Renal insufficiency 

Antimicrobial drugs excreted through the urinary tract may be toxic for patients with renal insufficiency if they are given in usual therapeutic doses, because serum concentrations in these patients may become dangerously high. Nephrotoxic and ototoxic drugs such as gentamicin or other aminoglycosides may damage the kidney, further decreasing the excretion of these drugs, leading to higher blood concentrations that may be ototoxic and may cause additional renal damage. In patients with renal insufficiency, therefore, an antimicrobial drug with minimal nephrotoxicity, such as a beta-lactam, is preferred. When nephrotoxic drugs must be used, renal function should be monitored. Measurements of serum creatinine or blood urea nitrogen (BUN) concentrations are useful as indices of renal function, but are not as accurate as measurements of creatinine clearance; serum creatinine and BUN concentrations may be normal even with significant loss of renal function.