Historically, serum creatinine was analyzed from a blood sample using a method called alkaline picrate. In addition to creatinine molecules, though, it also “counted” non-creatinine molecules that falsely elevated the resulting value by as much as 20%. This assay method was used for decades in the development of creatinine clearance estimates, such as the Cockcroft-Gault method.
Within the past 10-15 year, however, laboratories have largely moved to a new assay called IDMS (isotope dilution mass spectrometry). This method does not detect the non-creatinine molecules, which means that the IDMS value is often 10-20% lower than the more conventional assay. Because older equations, like Cockcroft-Gault, were created and validated using a non-IDMS assay, this poses a problem for estimating creatinine clearance (a surrogate for glomerular filtration rate) when using an IDMS-based lab assay.
Converting from IDMS to non-IDMS (Conventional)
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Colistin (in the form of colistimethate sodium, or CMS, in the United States) is an older, last-line agent for multidrug-resistant gram-negative infections. Because of colistin’s complex pharmacokinetics and for historical reasons, there is a paucity of data regarding its dosing in patients with severe gram negative infections, particularly for those with concurrent renal dysfunction.
In one of the largest pharmacokinetic analyses of colistin to date, Garonzik et al. published a detailed analysis of CMS dosing in critically ill patients. This analysis included dosing recommendations for patients with normal renal function, acutely changing renal function, intermittent hemodialysis (IHD), and continuous renal replacement therapy (CRRT).
ClinCalc is excited to announce our new colistin dosing calculator, which is based on the Garonzik pharmacokinetic recommendations. This calculator was developed in coordination with Julie Ann Justo, PharmD, MS, BCPS, AAHIVP — an Assistant Professor at the South Carolina College of Pharmacy who specializes in infectious diseases and HIV pharmacotherapy. Continue reading →
We’re releasing a major update to the calculation for our popular vancomycin calculator today. Briefly, the new update implements more advanced calculations when adjusting a vancomycin dose based on a trough level.
Drug Elimination during Vancomycin Infusion
When adjusting vancomycin based on a trough level, pharmacokinetic textbooks recommend estimating a vancomycin peak level using the following equation: Continue reading →
Amlodipine (Norvasc) is a great drug — but it takes 8 hours for the drug to reach its peak drug concentration, and about a week to reach steady-state full antihypertensive effect.