Dosing in Obese

Introduction

I can still recall a memorable incident during my provisional registered pharmacist year when I received a phone call from a doctor during an on-call shift.

  • The question seemed simple enough: whether dosage adjustments were needed for metronidazole in obese patient.
  • However, even today, we are still in the stage of figuring best ways to dose obese patients.

We are well aware that significant variations in pharmacokinetic and pharmacodynamic responses can occur among patients due to factors such as weight, age, genetics, concurrent diseases and other factors.

  • However, the "one dose fits all" (all patients receive the same dose) remains a common form of drug dosing.



Understanding Obesity

Body size is typically defined using the body mass index (BMI) classification.

  • Normal weight 18.5-24.00 kg/m2
  • Overweight 25-29.99 kg/m2
  • Obese class I 30-34.99 kg/m2
  • Obese class II 35-39.99 kg/m2
  • Obese class III ≥40 kg/m2

NOTE: Caution must be exercised when applying BMI to individuals with greater muscle mass, women or the elderly.

The lipophilicity or the affinity a molecule has to adipose tissues will vary between different drugs.

  • Lean body weight (LBW) and ideal body weight (IBW) correlate well with hydrophilic drugs that have a small volume of distribution and limited fat distribution.
  • Total body weight (TBW) tends to correlate well with lipophilic drugs that have a large volume of distribution.

Additionally, creatinine clearance was found not to be linear with TBW, but it has enhanced linearity when correlated with LBW or IBW.



References

In addition to conducting literature searches, I have personally found Demystifying Drug Dosing in Obese Patients, 2016 to be an excellent pharmacy reference. For day-to-day practice, you can quickly consult resources such as

Furthermore, the table of dosing adjustment in obesity in Critical Care Pharmacy Handbook, 2020 can also be a useful resource.

Dosing Approaches of Selected Drugs in Obese Patient

NOTE: The recommendations for dosing in obesity may vary across different references.



Summary

Generally, licensed dosing recommendations are based on clinical trials that often under-represent or exclude individuals with obesity.

  • Hence, evidence-based dosing guidelines for this population are lacking.

Having knowledge of the most appropriate weight-metric for each medication is essential to optimize treatment outcomes for critically ill obese patients.

  • The use of an incorrect weight metric, for weight-based dosing, could lead to treatment failure or drug toxicity.

Consequences of Using an Incorrect Weight Metric When Dosing Obese Patients



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