Why the Weight Descriptor Matters
A dose calculated “per kilogram” is incomplete unless the relevant kilogram is defined. In obesity, total body weight may differ substantially from the body-size measure that best predicts a drug’s volume of distribution or clearance. Choosing a descriptor therefore changes the dose implied by a model; it does not, by itself, establish which dose is pharmacologically appropriate.
The earlier material on obesity-related changes in clearance and distribution concerns how obesity can affect pharmacokinetics. Here the focus is narrower: defining common weight descriptors and using them consistently in a stated dose calculation. The appropriate descriptor can differ between drugs and between pharmacokinetic parameters for the same drug. It must be supported by drug-specific evidence, a validated model, or an applicable dosing recommendation.
Calculating the Descriptors
Several equations for IBW and LBW are in use. The following calculations use the Devine convention for IBW and the Janmahasatian equation for LBW. These are explicit operational definitions for the examples, not claims that the equations are interchangeable or optimal for every population.
In the IBW equations, the height is expressed in inches, with the baseline set at 60 inches; the constants and sex-specific forms are part of the convention. In the LBW equations, TBW is in kilograms and BMI is in kilograms per square metre. These equations use male and female categories as specified by their derivation. Applying them to an individual whose characteristics or clinical context are not represented by that convention requires clinical judgment; a calculated value should not be treated as a measured body composition.
Worked Example: Calculating Four Descriptors
Consider a hypothetical woman who is 170.18 cm tall (67 inches) and weighs 110.0 kg. Calculate IBW, LBW, and AdjBW using \(c=0.4\), alongside TBW.
TBW. The measured current weight is 110.0 kg.
IBW. Using the stated Devine convention:
BMI and LBW. First, \(170.18\ \mathrm{cm}=1.7018\ \mathrm{m}\), so:
Substituting this value in the stated female LBW equation gives:
AdjBW. Using \(c=0.4\):
The four results are TBW 110.0 kg, IBW 61.6 kg, LBW about 56.50 kg, and AdjBW 80.96 kg. Their differences are consequential in a dose equation, but the arithmetic alone does not decide which one is appropriate.
Connect the Descriptor to the Pharmacokinetic Parameter
A loading dose is primarily related to the volume of distribution \(V\), the target concentration, and bioavailability \(F\). For a one-compartment, linear model with an immediate input and a target concentration at the end of distribution, the dose is:
For an intravenous dose, \(F=1\). For an extravascular dose, the assumed bioavailability must be appropriate to the drug and formulation. The equation assumes that the target concentration is defined for the model and that the administered dose is not altered by nonlinear pharmacokinetics or a relevant absorption limitation.
A maintenance dose over interval \(\tau\), under linear pharmacokinetics at steady state, is related to clearance \(CL\) and target average steady-state concentration \(C_{\mathrm{ss,avg}}\):
Thus, a descriptor used to predict \(V\) informs a loading-dose calculation, while one used to predict \(CL\) informs a maintenance-dose calculation. They need not be the same. The equations do not imply that either parameter is proportional to any particular weight measure. That relationship must be specified and supported separately.
Worked Dose Calculations
Worked Example: Loading Dose Under a Specified Volume Model
For the hypothetical patient above, suppose a drug has a validated model for this dosing context in which \(V=0.65\ \mathrm{L/kg}\) multiplied by LBW. The intended target concentration is \(15.0\ \mathrm{mg/L}\), and the assumed oral bioavailability is \(F=0.80\). Calculate the loading dose, then compare it with the result of using TBW instead. Assume the linear one-compartment loading-dose equation applies.
Using LBW \(=56.50\ \mathrm{kg}\), the modeled volume is \(0.65\ \mathrm{L/kg}\times56.50\ \mathrm{kg}=36.725\ \mathrm{L}\). Therefore:
If TBW were used in the same volume-per-kilogram model, the calculated volume would be \(0.65\times110.0=71.50\ \mathrm{L}\), and the dose would be:
This large difference follows from the stated model and descriptor choice. It does not show that LBW is generally preferable: the conclusion depends on the hypothetical model being valid for this drug, population, route, and target.
Worked Example: Maintenance Dose With an Allometric Clearance Model
Suppose, for a different hypothetical drug, the established model predicts clearance from LBW as \(CL=4.00(\mathrm{LBW}/70)^{0.75}\ \mathrm{L/h}\), where the reference clearance is 4.00 L/h at 70 kg. For the same 56.50 kg LBW patient, find the dose every 12 hours for a target average steady-state concentration of \(10.0\ \mathrm{mg/L}\), assuming \(F=0.80\). Assume linear, time-invariant clearance and a dosing interval for which the average-concentration equation is appropriate.
First calculate clearance:
Then use the maintenance-dose equation with \(\tau=12\ \mathrm{h}\):
The units reduce to milligrams because \((\mathrm{mg/L})(\mathrm{L/h})(\mathrm{h})=\mathrm{mg}\). A practical regimen would also need consideration of dosage forms, safety, variability, and the drug’s applicable dosing guidance. The calculation is not evidence that LBW should predict clearance for other drugs.
Worked Example: Using Different Descriptors for Loading and Maintenance
Consider a hypothetical man who is 175 cm tall and weighs 125 kg. Suppose drug-specific evidence supports a volume model \(V=0.55\ \mathrm{L/kg}\) times LBW, but a clearance model \(CL=3.20(\mathrm{AdjBW}/70)^{0.75}\ \mathrm{L/h}\). For illustration, use the Devine and Janmahasatian equations above, \(c=0.4\), a loading target of \(12.0\ \mathrm{mg/L}\), and a maintenance target average concentration of \(8.0\ \mathrm{mg/L}\) every 24 hours. Assume \(F=1\) for loading and \(F=0.90\) for maintenance, with linear pharmacokinetics.
The height is \(175/2.54=68.8976\) inches. Thus:
BMI is \(125/(1.75^2)=40.8163\ \mathrm{kg/m^2}\). Using the male LBW equation:
Adjusted body weight is \(70.4645+0.4(125-70.4645)=92.2787\ \mathrm{kg}\). The loading dose uses LBW because that is the descriptor in the stated volume model:
The maintenance dose instead uses AdjBW in the stated clearance model:
The example deliberately assigns different descriptors to \(V\) and \(CL\), as drug-specific evidence may support. If LBW were incorrectly substituted into this example’s clearance model, the predicted clearance would be about \(3.362\ \mathrm{L/h}\), giving \(D_M\approx717.3\ \mathrm{mg}\) every 24 hours. That is about 14.6% lower than the model-specified result. Consistent arithmetic cannot compensate for an unsupported model choice.
How to Choose and Check a Descriptor
A defensible choice starts with the intended use: a loading dose, a maintenance dose, or another target. Identify the parameter that drives that calculation, then use the drug-specific relationship between that parameter and body size. A descriptor used in a published or institutional model should be interpreted with its exact definition, including the equation, units, population, route, and any limits on body size.
Useful diagnostics include checking whether the calculated descriptor is plausible, confirming that height and weight units were entered correctly, and comparing the patient’s size with the model’s development and validation range. If a weight-based prediction produces an implausible dose, investigate the model and its application rather than automatically capping or switching descriptors. Where concentration measurements or clinical response are available, they can help assess whether the regimen performs as intended, while accounting for timing, adherence, and other relevant sources of variation.
Common warning signs include extrapolating a linear mg/kg regimen far beyond the population in which it was evaluated, using an AdjBW correction factor without a drug-specific rationale, and assuming that a descriptor appropriate for volume is also appropriate for clearance. A descriptor calculation can also be wrong because of a unit error, an incorrect height convention, or an equation applied to a population for which it is unsuitable.
Alternatives, Limitations, and a Common Mistake
| Approach | What it does | Main caution |
|---|---|---|
| TBW-based dosing | Uses measured current weight directly. | Assumes the parameter or dose scales with TBW; this may overpredict in some contexts. |
| IBW-based dosing | Uses a height-based convention. | IBW is equation-defined, not a measured ideal or a universal predictor of drug disposition. |
| LBW-based dosing | Uses a calculated lean-mass descriptor. | Results depend on the LBW equation and its population; it is not direct measurement of lean mass. |
| AdjBW-based dosing | Uses IBW plus a fraction of excess TBW. | The correction coefficient is conventional and may not transfer across drugs or settings. |
| Drug-specific PK model | Predicts \(V\), \(CL\), or exposure from covariates supported by evidence. | Requires correct model implementation and caution outside its validated range. |
A common mistake is: “The patient has obesity, so use AdjBW with a correction factor of 0.4 for every dose.” The correction is then treated as though it were a universal pharmacological law. The correction is not universal: AdjBW is a constructed descriptor, and \(c=0.4\) does not establish how a particular drug’s \(V\) or \(CL\) relates to body size. The correction is to identify the dose target and use a descriptor-model relationship justified for that drug and parameter.
All weight descriptors have limitations. Equations compress complex body composition into a single number and may not transport well across populations, body shapes, age groups, or clinical states. The equations here do not replace drug-specific guidance, evaluation of the model’s applicability, or clinical monitoring. In particular, the examples assume linear pharmacokinetics and explicitly specified parameter models; real drugs may violate those assumptions.
Check Your Understanding
Answer each question, then compare with the brief solutions.
- A model predicts a drug’s clearance from LBW, while its volume model uses TBW. Which descriptor should enter the maintenance-dose calculation, and which should enter the loading-dose calculation under those models?
- For a patient with TBW \(=100\ \mathrm{kg}\), IBW \(=60\ \mathrm{kg}\), and \(c=0.4\), calculate AdjBW.
- Why does an AdjBW calculation with \(c=0.4\) not establish that the resulting weight is correct for a particular drug?
- A maintenance equation uses \(D_M=C_{\mathrm{ss,avg}}CL\tau/F\). If \(CL\) is in L/h, concentration is in mg/L, and \(\tau\) is in hours, what are the units of dose?
Solutions. 1. Use LBW for maintenance because the stated maintenance model predicts \(CL\) from LBW; use TBW for loading because the stated volume model uses TBW. 2. \(\mathrm{AdjBW}=60+0.4(100-60)=76\ \mathrm{kg}\). 3. The factor is a convention, not universal evidence about a drug’s volume or clearance; the descriptor-model relationship must be supported for that drug and use. 4. Milligrams, since \((\mathrm{mg/L})(\mathrm{L/h})(\mathrm{h})=\mathrm{mg}\), and \(F\) is dimensionless.