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The blood-to-plasma concentration ratio, almost always written Rb, is the ratio of a drug’s total concentration in whole blood to its total concentration in plasma at the same point in time: Rb = Cblood / Cplasma. It sounds like a bookkeeping detail — blood and plasma are just two ways of sampling the same circulating drug — but it is not interchangeable with 1, and treating it as though it were is one of the more consequential silent errors in pharmacokinetic analysis. Rb describes how a drug actually partitions between the cellular and non-cellular fractions of blood, and that partitioning determines whether a plasma-based clearance value can be used directly in a calculation that is physiologically defined in terms of blood, most importantly the hepatic extraction ratio.
What Rb Actually Measures
Whole blood is not a single homogeneous fluid. By volume it is roughly 45% cellular material — overwhelmingly red blood cells, with the hematocrit (Hct) describing exactly that fraction — and the remainder plasma. A drug molecule circulating in blood distributes between these two compartments according to its own physicochemical behavior: how well it crosses the erythrocyte membrane, whether it binds hemoglobin or carbonic anhydrase or other intracellular proteins once inside, and how strongly it binds plasma proteins outside the cell. Rb is the net, measurable result of that partitioning, and it can be decomposed as:
Cblood = Cplasma × (1 − Hct) + CRBC × Hct
Dividing through by Cplasma gives Rb = (1 − Hct) + Hct × (CRBC/Cplasma). This is why Rb is not a fixed physical constant of a molecule in the way molecular weight is: it is a function of hematocrit and of the drug’s own red-cell affinity, and it is normally reported at a standard, physiological hematocrit (about 0.45) precisely so values are comparable across studies. A drug that is completely excluded from red blood cells (CRBC = 0) has Rb = 1 − Hct, well below 1. A drug that distributes into red cells at exactly the same concentration as plasma has Rb = 1. A drug that is actively taken up or bound inside red cells — some calcineurin inhibitors and carbonic-anhydrase-binding compounds are the textbook examples — can have an Rb well above 1, sometimes far above it, which is precisely why clinical monitoring for those drug classes is done on whole blood rather than plasma.
Converting Plasma Clearance to Blood Clearance
Almost every clinical PK dataset is built from plasma concentrations, because plasma is what’s routinely sampled and assayed. Clearance calculated from that dataset, CLplasma, is a real and useful number, but it is defined relative to the plasma concentration, not the blood concentration. The two are related through a simple mass-balance identity: the rate at which drug leaves the body doesn’t depend on which fluid you used to measure concentration, so CLplasma × Cplasma must equal CLblood × Cblood. Substituting Rb = Cblood/Cplasma gives the standard conversion:
CLblood = CLplasma / Rb
When Rb is close to 1, the two clearance values are nearly identical and the distinction barely matters in practice. When Rb departs materially from 1 in either direction, the two values diverge, and which one is correct to use depends entirely on which physiological quantity the calculation is actually defined against. This conversion is the same one that has to be applied when reconciling in vitro intrinsic clearance generated from human hepatocytes or liver microsomes — which is inherently a blood-perfusion-referenced parameter once it’s scaled to whole-organ predictions — with an in vivo plasma clearance measured from a clinical study, a step that shows up directly in PBPK model parameterization and in bridging nonclinical to clinical predictions (see toxicokinetic exposure comparisons across species).
Why This Matters for the Hepatic Extraction Ratio
The hepatic extraction ratio, EH, is the fraction of drug removed from the blood in a single pass through the liver: EH = CLH / QH, where QH is hepatic blood flow. That denominator is the mechanistic reason Rb cannot be ignored: hepatic blood flow is exactly what its name says — a flow of whole blood, not plasma — because the liver is perfused by the portal vein and hepatic artery carrying whole blood, and every drug molecule the liver can possibly extract has to arrive there via that blood flow, including whatever fraction happens to be inside red cells rather than in the plasma at that moment. A hepatic extraction ratio is therefore only physiologically meaningful when the clearance term in the numerator, CLH, is expressed on a blood basis: CLH,blood, not CLH,plasma. The well-stirred model of hepatic clearance, the standard framework used to relate intrinsic metabolic clearance to organ-level clearance, is written explicitly in blood terms for this reason:
CLH = QH × fu,b × CLint / (QH + fu,b × CLint)
where fu,b is the unbound fraction of drug in blood. Because unbound fraction is usually measured in plasma (fu,p) via equilibrium dialysis or ultrafiltration, it also has to be converted, approximately, using fu,b ≈ fu,p / Rb. Skip either conversion and EH is calculated against the wrong flow-relative reference, which distorts a downstream classification that clinical pharmacologists rely on directly: whether a drug is high-, intermediate-, or low-extraction predicts whether its oral bioavailability is sensitive to hepatic blood flow (high-extraction drugs) or to enzyme activity and protein binding (low-extraction drugs), and that classification feeds first-pass metabolism estimates, IV-to-oral dose extrapolation, and predicted magnitude of metabolic drug-drug interactions — exactly the kind of parameter a population PK or physiologically based model has to get right before it can be trusted for a dosing decision.
How Rb Is Actually Measured
Rb is normally determined by an in vitro red-blood-cell partitioning assay: drug is spiked into fresh whole blood at a defined hematocrit (typically adjusted to a physiological value, since donor hematocrit varies), incubated at 37°C for long enough to reach distributional equilibrium, then centrifuged to separate plasma from the cellular fraction. Drug concentration is measured in the resulting plasma and compared with a matched whole-blood aliquot measured before centrifugation, usually by LC-MS/MS. Two methodological details matter for interpreting a reported value correctly. First, equilibration time is not a formality: some compounds cross the red-cell membrane slowly, and a partitioning assay read too early will underestimate Rb for those compounds. Second, Rb can be concentration-dependent if red-cell uptake is a saturable, capacity-limited process (active transport or binding to a finite number of intracellular binding sites) rather than simple passive partitioning — a single Rb value determined at one concentration does not automatically hold across the full therapeutic concentration range, and that nonlinearity is itself worth checking before it’s baked into a fixed conversion factor used throughout a model.
The Common Pitfall: Assuming Rb = 1
Because so many small, lipophilic, plasma-protein-bound drugs happen to have an Rb reasonably close to 1, it’s a common simplifying default in early PK modeling to assume it outright rather than measure it — and for a genuinely Rb-near-1 compound that shortcut costs little. The failure mode is applying the same default to a compound that actually partitions substantially into red cells without checking. The practical damage runs in a specific, traceable direction: if the true Rb is meaningfully above 1 and the analysis nonetheless uses CLplasma in place of CLblood in an extraction-ratio or blood-flow-limited calculation, the resulting EH is overstated relative to the true blood-referenced value, because CLplasma overstates CLblood whenever Rb > 1 (recall CLblood = CLplasma/Rb). A drug that is genuinely intermediate-extraction can get misclassified as high-extraction, which in turn skews predictions about how sensitive its oral exposure will be to liver blood flow changes (heart failure, cirrhosis, concomitant vasoactive drugs) versus enzyme-mediated drug interactions — two clinically distinct risk profiles with different monitoring implications. The reverse error, understating extraction for an Rb-below-1 compound, is just as real and just as easy to miss when Rb is assumed rather than measured. Either direction is silent: nothing in a standard PK software output flags that a hard-coded Rb of 1 was wrong, because the arithmetic runs cleanly regardless of whether the input was correct.
Where Rb Shows Up in Practice
Beyond the hepatic extraction ratio calculation itself, Rb is a required input parameter in physiologically based pharmacokinetic models submitted to regulators, because those models explicitly simulate blood flow through individual organs and therefore need a blood-referenced, not plasma-referenced, concentration at every compartment boundary. It is also a standard input to in vitro-derived drug-drug interaction risk assessments, where an unbound blood concentration (via fu,b) feeds directly into predicted inhibitor or inducer exposure at the enzyme site. And it is the underlying reason certain drug classes — calcineurin inhibitors used in transplant medicine being the most widely taught example — are monitored therapeutically using whole-blood trough concentrations rather than plasma: their Rb is high enough, and clinically consequential enough, that plasma sampling alone would misrepresent total systemic exposure. Anyone building or reviewing a pharmacometric analysis, a bioequivalence argument, or a first-in-human dose-escalation model should confirm explicitly which basis — blood or plasma — each clearance, extraction ratio, and unbound-fraction term in the analysis is defined on, rather than assuming they’re interchangeable.
Frequently Asked Questions
What does an Rb of exactly 1 mean?
It means the drug’s concentration in whole blood equals its concentration in plasma, which specifically requires that its concentration inside red blood cells equals its plasma concentration too (not that the drug is absent from red cells). At that point CLblood and CLplasma are numerically identical, and the distinction between the two bases stops mattering arithmetically, even though it still exists conceptually.
Can Rb be greater than 1?
Yes. Any compound that is actively taken up into red cells, binds an intracellular protein such as carbonic anhydrase, or is otherwise concentrated inside the erythrocyte relative to plasma will show Rb > 1, in some cases substantially so.
Can Rb be less than 1?
Yes, and this is common for drugs that are extensively bound to plasma proteins with little affinity for the red-cell interior, or that simply cross the erythrocyte membrane poorly. In the limiting case of complete exclusion from red cells, Rb approaches 1 minus hematocrit, meaningfully below 1.
Does hematocrit affect Rb?
Directly, since hematocrit is one of the two terms in Rb‘s defining equation. This is why Rb is reported at a standard, physiological hematocrit, and why it can shift in populations with materially altered hematocrit, such as anemia or polycythemia, even for the same drug.
Is Rb the same thing as plasma protein binding?
No, though the two are related. Plasma protein binding describes partitioning between free drug and protein-bound drug within the plasma compartment alone. Rb describes partitioning between the whole-plasma compartment and the red-cell compartment. A highly protein-bound drug can still have any Rb value depending on its independent red-cell affinity.
Working from the wrong basis rarely produces an implausible number — it produces a plausible-looking, quietly wrong one. Confirming which basis a clearance or extraction-ratio value is defined on, and applying Rb explicitly rather than assuming it away, is a small check with an outsized effect on downstream hepatic extraction, drug-drug interaction, and dose-scaling conclusions. For the broader modeling context this fits into, see CASRAI’s clinical research hub, and for the biopharmaceutics classification underlying a drug’s absorption and permeability behavior, see the Biopharmaceutics Classification System guide.
