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Purpose Creatinine-based estimates of glomerular filtration rate (GFR) have been the standard for classifying kidney function and guiding drug dosing for over 5 decades. There have been many efforts to compare and improve different methods to estimate GFR. The National Kidney Foundation recently updated the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations without race for creatinine (CKD-EPIcr_R) and creatinine and cystatin C (CKD-EPIcr-cys_R), and the 2012 CKD-EPI equation based on cystatin C (CKD-EPIcys) remains. The focus of this review is to highlight the importance of muscle atrophy as a cause for overestimation of GFR when using creatinine-based methods. Summary Patients with liver disease, protein malnutrition, inactivity, denervation, or extensive weight loss may exhibit markedly lower creatinine excretion and serum creatinine concentration, leading to overestimation of GFR or creatinine clearance when using the Cockcroft-Gault equation or CKD-EPIcr (deindexed). In some cases, estimated GFR appears to exceed the physiological normal range (eg, >150 mL/min/1.73 m2). Use of cystatin C is recommended when low muscle mass is suspected. One would expect discordance between the estimates such that CKD-EPIcys < CKD-EPIcr-cys < CKD-EPIcr ≈ Cockcroft-Gault creatinine clearance. Clinical evaluation can then occur to determine which estimate is likely accurate and should be used for drug dosing. Conclusion In the setting of significant muscle atrophy and stable serum creatinine levels, use of cystatin C is recommended, and the resulting estimate can be used to calibrate interpretation of future serum creatinine measurements.

Purpose Prior to the 2020 release of a joint consensus guideline on monitoring of vancomycin therapy for serious methicillin-resistant Staphylococcus aureus (MRSA) infections, clinicians had escalated vancomycin doses for 2 decades while targeting trough concentrations of 15 to 20 µg/mL, leading to an increased frequency of nephrotoxicity. For MRSA infections, the 2020 guideline recommends adjusting doses to achieve a 24-hour area under the concentration-time curve (AUC) of 400 to 600 µg · h/mL; however, monitoring of trough concentrations has been entrenched for 3 decades. Calculating dose regimens based on AUC will require obtaining an increased number of vancomycin serum concentrations and, possibly, advanced software. The aim of this investigation was to determine the relationship between AUC and trough concentration and the influence of dosing regimen on goal achievement. Methods The relationship between trough concentration and AUC was explored through derivation of an equation based on a 1-compartment model and simulations. Results 24-hour AUC is related to dosing interval divided by half-life in a nonlinear fashion. The target trough concentration can be individualized to achieve a desired AUC range, and limiting use of large doses (>15-20 mg/kg) can protect against excessive 24-hour AUC with trough-only monitoring. Conclusion After initially determining pharmacokinetic parameters, subsequent monitoring of AUC can be accomplished using trough concentrations only. Trough concentration may be used as a surrogate for AUC, although the acceptable target trough concentration will vary depending on dosing interval and elimination rate constant. This work included development of an AUC-trough equation to establish a patient-specific target for steady-state trough concentration.

Creatinine is an endogenous waste product resulting from the breakdown of creatine phosphate, a compound largely stored in skeletal muscle. The amount and rate of creatinine excretion (mg per 24 hours) are related to muscle mass; in fact, these measures have been used to estimate muscle mass.[1][1

The compatibility of ciprofloxacin injection with selected antimicrobials and aminophylline was studied. Ciprofloxacin, amikacin sulfate, aminophylline, clindamycin phosphate, gentamicin sulfate, and tobramycin sulfate were mixed separately in minibags containing 0.9% sodium chloride injection or 5% dextrose injection; admixtures were stored for up to 48 hours at either 4 degrees C or 25 degrees C. Ciprofloxacin was also combined separately with each of the other drugs and solutions and stored under the same conditions. In addition, ciprofloxacin was combined with metronidazole in ready-to-use mini-bags of the latter drug and stored at 25 degrees C. Drug concentrations were measured by fluorescence polarization immunoassay or high-performance liquid chromatography. All admixtures were also examined visually. Stability was defined as retention of at least 90% of the original drug concentration with no visual evidence of incompatibility. With one exception, drugs in all single-drug admixtures were stable for 48 hours. The drug concentration eight hours after amikacin was mixed in 0.9% sodium chloride and refrigerated was 89% of the original concentration. When ciprofloxacin was combined with gentamicin, metronidazole, or tobramycin, all of the involved drugs were stable for 48 hours. Compatibility of ciprofloxacin-amikacin admixtures depended on the fluid and storage temperature; all such admixtures were stable for at least eight hours. A precipitate formed immediately whenever ciprofloxacin was mixed with clindamycin and within four hours after ciprofloxacin was mixed with aminophylline. Ciprofloxacin injection was compatible with gentamicin, metronidazole, and tobramycin and incompatible with aminophylline and clindamycin. The compatibility of ciprofloxacin-amikacin admixtures depended on the i.v. solution and storage temperature.

We read with interest the recent article by Leong et al.[1][1] concerning the use of adjusted body weight (AdjBW) for determining clearance (CL) of vancomycin in overweight and obese patients. Several previous publications involving the pharmacokinetics of vancomycin in this population have