Determinants of gentamicin concentrations in critically ill patients: a population pharmacokinetic analysis

- Gentamicin pharmacokinetics are highly variable in critically ill patients.
- Albumin and creatinine clearance are major covariates for gentamicin concentration.
- Higher starting gentamicin doses may be needed for patients with hypoalbuminaemia.
- Serum creatinine and the Cockcroft-Gault formula overestimate gentamicin clearance.
- Urinary creatinine clearance correlates best with gentamicin clearance.

When treating critically ill patients with gentamicin for severe infection, peak concentrations (Cmax) determine clinical efficacy and trough concentrations (Cmin) determine toxicity. Despite administration of body weight-standardised starting doses, a wide range of Cmax is generally observed. Furthermore, in therapeutic drug monitoring, several measures of renal function are used to predict appropriate Cmin and gentamicin dosing intervals, but the most accurate predictor is not known. This study aimed to quantify the impact of several patient parameters on gentamicin Cmax values and to determine which measure of renal function best predicts gentamicin clearance (CL). Clinical data and serum gentamicin levels were retrospectively collected from all critically ill patients treated with gentamicin at our intensive care unit between 1 January and 30 June 2011. Data were analysed using non-linear mixed-effects modelling (NONMEM v.7.1.2). A two-compartmental model was developed based on 303 gentamicin concentration-time data from 44 critically ill patients. Serum albumin levels explained 25% of interindividual variability in the volume of distribution (Vd). Creatinine clearance calculated from the creatinine concentration in a 6-h urine portion (CalcCLCr) resulted in acceptable estimation of gentamicin CL, whilst serum creatinine (SCr) and creatinine clearance estimated by the Cockcroft-Gault formula (CGCLCr) overestimated gentamicin CL and therefore underestimated Cmin. In conclusion, low albumin concentrations resulted in a larger Vd and lower Cmax of gentamicin. These results suggest that use of a higher gentamicin starting dose in critically ill patients with hypoalbuminaemia may prevent underdosing. Urinary CalcCLCr is a better predictor of Cmin than SCr or CGCLCr.