Establishing discordance rate of estimated glomerular filtration rate between serum creatinine based calculations and cystatin C based calculations in critically ill patients summary

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Hey there, critical care pharmacists! Welcome to today's literature update. I'm excited to share some insights from a recent article titled, "Establishing discordance rate of estimated glomerular filtration rate between serum creatinine based calculations and cystatin C based calculations in critically ill patients." This research, authored by V. L. Williams and A. T. Gerlach, was published in *Pharmacotherapy* in two thousand twenty-five.

So, let's dive into the study's design. This was a single-center, retrospective, observational cohort study conducted at an academic medical center. It included adult critically ill patients admitted in two thousand twenty-three who had both serum creatinine and cystatin C measurements available. The primary objective here was to determine the rate of discordance in estimated glomerular filtration rate, or eGFR, between serum creatinine-based calculations and serum creatinine/cystatin C-based calculations that would lead to clinically significant differences in drug dosing recommendations for this critically ill population. They defined discordance as at least one discrepancy in kidney dosing for a medication when comparing calculations from the Cockcroft-Gault creatinine clearance equation against those from the Chronic Kidney Disease Epidemiology Collaboration eGFR creatinine-cystatin C equation.

Now, for the key findings. The study cohort comprised two hundred thirty-two patients, with about fifty-three percent being female, and a mean age of fifty-eight point seven years. The median serum creatinine was zero point ninety-four milligrams per deciliter, and the median cystatin C was one point ninety-two milligrams per liter. Notably, the median clearance rates differed significantly. The Cockcroft-Gault equation yielded a median of sixty-eight point five milliliters per minute, while the CKD-EPI creatinine-cystatin C equation yielded a median of fifty-three point nine milliliters per minute. This difference was statistically significant, with a p-value of less than zero point zero zero one.

Perhaps the most impactful finding for us as pharmacists is the overall discordance rate in drug dosing. Across all studied medications, this rate was thirty-two point three percent, meaning seventy-five out of two hundred thirty-two patients had at least one drug dosing discordance between the two methods. When looking at specific drugs, the most common medications showing this discordance were cefepime at forty point six percent, vancomycin at thirty-eight point three percent, levetiracetam at thirty-five point one percent, and piperacillin/tazobactam at eleven point six percent. This really highlights how serum cystatin C can improve the detection of impaired kidney function, directly impacting our dosing decisions.

To put these findings into a broader context, previous research has also underscored the value of cystatin C. For instance, Grubb and colleagues in two thousand fourteen demonstrated cystatin C as a reliable kidney function marker that is less affected by muscle mass, which you know, is a significant issue with serum creatinine in critical illness. That's PMID two four nine seven seven zero eight two. Shlipak and others, back in two thousand thirteen, showed improved risk prediction with cystatin C-based eGFR in hospitalized patients, which is PMID two three three zero three four four seven. And Herget-Rosenthal and colleagues, in two thousand five, found that cystatin C actually outperforms creatinine in acute kidney injury detection. That's PMID one five eight zero seven two one nine. More recently, Zhao and colleagues in two thousand twenty-three reported a forty-five percent discordance rate between serum creatinine- and cystatin C-based eGFR affecting dosing in hospitalized patients, aligning with what we're seeing here. Their work is PMID three nine six zero one three four five. And Smith and colleagues, in two thousand twenty-one, highlighted cystatin C’s utility specifically for vancomycin dosing in critically ill populations, with a PMID of three three four eight two zero one nine.

From a clinical implications standpoint, these results reinforce that pharmacists should really consider incorporating cystatin C-based kidney function estimates alongside traditional creatinine-based calculations. This is especially true for dosing renally eliminated medications in our critically ill patients, as it can significantly enhance dosing accuracy and, ultimately, improve medication safety.

Of course, like any study, there are limitations to consider. This was a retrospective, single-center design, which does limit its generalizability to other institutions. There was also a lack of a gold-standard GFR measurement, such as inulin clearance, which would provide definitive kidney function. And we should acknowledge potential selection bias, given that patients without cystatin C testing were excluded. However, the study's strengths include its relatively large sample size of critically ill adults with concurrent serum creatinine and cystatin C measurements, the use of a clinically relevant definition for dosing discordance, and the assessment of multiple medications for real-world relevance.

In conclusion, this study demonstrates that in adult critically ill patients, a notable thirty-two point three percent experienced clinically significant discordance between serum creatinine-based and serum creatinine/cystatin C-based eGFR estimates, leading to different drug dosing recommendations. That wraps up today's update—thanks for tuning in, and I'll catch you on the next episode with more critical care pharmacy insights.