In this episode, I’ll discuss whether kinetic GFR equations can be used in patients with shock.
A well-recognized problem when using equations such as cockcroft-gault or MDRD to estimate renal function is that validation studies for these equations all involved patients that were at steady-state renal function.
In critical illness serum creatinine is rarely at steady state and kinetic GFR equations have been developed to attempt to create an estimate of kidney function when the serum creatinine is not at steady state.
While methods such as iohexol clearance are reliable for GFR assessment in critically ill patients, this involves exogenous administration of iohexol and measurement over a 24 hour period, making it not a practical solution for estimating GFR at the bedside.
In the journal Critical Care Medicine, a group of authors published a retrospective study that assessed kinetic glomerular filtration rate formulas and compared them to iohexol clearance among patients with shock.
The authors compared 3 static and 7 kinetic GFR estimation formulas, as well as urinary creatinine clearance. The authors also applied various commonly used correction factors for volume of distribution and body weight for a total of 33 variants.
The best performing formula was the corrected Moran and Myers value. It was within 30% of iohexol-clearance–measured GFR for just under half of the patients and was within 10% for 1 in every 6 patients. Such poor accuracy is not fit for clinical use, and as the best performer, that meant that none of the other variants were acceptable either.
The equations and variants tested tended to overestimate GFR for patients in shock.
It is likely that the reason for poor performance of these kinetic equations is that they only take into account time between serum creatinine measurements, and other issues such as catabolism and nutritional intake affect the accuracy.
Unfortunately, kinetic GFR formulas such as Jelliffe, Chen, Chiou and Hsu, Moran and Myers, Yashiro, Seelhammer, and Brater are not suitable for use in critically ill patients with shock.
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