I completed a pharmacokinetic interpretation of a vancomycin trough level. The patient was a 28 year old male (62.3 kg) with refractory ascites and spontaneous bacterial peritonitis. The patient had an acute on chronic kidney injury with a fluctuating serum creatinine ranging from 120 to 150 mcrmol/L. Importantly for this patient, he was also cachectic and malnourished with very low muscle mass. In this patient population, it is important to remember than serum creatinine often provides a poor estimate (overestimate) of true renal function. As such, I elected to dose the patient at 750 mg IV q12h with a 1500mg loading dose. For a healthier 62kg patient with normal muscle mass and this degree of renal impairment I would have typically dosed at 1000mg IV q12h. The pre-4th dose trough level came back at 14 mg/L, close to our target of 15-20 mg/L. This level serves to illustrate that serum creatinine is a poor surrogate for renal function in cachectic patients as a healthy 28 year old 62kg patient with normal renal function would typically require 1000mg IV q8h dosing to achieve a trough level of around 14 mg/L, which is half the amount of vancomycin our patient was receiving. I assumed that the level was at steady state (this patient was unlikely to have a vancomycin elimination half-life >12h). Given the patient’s fluctuating renal function (was scheduled for a therapeutic paracentesis the following day which typically worsens his renal function for a few days due to large volume shifts) and the fact that he was afebrile and hemodynamically stable, I elected to finish a 5 day course of vancomycin 750mg IV q12h without further pharmacokinetic monitoring. I documented my assessment and recommendation in the form of a chart note.