What are PK/PD indices and why do they differ by class?

PK/PD indices link drug exposure to bacterial kill. Beta-lactams kill in a time-dependent way, so the fraction of the interval above the MIC matters most. Aminoglycosides are concentration-dependent, so the peak-to-MIC ratio drives kill. Quinolones and vancomycin track total exposure, so the 24-hour AUC divided by the MIC is the key target.

How is fT>MIC calculated?

Using a one-compartment IV model, the free peak concentration is the dose divided by volume of distribution and multiplied by the unbound fraction. The drug falls exponentially at rate ke, which equals ln(2) divided by half-life. The time above MIC is the natural log of the peak-to-MIC ratio divided by ke, capped at the interval, then expressed as a percentage.

What targets should I aim for?

Common teaching targets are 40 to 70 percent fT>MIC for beta-lactams depending on agent and severity, a Cmax/MIC near 8 to 10 for aminoglycosides, and AUC24/MIC near 125 for Gram-negative quinolones or 400 for vancomycin. Always confirm against current guidelines for your organism and patient.

Why does extended infusion change the result?

This model assumes an IV bolus. Extended or continuous infusions flatten the concentration curve, keeping levels above the MIC for longer and dramatically raising fT>MIC for the same daily dose. That is exactly why prolonged beta-lactam infusions are used in severe infection.

Can I dose a real patient from this?

No. It is an educational estimator using population-independent assumptions for Vd and clearance. Real patients vary enormously, especially in critical illness and renal impairment. Use validated dosing software and therapeutic drug monitoring for clinical decisions.

Antibiotic PK/PD Target Attainment Calculator

Antibiotic dosing is only partly about the total amount of drug given. What actually predicts bacterial kill is the shape of the concentration-time curve relative to the organism’s minimum inhibitory concentration, the MIC. This calculator reproduces the three pharmacodynamic indices used in clinical pharmacology and checks whether a proposed regimen reaches its target.

How it works

A one-compartment intravenous model is built from the inputs. The elimination rate constant comes from the half-life, and clearance follows from the volume of distribution:

ke   = ln(2) / t_half
CL   = ke x Vd
Cmax = (Dose / Vd) x free_fraction

For time-dependent beta-lactams, the time the free drug spends above the MIC within one interval is found by solving the exponential decay curve:

t_above = ln(Cmax_free / MIC) / ke      (capped at the interval)
fT>MIC% = 100 x t_above / interval

For concentration-dependent aminoglycosides the index is simply Cmax / MIC. For AUC-dependent quinolones and vancomycin the 24-hour exposure is scaled from the per-interval area:

AUC24     = (Dose / CL) x (24 / interval)
AUC24/MIC = AUC24 / MIC

Example and notes

Take piperacillin 4 g IV every 8 hours against an organism with MIC 4 mg/L, half-life 1 hour, and Vd 18 L. The free peak is high and the drug stays above the MIC for a large share of the interval, comfortably clearing a 50 percent fT>MIC target. Halve the dose or double the MIC and the time above MIC falls quickly, which is why dose, interval, and infusion strategy are tuned together. This tool uses fixed, population-independent kinetics and an IV-bolus assumption; it is a teaching aid, not a substitute for therapeutic drug monitoring.