# Creatinine Lag in AKI: The Kidneys-in-a-Bucket Thought Experiment

Andrew Bland, MD, FACP, FAAP · October 19, 2026

Imagine taking both kidneys out of the circulation and placing them in a bucket. The patient’s true GFR is immediately zero, but the creatinine already in the blood does not instantly change. Muscle continues producing creatinine, so its concentration rises over time.

![Diagram](https://urinenephrology.org/2025_UDPA_Lectures_Live/assets/creatinine-lag-zero-gfr-bucket-2026-10-19.png)

The changing creatinine does not require another change in GFR: filtration stays at zero throughout this thought experiment. The routine equation still returns positive numbers because it assumes steady-state creatinine. These values are not all normal, but every one overestimates actual filtration here.

| Sample | Creatinine (mg/dL) | Routine eGFR (mL/min/1.73 m²) | True GFR (mL/min) |
| --- | --- | --- | --- |
| 1 | 1.0 | 86 | 0 |
| 2 | 2.0 | 38 | 0 |
| 3 | 3.0 | 23 | 0 |
| 4 | 4.0 | 16 | 0 |

Illustrative 60-year-old man; 2021 CKD-EPI creatinine equation, rounded. These are successive hypothetical samples, not fixed time intervals or a predicted rate of rise. No dialysis is assumed. In real AKI, creatinine production, distribution volume, and residual clearance also affect the trajectory.

Routine eGFR assumes steady-state creatinine. In AKI, follow creatinine change, timing, urine output, and clinical context. Specialized kinetic estimates differ from routine laboratory eGFR.

## Sources

- [NIDDK: 2021 CKD-EPI creatinine equation](https://www.niddk.nih.gov/research-funding/research-programs/kidney-clinical-research-epidemiology/laboratory/glomerular-filtration-rate-equations/adults)
- [KDIGO AKI Appendix A: creatinine lag and nonsteady-state eGFR](https://kdigo.org/wp-content/uploads/2016/10/KDIGO-AKI-Suppl-Appendices-A-F_March2012.pdf)
- [NIDDK: clinical limitations of eGFR](https://www.niddk.nih.gov/research-funding/research-programs/kidney-clinical-research-epidemiology/laboratory/factors-affecting-egfr-accuracy/clinical-measurements)

[Return to the opening lecture](https://urinenephrology.org/2025_UDPA_Lectures_Live/case-based-lectures/opening-lecture.html#kidneys-in-a-bucket)