In the Intensive Care Unit (ICU), water gain and loss in 24 h can be measured from:
- Water gain: drinking, tube feeding or intravenous infusion
- Water loss: urine
There can be substantial additional loses via excessive sweating, vomiting and diarrhea, and those are not usually measured, but only estimated and may or may not be recorded.
If the question is, what can be a possible difference between total water gain and loss in 24 hours, I'd say roughly 10 liters in each direction, but this is just what is physiologically possible and not likely what they would allow to occur in the ICU, where +2 to -2 liters would be more realistic.
If we are talking about absolute amounts of water gain and loss in 24 hours, they could be both up to about 30 liters.
Example 1: Treatment of severe dehydration. A patient who loses 30 liters of water per day by diarrhea due to cholera, would be treated by 30+ liters of fluid in that day.
Example 2: Excessive urine loss (polyuria). A person gets 5 liters of fluid in a day, but due to excessive urination caused by diabetes mellitus, he loses 10 liters of fluid. Again, in the ICU they would not likely allow that such great negative water balance would develop.
Example 3: Water intoxication. Water intoxication can happen, for example, when dehydration is treated by excessive amounts of water, let's say, by 5 liters in 2-3 hours - such mistakes should be rare in the ICU, but occasionally occur in everyday life... In one case, a 22-years-old man drank 6 liters of water in 2-3 hours and developed severe hyponatremia. It's probably not survivable to drink 10 liters of water in 2-3 hours for a person who is already well hydrated at the beginning of drinking.
Example 4: Hyperhydration with edema. In kidney or liver patients with impaired water balance regulation, water can accumulate in the body (which otherwise doesn't in significant amounts), which can result in generalized edema with 10+ liters of excessive fluid in the body, or in case of ascites (accumulation of fluid in the abdominal cavity), up to about 30 liters, as mentioned in this article. Anyway, such big amounts of water do not likely accumulate within 24 hours.