Urine Output Calculator

Key facts about urine output

In order to calculate the urine output rate in mL/kg/hr and fluid balance, the following variables are considered: patient weight, fluid intake and the total urine output per period of time, usually a 24h period is considered. 

The urine output calculator uses the following equations:

■ Urine output in mL/kg/hr = Total urine output in mL / (Weight in kg x Hours);

■ Fluid balance in mL = Fluid intake in mL - Total urine output in mL.

Urine output can offer information on whether the kidneys function normally, in terms of blood filtration and excretion of waste products through urine.

Total urine output for an adult, during 24h is of 800 to 2000 mL, this is equivalent to around 1 mL/kg/hr. In an adult weighing 60 kg, this means 60 mL/hr.

Urinary output (the 24-hour urine collection test) is one of the most common laboratory tests, which checks for creatinine, sodium, potassium, protein presence and nitrogen levels.

Urinary output is however, impacted by other factors such as:

■ Existence of chronic kidney disease;

■ Dehydration;

■ Certain drugs;

■ Caffeine and alcohol intake;

Fluid balance

FB consists in the difference between declared fluid intake and total urine output (water lost should be equal to water in).

Its mechanism of control is osmoregulation, electrolyte concentration and hormonal regulation through anti diuretic hormones. Some also include here behaviour in order to maintain euvolemia.

When referring to input fluid, we refer to drinking, eating and parenteral intake. Output fluids are those of respiration, perspiration, expectoration, urine and defecation.

Oliguria

The main causes of oliguria include:

■ renal causes;

■ septic shock;

■ drug toxicity;

■ myoglobulin release.

Oliguria, or hypo-perfusion, starts at:

■ In adults: urine output

■ In children:

Acute oliguria is less than 400 mL of urine output per day. The mechanism involves a sudden drop in glomerular filtration rate which in turn determines an increase in plasma urea and urine creatinine concentration. Development of acidosis and hyperkalemia follow, along with retention of salt and water.

Early identification of oliguria and decreases in GFR means that further kidney damage can be prevented.

According to RIFLE (Risk, Injury, Failure, Loss of kidney function, and End-stage kidney disease) classification definition of the Acute Dialysis Quality Initiative, the conditions for acute kidney injury (AKI) are:

■ Less than 0.5 mL/kg/hr for more than 6h at a time (high risk AKI);

■ Less than 0.5 mL/kg/hr for more than 12h at a time (positive AKI diagnosis);

■ Less than 0.3 mL/kg/hr for more than 24h at a time or anuria for 12h (acute renal failure).

Polyuria on the other hand, is defined as abnormal volumes of urine, larger than normal, usually (if not triggered by high fluid intake) a symptom of diabetes insipidus, some forms of kidney disease or prolonged use of diuretic medication.

References

1. Klahr S, Miller SB. (1998) Acute Oliguria. N Engl J Med. 1998; 338:671-675.

2. Gowda S, Desai PB, Kulkarni SS, Hull VV, Math AAK, Vernekar SN. Markers of renal function tests. N Am J Med Sci. 2010; 2(4): 170–173.

3. Chau et al. Fluid balance, change in serum creatinine and urine output as markers of acute kidney injury post cardiac surgery: an observational study. Can J Kidney Health Dis. 2014; 1: 19.