Urinary urea: are currently available methods adequate for revival of an almost abandoned test?

Measurement of urinary urea excretion has been suggested as a means of estimating nitrogen balance in hospitalized patients who are malnourished. Because proficiency-testing surveys show gross variations in mean urea as determined by various automated methods and extremely poor precision occasionally, we compared urinary urea measurements and ammonia interference in three widely used methods. The coupled urease/glutamate dehydrogenase method (used in the DuPont aca) showed positive interference from ammonia, as expected; with the diacetylmonoxime (Technicon (12/60) and the urease conductivity (Beckman ASTRA) methods we saw no such interference. Generally, interference by ammonia is less than 10%, but (rarely) it may exceed 25%. However, if urine specimens are properly diluted and potential sources of interference recognized, all three methods appear capable of providing clinically useful data.