Hematocrit Effects On The Blood-To-Plasma Ratio

Why Hematocrit Changes the Blood-to-Plasma Ratio

Whole blood contains plasma and red cells, and a drug can have different concentrations in those two components. The whole-blood concentration is therefore a volume-weighted concentration: it depends both on how much blood volume is occupied by red cells and on how the drug partitions between red cells and plasma. This explains why the blood-to-plasma ratio can differ between samples with different hematocrit values, even when the underlying red-cell partitioning is unchanged.

The central calculation is a mass balance. It requires a clear definition of each concentration and an explicit assumption that the red-cell-to-plasma concentration ratio is applicable to the conditions being compared. The result is useful when parameterizing PBPK models or interpreting blood and plasma measurements, but it should not be treated as a universal correction independent of drug and sample conditions.

Definition: Let \(H\) be hematocrit, expressed as a fraction of whole-blood volume occupied by red cells. Let \(C_p\) be the drug concentration per unit plasma volume, \(C_{RBC}\) the concentration per unit red-cell volume, and \(C_b\) the concentration per unit whole-blood volume. Define the red-cell-to-plasma partition coefficient as \(K_{RBC/p}=C_{RBC}/C_p\), and the blood-to-plasma ratio as \(R_{b/p}=C_b/C_p\). The concentrations must use compatible units and refer to the same drug and sampling conditions.

Deriving the Hematocrit Relationship

Consider a blood sample with total volume \(V\). Under the two-component approximation, red cells occupy volume \(HV\), while plasma occupies volume \((1-H)V\). The drug amount in the sample is the amount in the red cells plus the amount in the plasma. Dividing that total amount by the whole-blood volume gives \(C_b\).

\begin{aligned} \text{Drug amount in sample} &= C_{RBC}(HV)+C_p\bigl((1-H)V\bigr),\\ C_b &= \frac{C_{RBC}HV+C_p(1-H)V}{V}\\ &= H C_{RBC}+(1-H)C_p. \end{aligned}

Dividing both sides by \(C_p\), and substituting \(K_{RBC/p}=C_{RBC}/C_p\), gives the desired relationship.

R_{b/p}=\frac{C_b}{C_p}=(1-H)+H K_{RBC/p} =1+H\bigl(K_{RBC/p}-1\bigr).

Key result: If \(K_{RBC/p}\) is fixed, \(R_{b/p}\) changes linearly with hematocrit. At \(H=0\), the blood-to-plasma ratio is 1 because the sample is all plasma in this idealized two-component model. At \(H=1\), it is \(K_{RBC/p}\), the limiting all-red-cell value. For intermediate hematocrit, the ratio is a volume-weighted average of 1 and \(K_{RBC/p}\).

The assumptions matter. This derivation treats blood as plasma plus red cells, so it neglects the small volume of other cellular components and any trapped plasma counted as packed cells. It assumes that \(C_p\) and \(C_{RBC}\) describe concentrations in their respective compartments, and that the sample is sufficiently mixed for the whole-blood concentration to be represented by the total amount divided by total volume. To predict a change in ratio using the same \(K_{RBC/p}\), one must additionally assume that the partition coefficient remains applicable across the hematocrit values being compared.

Worked Calculations

Worked Example: Calculating the Ratio From Red-Cell Partitioning

Suppose a hypothetical drug has \(K_{RBC/p}=2.40\) under the conditions of interest, and a blood sample has hematocrit \(H=0.45\). Calculate \(R_{b/p}\).

The red-cell concentration is 2.40 times the plasma concentration. Using the volume-weighted relationship,

R_{b/p}=(1-H)+H K_{RBC/p} =(1-0.45)+(0.45)(2.40) =0.55+1.08 =1.63.

The predicted whole-blood concentration is 1.63 times the plasma concentration. This value is greater than 1 because the drug concentration in red cells exceeds that in plasma, and red cells occupy 45% of the sample volume.

Worked Example: Inferring Partitioning and Predicting Another Hematocrit

In a hypothetical sample with \(H=0.40\), a drug has a measured \(R_{b/p}=1.30\). Assume the two-compartment relationship applies. First infer \(K_{RBC/p}\), then predict the ratio at \(H=0.25\) and \(H=0.50\), assuming the inferred partition coefficient remains constant.

Rearrange the relationship to isolate \(K_{RBC/p}\):

K_{RBC/p}=\frac{R_{b/p}-(1-H)}{H} =\frac{1.30-(1-0.40)}{0.40} =\frac{1.30-0.60}{0.40} =1.75.

At \(H=0.25\), the predicted ratio is

R_{b/p}=(1-0.25)+(0.25)(1.75) =0.75+0.4375 =1.1875\approx1.188.

At \(H=0.50\), it is

R_{b/p}=(1-0.50)+(0.50)(1.75) =0.50+0.875 =1.375.

The predicted ratio rises with hematocrit because \(K_{RBC/p}=1.75\) is greater than 1. The predictions are conditional on the partition coefficient remaining unchanged; they are not guaranteed merely because the hematocrit values are known.

Worked Example: Red-Cell Exclusion and a Falling Ratio

For a hypothetical drug, suppose \(K_{RBC/p}=0.60\). Calculate the ratio at hematocrits of 0.30 and 0.50, and explain the direction of change.

At \(H=0.30\),

R_{b/p}=(1-0.30)+(0.30)(0.60) =0.70+0.18 =0.88.

At \(H=0.50\),

R_{b/p}=(1-0.50)+(0.50)(0.60) =0.50+0.30 =0.80.

The ratio decreases by \(0.08\) as hematocrit increases by \(0.20\). Red cells have a lower drug concentration than plasma, so increasing the red-cell volume fraction pulls the whole-blood concentration relative to plasma downward. The result does not mean that the plasma concentration itself must decrease; it describes the ratio of whole-blood to plasma concentration under the stated assumptions.

Interpreting the Direction and Size of the Effect

The final form, \(R_{b/p}=1+H(K_{RBC/p}-1)\), makes the direction of the hematocrit effect explicit. For a fixed \(K_{RBC/p}\), the change in ratio between two hematocrit values is

R_{b/p}(H_2)-R_{b/p}(H_1) =(H_2-H_1)\bigl(K_{RBC/p}-1\bigr).

Thus, if \(K_{RBC/p}>1\), the ratio increases with hematocrit; if \(K_{RBC/p}<1\), it decreases; and if \(K_{RBC/p}=1\), it remains 1 at every hematocrit. The size of the change depends on both the hematocrit difference and how far the partition coefficient is from 1. This is a direct consequence of the mass-balance expression, not an additional empirical rule.

Hematocrit should be entered as a fraction, not as a percentage number. For example, 45% is \(H=0.45\). Entering 45 into the equation would not represent a volume fraction and would produce an impossible result. Also check that ratios are dimensionless and that \(C_b\), \(C_p\), and \(C_{RBC}\) use consistent concentration units.

When the Calculation May Not Transfer

The equation is a mixture relationship, but using it to transfer a measured ratio from one hematocrit to another relies on a biological assumption: \(K_{RBC/p}\) is the same in both conditions. That assumption may be poor when the drug’s red-cell partitioning depends on concentration, pH, temperature, binding-site occupancy, or other conditions that differ between samples. If the partition coefficient changes, the appropriate calculation at each condition is \(R_{b/p}=(1-H)+H K_{RBC/p}\) using the condition-specific value, rather than a single fixed value.

A useful diagnostic is to calculate \(K_{RBC/p}\) from each measured pair of hematocrit and blood-to-plasma ratio. If values inferred from comparable samples are substantially inconsistent, a fixed partition coefficient may not describe the data, or measurement and sample-handling differences may be important. The equation also assumes the hematocrit used corresponds to the sample in which the concentrations were measured. A ratio measured in one matrix or at one sampling condition should not be transferred automatically to a different one.

This calculation is different from directly measuring \(R_{b/p}\) in a whole-blood and plasma sample pair. Direct measurement can reflect sample-specific conditions but does not, by itself, explain how much of the observed ratio is attributable to hematocrit. Conversely, the mixture equation can describe the expected hematocrit effect when partitioning is known, but it cannot establish that partitioning is constant. Simply applying one measured blood-to-plasma ratio to every hematocrit is a simpler alternative, but it ignores the volume-mixing effect whenever \(K_{RBC/p}\ne1\).

Common Pitfalls

Using hematocrit as a whole percentage number. The incorrect substitution \(H=45\) for 45% treats a fraction as a number out of 100. Convert to \(H=0.45\) before calculating.
Using the red-cell partition coefficient as the blood-to-plasma ratio. \(K_{RBC/p}=C_{RBC}/C_p\) compares two compartments; \(R_{b/p}=C_b/C_p\) compares whole blood with plasma. They are equal only in the limiting case \(H=1\) in this model.
Assuming every ratio rises with hematocrit. That is true only when \(K_{RBC/p}>1\). When \(K_{RBC/p}<1\), the ratio falls as hematocrit rises.
Treating a predicted change as proof of a biological mechanism. The calculation shows what follows from volume mixing with fixed partitioning. It does not prove that the partition coefficient is unchanged across samples or that hematocrit caused every observed difference.

Key takeaway: The hematocrit-weighted relationship is \(R_{b/p}=(1-H)+H K_{RBC/p}\). Use it to calculate or interpret a ratio change only after checking that hematocrit is a fraction, concentrations are defined on the appropriate compartment volumes, and the red-cell partition coefficient is applicable to the conditions being compared.

Check Your Understanding

Try each calculation using hematocrit as a fraction. Complete solutions follow the questions.

A hypothetical drug has \(K_{RBC/p}=1.60\) at \(H=0.35\). Calculate \(R_{b/p}\).
A measured ratio is \(R_{b/p}=0.92\) at \(H=0.20\). Under the two-compartment relationship, what value of \(K_{RBC/p}\) is implied?
For \(K_{RBC/p}=2.20\), calculate the ratios at \(H=0.30\) and \(H=0.45\). What is the change?
What happens to \(R_{b/p}\) across hematocrit values when \(K_{RBC/p}=1\), and why?

Solutions. (1) \(R_{b/p}=(1-0.35)+(0.35)(1.60)=0.65+0.56=1.21\). (2) \(K_{RBC/p}=[0.92-(1-0.20)]/0.20=(0.92-0.80)/0.20=0.60\). (3) At \(H=0.30\), \(R_{b/p}=0.70+(0.30)(2.20)=1.36\). At \(H=0.45\), \(R_{b/p}=0.55+(0.45)(2.20)=1.54\). The change is \(1.54-1.36=0.18\). (4) The ratio is \(R_{b/p}=(1-H)+H(1)=1\) for every \(H\), because the drug concentration is equal in red cells and plasma, so changing their volume proportions does not change the whole-blood-to-plasma ratio.