The nitrogen to creatinine ratio in untimed samples of urine as an index of protein catabolism after surgery.

The loss of nitrogen after surgery is said to be related to body size as well as to the magnitude of the metabolic effects of surgery. Urine nitrogen should be 'corrected' for body size before it can be taken as a guide to the severity of surgery in the individual. Twenty-three patients were studied after elective abdominal surgery of moderate severity, when they were having only 3 litres of dextrose saline daily. The 24-hr urine nitrogen excretion (mean of days 3 and 4) was correlated with 3 indices of body size, body weight (r = 0.614), fat-free mass (r = 0.743) and 24-hr creatinine excretion (r = 0.780). Nitrogen excretion was corrected for body size by calculating the ratio of nitrogen to each index. For each index the s.d. of this corrected nitrogen excretion at the mean value of the index was less than the s.d. of the uncorrected data. The nitrogen : creatinine ratio in an untimed urine sample was closely related to the ration in the 24-hr urine (r = 0.914). These results demonstrate that nitrogen excretion after surgery is related to body size and can be corrected for body size by calculating the nitrogen : creatinine ratio in an untimed urine.