PK/PD targets and the MIC: what the indices actually mean
Bacteria do not read the prescription label. They experience a rising and falling concentration of drug around them. Pharmacokinetic/pharmacodynamic (PK/PD) modeling is mostly the craft of putting that time course next to a susceptibility number, usually the MIC, and asking whether the exposure looks adequate.
This lesson stays practical. You will leave with a clear picture of what MIC can and cannot carry, what the classic indices mean, and how to choose one without turning the choice into folklore.
Teaching intuition for modeling. Not a dosing protocol and not a substitute for clinical guidelines.
Start with the awkward object: the MIC
The minimum inhibitory concentration is the lowest concentration that stops visible growth in a defined lab assay. That sentence already hides three compromises.
First, “visible growth” is an assay endpoint, not a full story of killing. Second, the assay has a floor and a set of conditions. Change those conditions and the number can move. Third, the pathogen in a patient is not living in Mueller-Hinton broth with perfect mixing and a fixed inoculum.
Still, MIC is useful because it compresses susceptibility into something a simulator can compare with an exposure curve. The healthy habit is to treat MIC as a scenario input with uncertainty, not as a moral fact about the organism.
Pause: what question is MIC answering?
MIC answers roughly: under this assay, what concentration prevents visible growth? It does not by itself answer: will every cell die, how fast will they die, or will a small surviving fraction rebound later?Three index families, said simply
Most antibacterial PK/PD targets fall into one of three families. The leading f means free (unbound) drug.
| Index | Plain meaning | Often used for |
|---|---|---|
| fT>MIC | How much of the dosing interval free concentration stays above MIC | Many beta-lactams |
| fAUC/MIC | Total free exposure over time, scaled by MIC | Fluoroquinolones; vancomycin discussions often use AUC logic |
| fCmax/MIC | Peak free level relative to MIC | Some concentration-dependent patterns |
If your model outputs total plasma concentration but the target is written for free drug, the units are already arguing with each other. Fix that before you admire the PTA curve.
fT>MIC is literally the share of time the free curve sits above the MIC line.
Choosing an index without mystique
Ask three questions before you code a target.
- What does the drug class literature actually use for this organism and indication style: time above threshold, integrated exposure, or peak?
- Are you modeling free or total concentrations, and does the cited target match that choice?
- Is success one condition or two? Combinations may need joint rules.
A published target such as “100% fT>MIC” or “fAUC/MIC at least 125” is a decision rule for simulation. It is not a promise that every patient above the line is cured and every patient below it fails.
Time-driven thinking cares how long the free level stays above a threshold. Flattening troughs (for example with longer infusions) can matter more than chasing a higher peak.
Exposure-driven thinking cares about the area under the free curve relative to MIC. Missing a little time above MIC can be less important than overall integrated exposure, depending on the drug.
Peak-driven thinking cares whether the highest free concentration clears a multiple of MIC. The shape after the peak still matters clinically, but the index itself is anchored on Cmax.
A working habit for modelers
Write the index, the free/total state, and the MIC source in one line whenever you start a run. If you cannot fill that line, the simulation is not ready for interpretation yet.
Tolerance and persistence can still spoil a “good” MIC story. We will separate those ideas in a later lesson. For now, keep the simpler rule: pick the index that matches the pharmacology, then keep the units honest.