Question 1

What is the peak-to-trough ratio (PTR)?

Accepted Answer

The peak-to-trough ratio (PTR), also called the index of fluctuation or swing, is the ratio of the peak plasma concentration (Cmax) to the trough plasma concentration (Cmin) at steady state: PTR = Cmax,ss / Cmin,ss. It quantifies the degree of concentration fluctuation within a dosing interval. A PTR close to 1.0 indicates minimal fluctuation (e.g., IV continuous infusion). High PTR indicates large swings between peak and trough — the concentration spends significant time near the trough (potentially below MEC) or near the peak (potentially above MTC). PTR is particularly important for narrow therapeutic index drugs and for drugs with concentration-dependent pharmacodynamics.

Question 2

How is the peak-to-trough ratio calculated?

Accepted Answer

For a one-compartment model with first-order kinetics, the PTR at steady state is: PTR = exp(ke × τ) = exp(0.693/t½ × τ), where ke is the elimination rate constant (= 0.693/t½), and τ is the dosing interval. This assumes instantaneous IV bolus or that the dose is absorbed rapidly. For oral administration, the actual peak is lower than the IV bolus peak (due to first-order absorption), and the trough is the same — so the oral PTR is lower than the IV PTR for the same dose and interval. As a practical approximation: if the dosing interval equals one half-life, PTR = 2.0; if the interval equals two half-lives, PTR = 4.0.

Question 3

Why does PTR matter for narrow therapeutic index drugs?

Accepted Answer

For drugs with narrow therapeutic windows, the therapeutic range (MEC to MTC) is small. A high PTR means concentrations swing widely — potentially exceeding the MTC at peak (causing toxicity) and falling below the MEC at trough (causing loss of efficacy). For example, aminoglycosides given 8-hourly have high PTR — trough must be monitored to prevent accumulation/nephrotoxicity. Extended-release formulations of NTI drugs (e.g., extended-release carbamazepine, lithium, theophylline) reduce PTR, providing more consistent concentrations and a better safety margin. FDA requires PTR characterisation for NTI drug applications.

Question 4

How do extended-release formulations reduce PTR?

Accepted Answer

Extended-release (ER) formulations slow the rate of drug release and absorption, reducing the absorption rate constant (ka). This reduces the peak concentration (lower Cmax) while maintaining similar trough concentrations (the trough is determined by the dose and interval, not the release rate). The net effect is a lower PTR — the concentration profile is "flattened." This is clinically beneficial for NTI drugs because it reduces peak-related toxicity while maintaining troughs above MEC. ER formulations also improve adherence by reducing dosing frequency. However, ER formulations can be problematic if swallowed whole versus crushed, as dose-dumping may produce very high peaks.

Drug	t½	Interval	Approx PTR	Clinical implication
Gentamicin (ODA)	2–3 h	24 h	~32–128	Intentionally high PTR; high peak kills; low trough avoids accumulation nephrotoxicity
Vancomycin (q8–12h)	4–8 h	8–12 h	~2–4	Moderate fluctuation; AUC/MIC now preferred over trough monitoring
Carbamazepine IR (TID)	8–24 h (autoinduced)	8 h	Variable	Significant fluctuation with IR; ER formulation reduces PTR, improves tolerability
Lithium (IR, TID)	18–24 h	8 h	~1.3–1.5	Relatively low PTR; but SR formulation used to further reduce GI effects at peak
Amlodipine (OD)	~35–50 h	24 h	~1.3–1.5	Very low PTR; very long t½ means minimal fluctuation even with once-daily dosing
Short-acting morphine (IR, q4h)	2–3 h	4 h	~4–8	High PTR with IR; modified-release preferred for chronic pain to reduce peaks and troughs

Peak-to-trough ratio

Formula

Clinical examples: PTR and dosing implications

Frequently asked questions