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Kidney Week

Abstract: SA-PO160

How to Estimate Kidney Function in AKI via the Basic Clearance: Role of the True Average (Creatinine)

Session Information

Category: Acute Kidney Injury

  • 102 AKI: Clinical, Outcomes, and Trials

Author

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

The basic clearance formula U×V/P has a single Plasma creatinine ([Cr]), so it seems to apply only in the steady state. Can U×V/P be used in the non-steady state when there are multiple values of P? We postulate that if all the P's in a [Cr] trajectory are represented by a "true average" [Cr], then dividing by this one P will recreate the kinetic GFR.

Methods

Working from any differential equation that models creatinine kinetics, we can take the novel step of using the fundamental theorem of calculus at the start. This produces a definite integral that calculates the average (not the simple mean) of a [Cr] vs. time function, our candidate true average [Cr]. It ends up in the denominator after solving for kinetic GFR, fitting the template of U×V/P.

Results

To use the true average [Cr] to compute kinetic GFR, we present two techniques, a graphical one and a numerical one—Newton’s method. Both yielded identical answers for kinetic GFR, verifiable by a gold standard technique. But the true average method arrived at the answer faster than the gold standard and without any false solutions. In analyzing a recent case, the kinetic GFR aided clinical decision-making on a 74-year-old man whose creatinine rose subacutely from 1.10 to 9.57 mg/dL. Intermittent dialysis was done, giving the [Cr] plot a sawtooth pattern (Fig., in purple). Despite an overall decline in [Cr] at first, the kinetic GFR (in blue) showed no improvement in his kidney function, since the "valleys" in between dialyses sank down to the same low level. Later, a renal biopsy (3/26) revealed acute interstitial nephritis, and prednisone was started (3/28). The next kinetic GFR valley (3/29) was slightly higher, hinting at an early renal recovery. The valleys kept increasing (3/31-4/5), telling us that the steroid was working and so dialysis was stopped.

Conclusion

The clearance paradigm applies to the non-steady state as well if the true average [Cr] is the divisor, providing a fundamental strategy to deduce the kinetic GFR from the plasma [Cr] trends occurring in real-life acute kidney injury or renal recovery.