Every eGFR equation assumes steady state - that creatinine production and excretion are in balance, so that the concentration reflects the filtration rate. In acute kidney injury they are not, and the reported number lags reality by a day or more. A patient whose creatinine has doubled overnight may still have an eGFR printed beside it in the 40s; their actual filtration rate at that moment may be close to zero.
The consequences are practical and immediate. Drug doses calculated from a lagging eGFR are overdoses, and this is where gabapentin, metformin, direct oral anticoagulants, low molecular weight heparin and renally cleared antibiotics cause avoidable harm. Contrast decisions made on a stale number are made on the wrong number. And a falling creatinine during recovery overestimates function in the other direction, which is when patients get discharged on doses they cannot yet clear.
What to do instead: use the trend and the direction of the creatinine, not the eGFR, while it is unstable; dose by the clinical picture and urine output; and where a level can be measured, measure it. Note in the chart that the eGFR is unreliable, because the number is printed automatically and the next clinician will otherwise use it.
- Ignore the printed eGFR whenever creatinine is rising or falling; use the trend
- Reduce or hold renally cleared drugs on the direction of travel, not the reported eGFR
- Watch urine output - it moves before creatinine does
- Measure drug levels where available rather than calculating from an unstable estimate
- Write in the notes that the eGFR is invalid in acute injury; it will be quoted otherwise
Why it matters
The number is printed on every result and looks authoritative at exactly the moment it is wrong.
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