Effect site versus plasma: where the bug actually lives
Most antibiotic PK models begin in plasma because plasma is what we can sample often. Many infections, though, are not happening in a blood vial. Pneumonia asks about lung concentrations. Meningitis asks about CSF. Abscesses and bone infections have their own stubborn geography.
This lesson is about the gap between what we measure and where the bacteria sit, and how to keep that gap from becoming an invisible leap of faith.
Modeling intuition about site of infection. Not a catalog of tissue penetration coefficients for every drug.
Plasma is a proxy, sometimes a good one
If free plasma exposure tracks free exposure at the effect site closely enough for your question, plasma-based PTA can be a fair first tool. If the site is protected, poorly perfused, or governed by transporters, plasma can flatter the regimen.
A simple mental split helps:
- Plasma-centric question: given systemic PK, how often do we clear a plasma free-drug target?
- Site-aware question: given systemic PK plus a site model or site observations, how often does the effect-site exposure clear the relevant target?
Those are different claims. Both can be useful. They should not share one sentence in the abstract as if they were identical.
A sentence worth refusing
“Plasma fT>MIC was high, therefore lung exposure was adequate.” That “therefore” needs evidence about penetration, timing, and free fraction at the site.A site curve can be lower, delayed, or both. Plasma success does not automatically copy over.
Common site stories modelers meet
Epithelial lining fluid (ELF) for pneumonia work: ratios to plasma vary by drug and sampling time.
CSF for meningitis: barrier and inflammation change penetration.
Tissue or abscess: averages may hide necrotic cores where drug arrives late.
In each case you need either direct site observations, a justified transfer model, or an honest plasma-only limitation statement.
Defensible when you clearly claim a plasma free-drug endpoint. Weak when the abstract quietly upgrades that claim into site adequacy.
Apply a published penetration ratio or time-varying factor, cite it, and sensitivity-test lower ratios. Keep free/total bookkeeping intact at the site if data allow.
Best case: measured site concentrations with timing. Still watch assay method, inflammation state, and whether samples represent the infected subcompartment.
How to write this without hand-waving
In methods, name the compartment your target refers to. In limitations, say what you did not model. If donor ELF or CSF data come from a different population than your primary PK, treat them with the same provenance care you would use for a borrowed clearance equation.
Two abstracts, only one is honest
Honest: “We evaluated free plasma target attainment for regimen X in a virtual ICU population. Site-of-infection concentrations were not modeled.”
Overstated: “We showed regimen X provides adequate lung exposure,” after only plasma PTA.
The first abstract can still be scientifically valuable. The second asks readers to invent a penetration model you never wrote down. If you do have ELF or CSF data, bring them in with dates, methods, and sensitivity to lower penetration. If you do not, keep the claim inside plasma and sleep better.
Timing matters too. A site-to-plasma ratio from a late sampling window may not describe early therapy. If your PTA question includes the first hours, a single steady ratio can be the wrong proxy.
Takeaway
Bacteria live at an effect site. Plasma is often the ladder we climb to estimate that site, not the destination itself. Keep the proxy relationship visible, or your PTA will answer a cleaner question than the infection deserves.
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