Apparent oral clearance
CL/FThe ratio of dose to AUC after oral administration, combining hepatic/systemic clearance and oral bioavailability. It cannot separate true clearance from bioavailability without an intravenous reference.
Definition
After an oral dose, the area under the plasma concentration-time curve (AUC) reflects only the fraction of dose that reaches systemic circulation:
Where F is oral bioavailability (0–1). Because bioavailability cannot be determined from oral data alone, CL and F are mathematically inseparable — only their ratio CL/F is identifiable.
If an intravenous reference arm is available, true clearance is CL = DoseIV / AUCIV, and bioavailability is recovered as F = CL / (CL/F).
Clinical Implications
CL/F is widely reported in pharmacokinetic summaries and package inserts for orally administered drugs. Common applications and limitations:
- Population PK models — covariates such as weight, sex, renal function, and genetic polymorphisms are related to CL/F, not directly to CL.
- Drug-drug interactions — inhibitors of gut-wall CYP3A4 (e.g., grapefruit) reduce first-pass extraction and increase F, decreasing the observed CL/F even though hepatic CL is unchanged.
- Dose individualisation — Bayesian forecasting tools use CL/F from population models to predict individual AUC from oral dosing records, enabling dose adjustment for TDM.
- Food effects — high-fat meals that increase F (e.g., for poorly soluble BCS Class II drugs) lower CL/F, increasing exposure at the same dose.