Kidney Week

Abstract: SA-PO0065

Four-Hour Urine Is Accurate in Measuring Creatinine Clearance in AKI

Session Information

• AKI: Clinical Diagnostics and Biomarkers
  November 08, 2025 | Location: Exhibit Hall, Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: Acute Kidney Injury

• 102 AKI: Clinical, Outcomes, and Trials

Authors

• Mellas, John, Nephrology and Hypertension Specialists, St Louis, Missouri, United States

John Mellas, MD

• Mellas, Dean, Southern Illinois University School of Medicine, Springfield, Illinois, United States

Dean Mellas, MD

Background

In AKI rmethods to rapidly measure creatinine clearance (K) are fraught with poor precsion and reproducibilty.
Measuring K requires a collection of urine over a precise time and is signified by the equation; K = U x V / C. This is true in the steady state but has not been validated in AKI.
The kinetics of creatinine in AKI show that after injury there is an abrupt fall in K with a gradual rise in C and an abrupt decrease followed by a gradual rise in U x V, followed by an eventual new steady state corresponding to K (fig). A urine-based measure of K may be accurate if the duration of urine collection is long enough to represent an accurate sample while being short enough to give a value for K. A four-hour urine has been described as accurate and reproducible.

Methods

A modeled kinetic measure of K in AKI is as follows:
K = P/C – [V x dC/dt/C], where P = creatinine production
Hence; C₂ = P/K₂ + [(C₁ – P/K₂] x e ^-Kt/V
with known K₁, K₂, and C₁, C₂ is calculated
Actual U x V = [U x V]* ( steady state) – [Vd x (C₂ – C₁)] (Vd = creatinine distribution volume)
K_{2 ⁼} U x V / C₂.
Measure K₂ at any two time intervals and determine if all calculated values of K₂ compare favorably to its known value.

Results

When the values were compared to six modeled patients with various values for K₁ and K_2, it was found, using Bland Altman plots, there was a high level of agreement between calculated and actual K. Data points fell within the limits of agreement of 10%, defined as the mean difference ± 1.96 times the standard deviation of the differences.

Conclusion

It has been mathematically shown that a 4 hour urine collection measuring K by the traditional formula of K = U x V / C , is an accurate measure of K in AKI predicting the extent of kidney injury and the trajectory of the level of azotemia well in advance of any change in serum creatinine.

Kinetics of creatinine in AKI

Funding

• Private Foundation Support

Digital Object Identifier (DOI)

• doi: 10.1681/ASN.2025jjvze5ze