Clinical strategies: Underweight

Patients who are underweight or overweight are at risk of receiving incorrect doses of aminoglycosides. The reasons for this are different, depending on if the patient is underweight or overweight. The clinical strategy for underweight patients will be examined in this step and for overweight patients in the next step.

Let’s return to the Cockcroft-Gault equation, which calculates creatinine clearance rates (CrCl).

If you require a screen reader-compatible version this is available as a PDF.

In week 2, you learnt how this equation estimates renal function, and the results inform aminoglycoside dosing. You also learnt that for non-obese patients, their actual body weight (ABW) or ideal body weight (IBW) is used, whereas for obese patients, an adjusted body weight, known as the dose-determining weight (DDW), is used instead.

Patients who are underweight are at risk of being given doses that are too large and dosed too frequently. For those who are underweight, their ABW should be used to calculate their renal function and aminoglycoside dose. Using the IBW in these circumstances will result in an overdose due to the overestimation of CrCl.

Calculating CrCl and determining the dose and interval can also be challenging in those who are malnourished, older, frail, or underweight. These individuals often have reduced muscle mass, resulting in low serum creatinine (SeCr) concentrations and leading to an overestimation of renal function. Look back at the Cockcroft-Gault equation discussed earlier and see how an abnormally low SeCr can lead to a mathematical overestimation of renal function.

Enhanced monitoring is always warranted in underweight patients who have been administered aminoglycosides, even if care was taken to ensure the right weight and renal function were calculated. Changes in water distribution due to low body mass result in a reduced volume of distribution (Vd), compared to people of healthy weight and those who are overweight. This can lead to higher than expected aminoglycoside blood concentrations, which as discussed last week can lead to kidney damage and audio-vestibular disturbances.

In the next step, you will learn more about the clinical strategy for overweight patients.