Lessons · Medicine foundations · urine output in mL per kg per hour
Urine output: millilitres per kilogram per hour
Urine output is compared per kilogram of body weight and per hour, so a volume collected over a stretch of time is divided twice: once by the hours, once by the weight.
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What it is for
A fluid balance chart gives you a volume and a span of hours, and the figure anyone actually quotes is per kilogram per hour, because 200 mL means something different for a 40 kg person and a 100 kg one. Two divisions, and the unit at the end tells you whether you did both.
How to think about it
Read the unit backwards and it tells you the arithmetic: millilitres per kilogram per hour means millilitres, then divided by kilograms, then divided by hours. Do the hours first if you like; division does not care about the order. What matters is that both happen.
Worked example
A chart gives 240 mL over 4 hours in an 80 kg adultThree figures: a volume, a span, a weight.
240 / 4 = 60 mL/hHours first. Sixty millilitres an hour.
60 / 80 = 0.75 mL/kg/hThen per kilogram. Three quarters of a millilitre per kilogram per hour.
Your turn
A chart gives 180 mL over 3 hours in a 60 kg adult. Write the line that gives the output in millilitres per kilogram per hour.
180 / 3 / 60 = mL/kg/h
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The trap
Dividing by the weight and stopping, so the answer is per kilogram but not per hour. The number looks reasonable and means nothing, because the span it was collected over has vanished from it.