Urine Output Calculator

How does this urine output calculator work?

This health tool determines the urine output rate in mL/kg/hr in order to offer information on fluid balance based on total urine output and fluid intake.

Kidneys perform the essential task of filtering blood and excreting waste products through urine. The normal values for total urine output range between 800 to 2000 mL in adults with normal fluid intake of 2L during 24 hours.

Normal urine output per hour values for adults need to be around 1 mL/kg/hr. Considering an average weight of 60 kg, this means 60 mL/hr for normal kidney perfusion.

The urine output calculator uses the following two formulas:

■ 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.

While urinary output is largely influenced by fluid intake and overall kidney function (types of chronic kidney disease), there are other factors that affect it:

■ Dehydration;

■ Perspiration;

■ Caffeine, alcohol intake;

■ Medicaments.

In order to assess kidney function, the 24-hour urine collection test is one of the most commonly used methods. As part of the test, the following are determined:

■ Creatinine;

■ Sodium;

■ Potassium;

■ Nitrogen;

■ Protein.

Polyuria is defined as abnormally large volumes of urine, a condition seen in diabetes insipidus. Increased urine volume also occurs in some forms of kidney disease, usage of diuretic medication or during periods of high fluid intake.

Oliguria guidelines

Oliguria diagnosis values (hypoperfusion starts at):

■ In adults: urine output

■ In children:

Acute oliguria, defined as less than 400 mL of urine per day, is often one of the earliest symptoms of damaged renal function.

Early identification of the sudden decrease in glomerular filtration rate and treatment implementation within the therapeutic window can help prevent further impairment.

Amongst the causes of oliguria there are renal causes, septic shock, drug toxicity, myoglobulin release. The mechanism involves a sudden drop in GFR rate which results in an increase in plasma urea and creatinine concentration, development of acidosis and hyperkalemia, plus retention of salt and water.

According to the RIFLE (Risk, Injury, Failure, Loss of kidney function, and End-stage kidney disease) classification definition of the Acute Dialysis Quality Initiative, the following conditions are consistent with acute kidney injury in adult patients:

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

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

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

Fluid balance

The second determination in the above calculator uses the difference between declared fluid intake and total urine output to check the fluid balance.

FB is controlled through osmoregulation, electrolyte concentrations, hormonal regulation through anti diuretic hormones and behavior in order to maintain euvolemia. The principle is that water lost should be equal to water in.

Input fluid consist of that taken through drinking, eating and parenteral intake. Output fluids are those of respiration, perspiration, expectoration, urine and defecation.

References

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

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

3) Marriott HL. (1950) The Maintenance of Fluid Balance: Based on lectures given at The Royal College of Surgeons, 1949-50. Ann R Coll Surg Engl; 7(5): 339–356.

4) Lopes JA, Jorge S. (2012) The RIFLE and AKIN classifications for acute kidney injury: a critical and comprehensive review. Oxford Journals Medicine & Health Clinical Kidney Journal 6(1): 8-14

5) Wang N, Jiang L, Zhu B, Wen Y, Xi XM; Beijing Acute Kidney Injury Trial (BAKIT) Workgroup. (2015) Fluid balance and mortality in critically ill patients with acute kidney injury: a multicenter prospective epidemiological study. Crit Care; 19:371.