The KDIGO definition has two limbs, and one of them is routinely ignored. A rise in serum creatinine of 26.5 micromol/L within 48 hours, or a 1.5-fold rise within 7 days, is the limb everyone uses. Urine output below 0.5 mL/kg/h for 6 to 12 hours is the limb that gets left out, and it is usually the earlier one.
Creatinine is a lagging indicator. It has to accumulate before it moves, so a patient can lose most of their glomerular filtration and still have a creatinine inside the reference range for a day or more. Hourly urine output, in anyone catheterised or ill enough to be monitored, declares itself first - and it is free.
So the habit is to chart urine output in millilitres per kilogram per hour rather than as a shift total, and to treat six consecutive hours below 0.5 as an event that requires a response: review the drug chart for nephrotoxins and renin-angiotensin blockade, assess volume status properly rather than by weight alone, and rule out obstruction with a bladder scan before anything else. Half the acute kidney injury that gets diagnosed late was visible in the fluid balance chart the day before.
- Chart urine output as mL/kg/h, not as a total per shift - the threshold is per kilogram and per hour.
- Treat six hours below 0.5 mL/kg/h as stage 1 acute kidney injury and act on it, whatever the creatinine says.
- Review the drug chart first: NSAIDs, aminoglycosides, contrast, and renin-angiotensin blockade in a hypovolaemic patient.
- Bladder scan before assuming intrinsic injury - obstruction is the reversible cause that gets missed.
- Do not use a normal creatinine to reassure yourself in the first 24 hours; it lags the injury.
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