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.

What is the Antibiotic PK/PD Target Attainment Calculator?

Accepts dose, dosing interval, half-life, MIC, and volume of distribution to compute PK/PD indices: fT>MIC for beta-lactams, Cmax/MIC for aminoglycosides, and AUC24/MIC for quinolones and vancomycin, then checks target attainment. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Antibiotic PK/PD Target Attainment Calculator

Name: Antibiotic PK/PD Target Attainment Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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 × Vd
Cmax = (Dose / Vd) × 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 × 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) × (24 / interval)
AUC24/MIC = AUC24 / MIC

Which index applies to which drug class?

Drug class	PK/PD index	Typical target (illustrative)
Beta-lactams (penicillins, cephalosporins, carbapenems)	fT>MIC	~40–70% of interval
Aminoglycosides (gentamicin, amikacin)	Cmax/MIC	~8–10
Fluoroquinolones (ciprofloxacin, levofloxacin)	AUC24/MIC	~125 (Gram-negative)
Vancomycin	AUC24/MIC	~400–600

These are representative teaching targets. Clinical guidelines and the specific organism, infection site, and patient population all influence what is actually appropriate — always verify against current authoritative guidance.

Worked example (illustrative)

Consider an illustrative regimen: 4 g of a penicillin IV every 8 hours (interval = 8 h), with a half-life of 1 hour, Vd of 18 L, free fraction of 0.6, and MIC of 4 mg/L.

ke = ln(2) / 1 ≈ 0.693 h⁻¹
CL = 0.693 × 18 ≈ 12.5 L/h
Cmax = (4000 mg / 18 L) × 0.6 ≈ 133 mg/L (free peak)
t_above = ln(133 / 4) / 0.693 ≈ 5.1 h
fT>MIC = 5.1 / 8 = 64% — exceeds a 50% target

Doubling the MIC to 8 mg/L drops fT>MIC to around 50%, which explains why extended or continuous infusions are used for organisms with higher MICs: they redistribute the same total dose to maintain drug above the MIC for a greater fraction of the interval.

Limitations to keep in mind

This model assumes an IV bolus and population-average pharmacokinetics. Real patients — especially those in critical illness, with renal impairment, or with altered protein binding — differ substantially. Extended infusions, continuous infusions, and oral bioavailability are not modelled. Use this calculator for education and preliminary exploration, and rely on therapeutic drug monitoring and validated dosing software for clinical decisions.