Clinical strategies: Overweight

In this step, you will learn about the clinical strategy for using aminoglycosides in patients who are overweight.

As you learned in Week 2 and recapped in the previous step, when calculating CrCl using the Cockcroft-Gault equation in overweight patients, an adjusted body weight, known as the DDW, is used instead of their actual body weight (ABW), which informs aminoglycoside dosing.

Why does dosing need to change?

As body mass increases, the total amount of body water also increases. However, only a certain proportion of this is extracellular. Aminoglycosides do not distribute well into adipose tissue. Therefore, dosing strategies based on ABW will result in overdose in obese patients.

Overestimation of the patient’s glomerular filtration rate (GFR), and therefore aminoglycoside clearance, is also more likely in overweight and obese patients. Adipose tissue does not generate creatinine. SeCr levels are used to calculate CrCl, which acts as an estimation of GFR and renal function. Therefore, using the ABW can lead to vast overestimations of renal function. Despite this, the “correct” weight to use when calculating the appropriate dosage for aminoglycosides is still open for debate.

In general, most clinicians follow these principles:

• For people who are overweight, but not 20% greater than their IBW, the IBW should be used to calculate creatinine clearance.
• For people whose actual body weight is more than 20% greater than their ideal body weight, the DDW should be calculated and used to calculate creatinine clearance

Equation for calculating DDW in patients whose weight is >20% greater than than their IBW

As mentioned above, the DDW needs to be calculated to avoid overdosing.

Step 1 – calculate the IBW

Step 2 – calculate the DDW

This is done using the formula below, which takes 40% of the excess body mass and adds it to the IBW.

Remember, if an individual’s ABW is greater than their IBW but not by as much as 20%, then their ABW should be used to calculate aminoglycoside dosing.

However, it is important to look at the patient, not just the numbers, as those who weight train may well appear “overweight” on paper, but their body mass is made up of muscle rather than adipose tissue, and thus, they should be dosed as per their ABW.

In the next step, you will have the opportunity to apply what you have learnt in this section and the previous section, which examined the clinical strategy for extremes of age in our case study.