We present a case of renal impairment in an emaciated HIV-infected male that initially went unrecognized because of reliance on serum creatinine and estimated glomerular filtration rate (eGFR). Inaccurate vancomycin dosing led to toxic drug levels (66 mg/l), associated with acute and severe worsening of kidney function. This occurred in the context of escalating doses of vancomycin given in the presence of changing kidney function, albeit kidney function that always remained well within the normal range (serum creatinine 29 - 42 mumol/l). In the absence of other plausible explanations, a presumptive diagnosis of vancomycin nephrotoxicity was made. Given the rarity of this diagnosis in the current era, we discuss the pathophysiology of vancomycin nephrotoxicity. We also explore the potential reasons for inaccuracy of GFR prediction equations in the HIV population, and discuss the potential pitfalls associated with application of eGFR or even serum creatinine without appropriate understanding of their limitations. We believe our case highlights a number of important teaching points: Vancomycin nephrotoxitiy is rare but can occur in the setting of kidney dysfunction. Current assessment of kidney function using creatinine and eGFR requires awareness of the clinical caveats in which these measures may be misleading. Acute changes in kidney function, irrespective of the test used, should be contextualized to the individual situation. Persons with HIV and low muscle mass constitute a specific subgroup in whom assessment of kidney function may be problematic using creatinine. We support ongoing efforts to develop or refine equations for specific unique and easily identifiable populations.