Nitrogen Balance Calculator

This calculator may be used to calculate a patient's nitrogen balance based upon a 24 hour urine urea nitrogen (UUN) collection.

24 hour nitrogen intake (gm)
24 hour urinary nitrogen output (gm)
Abdominal fluid output over past 24 hours (if applicable) (in liters)

Calculating Nitrogen Balance

Calculation of a patient’s nitrogen balance can be used as an indicator of whether sufficient nutrition is being supplied to prevent further breakdown of muscle protein. A patient who is in “positive nitrogen balance” excretes less nitrogen than they consume and is incorporating nitrogen into newly formed protein (i.e. muscle). A patient who is in “negative nitrogen balance” excretes more nitrogen than they consume and is continuing to use muscle protein as an energy source. The goal of nutritional support is positive nitrogen balance.

Nitrogen Balance = Total protein intake (grams)/6.25 - (UUN + 4 grams)
where 6.25 = 6.25 grams of protein per gram of nitrogen, UUN = grams of nitrogen excreted in the urine over a 24 hour period of time, 4 = 4 grams of nitrogen lost each day as “insensible losses” via the skin and gastrointestinal tract

As with any equation that attempts to estimate a physiologic parameter, nitrogen balance is only a crude estimate. It is not valid in the presence of burns, fistulae, wounds with high protein output, or renal failure. Nitrogen balance measurements are also time consuming as they require a 24 hour urine collection for measurement of UUN (although some evidence suggests that accurate measurements may be obtained with only an 8 hour urine collection). Because of the inaccuracy associated with determination of protein requirements, the current recommendations are that an estimate of 1.5 grams of protein/kg/day should be used in calculating protein administration for surgical patients. For comparison, a healthy person requires approximately 0.8 grams of protein/kg/day. Administration of more than 1.5 grams/kg/day exceeds the body’s ability to incorporate protein and does little to restore nitrogen balance. The exception to this is the burn patient who is losing large quantities of protein through their burn wounds. These patients may require protein administration of up to 2-2.5 grams/kg/day.