How this instrument works
The rate is urine volume divided by weight times the number of hours the collection covers, expressed in millilitres per kilogram per hour. Charting output per kilogram rather than as a raw hourly volume corrects for an obvious unfairness: a small patient producing 30 mL an hour is doing proportionally far more urinary work than a large patient producing that same 30 mL, because the smaller patient's kidneys are filtering and excreting relative to a much smaller blood volume and body mass. Dividing by weight puts people of very different sizes onto one comparable scale, which is why nephrology tracks the rate this way rather than a raw hourly figure.
The KDIGO Clinical Practice Guideline for Acute Kidney Injury, published in 2012, sets the oliguria threshold at under 0.5 mL/kg/hr. That line isn't a rough rule of thumb — it's one of the formal criteria KDIGO uses to stage AKI, alongside a rise in serum creatinine and a fall in glomerular filtration rate. Output sustained below that threshold for six hours or longer is, by itself, sufficient to diagnose AKI stage 1, with no creatinine change required. Reduced output can show up before a creatinine rise appears on a blood panel, part of why bedside tracking still matters even where labs run frequently.
A single low reading isn't automatically alarming — a short collection window, a recently emptied bladder, or a missed measurement can all drag one hour's number down without reflecting six sustained hours. The KDIGO criteria specifically require the threshold to hold across a defined stretch of time, not a single spot check, and this instrument reports one rate from whatever volume and period you enter, leaving the judgment about sustained duration to whoever is charting it.
- Collect and measure Urine volume in mL over a defined period, ideally from an indwelling catheter for accuracy.
- Enter the patient's Weight in kilograms.
- Enter the collection period as Hours.
- Read the rate in mL/kg/hr; under 0.5 sustained for 6+ hours meets the KDIGO oliguria criterion for AKI stage 1.
Worked example — three collections, three patients
100 mL collected over 4 hours from a 70 kg patient: 70 × 4 = 280, and 100 ÷ 280 gives a rate of 0.357 mL/kg/hr — below the 0.5 threshold, and worth watching if it holds for the rest of a 6-hour window.
Triple the volume with everything else unchanged — 300 mL over the same 4 hours in the same 70 kg patient: 70 × 4 = 280 again, and 300 ÷ 280 gives 1.071 mL/kg/hr, comfortably above the oliguria line.
A smaller collection tells a starker story: 20 mL over 6 hours in a 60 kg patient. 60 × 6 = 360, and 20 ÷ 360 gives just 0.056 mL/kg/hr — severely reduced, and because the full 6-hour window is already covered, this reading alone satisfies the KDIGO sustained-oliguria criterion.
Questions
What is the KDIGO threshold for oliguria?
Under 0.5 mL/kg/hr. That's the specific cutoff the 2012 KDIGO Clinical Practice Guideline for Acute Kidney Injury sets for reduced urine output, expressed per kilogram of body weight rather than as a flat volume, precisely so it applies fairly across patients of very different sizes.
Is sustained low output actually a diagnostic criterion for AKI, or just a warning sign?
It's an actual diagnostic criterion. KDIGO stages acute kidney injury using a rise in serum creatinine, a fall in glomerular filtration rate, or urine output under 0.5 mL/kg/hr sustained for 6 hours or more. Meeting that output criterion alone, with no change in creatinine at all, is sufficient to diagnose AKI stage 1 — it isn't a soft warning layered on top of the 'real' criteria, it is one of the real criteria.
Why divide by weight instead of just tracking urine volume in mL per hour?
Because a raw hourly volume treats every patient the same regardless of size, and that isn't a fair comparison. A 45 kg patient and a 110 kg patient producing an identical 30 mL an hour are not doing the same amount of physiological work — relative to body mass, the smaller patient's kidneys are filtering far more intensely. Dividing by weight converts a raw figure into a rate that means roughly the same thing across different body sizes.
What counts as anuria rather than oliguria?
Anuria generally refers to essentially no output at all, commonly defined as under 100 mL over 24 hours, and it points toward a more severe or more mechanically obstructive problem than oliguria typically does. Oliguria, by contrast, is reduced but present output, under the 0.5 mL/kg/hr KDIGO threshold — different points on the same spectrum rather than interchangeable labels.
Does a single low reading always mean kidney injury?
No. A brief collection window, a recently emptied bladder, or a missed measurement can all pull one hour's rate down without reflecting a genuine sustained drop. The KDIGO criterion specifically requires the threshold to hold for 6 hours or longer, so one low number is a prompt to keep watching and confirm the trend, not a stand-alone diagnosis.
How is urine output actually measured in a hospital setting?
Most reliably through an indwelling urinary catheter connected to a graduated collection chamber, read at fixed intervals — hourly in intensive care, less often on a general ward. Without a catheter, output is estimated from timed voids into a collection container, less precise but still useful for tracking a trend over several hours.
References
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2(1):1-138
- National Kidney Foundation — Acute Kidney Injury (AKI)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.