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Kidney Week

Abstract: FR-PO011

Improving the Kinetic GFR by Taking Volume Changes into Account

Session Information

Category: Acute Kidney Injury

  • 102 AKI: Clinical, Outcomes, and Trials

Author

  • Chen, Sheldon, MD Anderson, Houston, Texas, United States
Background

The kinetic glomerular filtration rate (GFR) equation was developed to estimate the kidney function even as the serum creatinine is changing. One shortcoming of the equation is that it assumes a constant volume of distribution for the creatinine. The volume in hospitalized patients can fluctuate quickly and drastically. For example, in hypotension and multiorgan failure, aggressive volume resuscitation would dilute the creatinine concentration, slowing down a rise in creatinine and making the acute kidney injury (AKI) appear milder.

Methods

To undo the confounding effects of volume change, we improved the kinetic GFR model by introducing variables to describe the ongoing influence of volume change. In the upgraded model, we solved its differential equation to yield a formula that predicts how the serum creatinine would evolve, as determined by the creatinine generation rate, creatinine clearance rate (~GFR), and now a volume change rate. Clinically, the starting and ending creatinines are measured by the lab, so the equation is rearranged to solve for the kinetic GFR.

Results

The new kinetic GFR equation was tested in multiple patients, some of whom had extreme changes in volume. In all cases, incorporating volume information resulted in the calculated GFR being correctly adjusted, concordant with: 1) Volume Gain → A) AKI looks less severe; GFR is actually lower, or B) renal recovery looks very robust; GFR is actually not as high, and 2) Volume Loss → A) AKI looks quite severe; GFR is actually not as impaired, or B) renal recovery looks unimpressive; GFR is actually higher than believed. Case: a 66-year-old woman has ischemic lactic acidosis and hypotension (87/53 mm Hg). She was given normal saline at 300 mL/h. NS plus the many intravenous drips (pressors and antimicrobials) added up to a net volume gain of 5 liters in 12 hours. With oliguria, her serum creatinine rose from 0.84 to 1.25 mg/dL over a 16-hour period, encompassing the above 12 hours. The old equation would calculate a kinetic GFR of 48.3 mL/min. The new equation would calculate a kinetic GFR of 38.6 mL/min. The difference of 9.7 mL/min was due to massive volume gain, which masked the true severity of her AKI.

Conclusion

Body volume changes can significantly affect creatinine kinetics. The new volume-adjusted equation improves the precision of kinetic GFR estimation, which could aid in diagnosis and medication dosing.