and trough levels?

Accepted Answer

The peak level (Cmax) is the highest plasma drug concentration achieved within a dosing interval, typically measured 1-2 hours after an oral dose or 30-60 minutes after completion of an IV infusion. The trough level (Cmin) is the lowest concentration, measured immediately before the next scheduled dose. Peak levels are most relevant for drugs with concentration-dependent efficacy (aminoglycosides — higher peaks produce greater bactericidal killing) and for assessing acute toxicity risk. Trough levels ensure that drug concentrations do not fall below the minimum effective concentration and help detect drug accumulation. For some drugs, both peak and trough are needed to calculate AUC (vancomycin Bayesian dosing), while for others only the trough is monitored (tacrolimus, cyclosporine, lithium).

Question 7

How do drug interactions affect trough levels?

Accepted Answer

Drug interactions that inhibit or induce drug-metabolising enzymes or transport proteins directly affect trough levels. CYP inhibitors slow drug elimination, raising trough concentrations and potentially causing toxicity. CYP inducers accelerate elimination, lowering troughs — potentially causing loss of efficacy. For renally cleared drugs (aminoglycosides, vancomycin, lithium), drugs that alter renal blood flow (NSAIDs, ACE inhibitors) or tubular secretion (trimethoprim blocks creatinine secretion — falsely elevates creatinine but does not affect vancomycin) can impact trough. Protein-binding displacement interactions change the free fraction, which may affect total-drug trough interpretation.

Drug	Target	Draw timing	Notes
Vancomycin	AUC/MIC 400–600 mg·h/L	Peak + trough at steady state	AUC/MIC replaces trough-only per 2020 guidelines
Gentamicin (ODA)	Trough <1 mg/L	6–14 h post-dose (Hartford)	Hartman or Nicolau nomogram for ODA
Tacrolimus	5–15 ng/mL (early); 3–8 (maint)	Pre-dose (12 h post-dose for BD)	LCMS preferred over immunoassay for accuracy
Cyclosporin	100–300 ng/mL	12 h pre-dose (C0)	Varies by organ, time post-transplant
Phenytoin	10–20 mg/L (total)	Pre-dose (trough)	Correct for albumin (Winter-Tozer eq.) in hypoalbuminaemia
Lithium	0.6–1.0 mmol/L	12 h post-dose (standard)	0.8–1.2 for acute mania; renal function monitoring essential
Digoxin	0.5–0.9 ng/mL (HF)	6–8 h post-dose (distribution complete)	Draw before 6 h → overestimates true level

Trough level

Therapeutic Drug Monitoring and Peak and Trough Levels

Target trough ranges for common drugs

Frequently asked questions