![]() ![]() These medications falsely increase creatinine because they inhibit the active secretion of creatinine in the proximal tubule, but it does not affect the glomerular filtration of creatinine. The most pertinent medication that can increase serum creatinine is trimethoprim (a component of Bactrim or Septra) and cefoxitin (an antibiotic sometimes used for perioperative prophylaxis). Certain medications can falsely elevate serum creatinine.While the difference isn’t usually clinically relevant, it’s an important distinction. The kidney filters creatinine in the glomerulus (which is what GFR measures), but it also actively secrets creatinine in the proximal tubule (which should not be counted toward GFR). Technically, creatinine clearance is a surrogate for GFR (glomerular filtration rate), but creatinine clearance slightly overestimates GFR. Creatinine clearance and GFR are different, but often (incorrectly) used interchangeably.These methods aren’t validated in a large number of patients and aren’t likely to be terribly accurate, but they are the only methods available. There are two methods to estimate “unstable” renal function (Jelliffe 1972 and Chiou 1975). Many hospitalized patients do not have a stable renal function, which means that these traditional equations are not appropriate for these patients. All traditional equations (C-G, MDRD, CKD-EPI) require that a patient have a “stable” renal function (usually two similar values drawn more than 24 hours apart). You can (roughly) estimate creatinine clearance in patients with unstable renal function.It is possible to “convert” between older methods and the newer IDMS, but this practice increases the complexity and number of corrections required to calculate a creatinine clearance. While it’s great that the assay is more accurate, older methods of estimating renal function (specifically Cockcroft-Gault) will return a lower value with IDMS than with previous assay methods. ![]() This bias can be as high as 20%, and is especially problematic with a normal creatinine (closer to 1 mg/dL). Older methods detected non-creatinine chromagens, which falsely elevated the amount of creatinine detected. Isotope Dilution Mass Spectrometry (IDMS) is the newest assay for measuring serum creatinine. CKD-EPI was designed to be as accurate as MDRD at lower renal function, but to have better precision in patients with normal renal function. In fact, the newer CKD-EPI equation was specifically developed for this problem. Because MDRD was developed in patients with renal dysfunction, it loses precision in patients with normal renal function.
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