The distribution clearance parameter (Q) in two-compartment pharmacokinetic models — describing how rapidly a drug moves between the central compartment (blood, highly perfused organs) and the peripheral compartment (muscle, fat, slowly perfused tissues). Determines the duration and steepness of the alpha distribution phase.
Definition
In a two-compartment model, the rate of
drug transfer between the central (V1) and peripheral (V2)
compartments is described by the intercompartmental clearance Q
(also called distribution clearance or Qd):
Transfer rate (1→2) = Q × C1
Transfer rate (2→1) = Q × C2
Q = k12 × V1 = k21 × V2
Where k12 is the first-order transfer rate constant from central to
peripheral compartment, and k21 is the reverse. At distribution
equilibrium: C1/C2 = V2/V1.
The total volume at steady state is Vss = V1 + V2 —
see our volume of distribution glossary.
Clinical Significance
Intercompartmental clearance determines how quickly a drug equilibrates between blood
and tissues — with direct consequences for sampling timing, toxicity patterns, and
dosing strategy:
Vancomycin (Q ≈ 0.5–1 L/h): The alpha distribution phase lasts
~1–2 hours post-infusion. Post-distribution samples (≥2h post-infusion or trough)
are required for meaningful TDM.
AUC-guided vancomycin monitoring uses population PK models that explicitly
parameterise Q.
Aminoglycosides (low Q to renal cortex): Gentamicin and tobramycin
distribute slowly into renal proximal tubular cells. This accumulation drives
nephrotoxicity — extended-interval dosing (24h) exploits the slow Q to allow trough
levels to near zero before the next dose, reducing tubular accumulation.
Digoxin (very slow Q, ~6–8h): Distributes extensively to myocardium
(peripheral compartment). Serum levels during the alpha phase are not representative
of cardiac tissue concentrations — TDM samples must be collected ≥6h post-dose.
Propofol (very high Q): Near-instantaneous CNS equilibration explains
rapid induction of anaesthesia within one arm-brain circulation time.
FAQ
What is intercompartmental clearance (Q)?
Intercompartmental clearance (Q, also written Q10 or Qd) is the parameter in a two-compartment pharmacokinetic model that describes the rate of drug transfer between the central and peripheral compartments. It has units of volume per time (e.g., L/h), analogous to elimination clearance (CL). A high Q means drug moves rapidly between compartments, reaching equilibrium quickly and producing a short, steep alpha phase. A low Q means distribution is slow, producing a prolonged alpha phase and delayed equilibration. Q is related to the micro rate constants: Q = k12 × V1 = k21 × V2, where k12 is the rate constant for transfer from central to peripheral and k21 is the reverse.
How does intercompartmental clearance relate to the alpha and beta phases?
The hybrid rate constants alpha (α) and beta (β) that describe the biexponential plasma concentration curve are functions of both intercompartmental clearance (Q) and elimination clearance (CL). Alpha represents the faster, distribution-dominated phase — its rate is determined primarily by Q relative to V1. Beta represents the slower, elimination-dominated terminal phase. When Q is large relative to CL, distribution equilibrates quickly and the alpha phase is brief; when Q is small, the alpha phase is prolonged. Understanding this helps interpret TDM samples: samples taken during the alpha phase reflect the distribution process, not the elimination kinetics, and can give misleading concentration readings.
Which drugs have clinically important intercompartmental clearance?
Drugs with clinically significant distribution phases include: vancomycin (Q ~0.5–1 L/h — alpha phase ~1–2h; TDM samples drawn before this window are spuriously high); aminoglycosides (gentamicin, tobramycin — slow Q drives renal cortex accumulation and nephrotoxicity); digoxin (Q very slow — distribution takes 6–8h; early samples falsely elevated); propofol (very high Q — rapid CNS distribution explains fast onset); lidocaine (rapid initial CNS distribution explains toxicity with fast IV bolus); and many oncology drugs including cisplatin and anthracyclines where tissue binding contributes to the peripheral compartment.
How is intercompartmental clearance estimated in practice?
Intercompartmental clearance is estimated by fitting a two-compartment model to serial plasma concentration-time data collected after IV administration. Non-compartmental analysis cannot directly estimate Q — compartmental curve fitting (using software such as NONMEM, Phoenix WinNonlin, or Monolix) is required. In clinical practice, population PK models for specific drugs (e.g., vancomycin, tacrolimus) encode population estimates of CL, V1, Q, and V2 alongside covariates (weight, creatinine, albumin). Bayesian software then uses these priors and observed drug levels to estimate individual PK parameters for dose optimisation.