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Why "urea nitrogen" when urea is measured?

To the Editor:

The paper by Morishita et al. (1) regarding the analysis of urea is confusing. In the text, the authors state that they are measuring urea nitrogen (UN), and it may seem so, because their calibrators were "physiological saline, with and without 17.86 mmol/L UN". However, the abstract states that analytical recovery of added urea was studied with patient sera containing 6.14 mmol/L urea. In contrast, the text describes patient sera with a UN concentration of 6.14 mmol/L. Because urea contains two nitrogens, these two sentences seem to be incompatible.

I think that if one is measuring urea, one should give the results in SI units (mmol/L) for the whole compound and not for the urea nitrogen. Otherwise, ambiguities arise in the interpretation of the results. Urea should be measured, and "urea nitrogen" should be abandoned as an analyte in clinical chemistry work.


(1.) Morishita Y, Nakane K, Fukatsu T, Nakashima N, Tsuji K, Soya Y, et al. Kinetic assay of serum and urine for urea with use of urease and leucine dehydrogenase. Clin Chem 1997;43: 1932-6.

Bertil Kagedal

Dept. of Clinical Chemistry

University Hospital

S-581 85 Link ping, Sweden

Author for correspondence. Fax 46-13-22 32 40; e-mail

Editor's note: An author in Reference 1, aware that urea nitrogen contains two N, indicates that "NZ (nitrogen molecule)" was used for the calculation. He apologizes for the confusion.
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Title Annotation:Letters
Author:Kagedal, Bertil
Publication:Clinical Chemistry
Article Type:Letter to the editor
Date:Apr 1, 1998
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