The colorimetric determination of urea in the blood of normal and uremic rats.

In connection with other studies in this laboratory (1, 2),’ it became necessary to determine the concentration of urea in the blood of individual new-born rats. In view of the small quantities of blood available and the need for a considerable number of analyses, the calorimetric method developed by Archibald (3) appeared to be particularly suitable. This method is based upon the formation of a colored compound when strongly acid solutions of urea and cu-isonitrosopropiophenone are heated together. As applied to human blood, this procedure wa+s reported by Archibald to yield results which corresponded closely to those determined with a manometric urease method. Inasmuch as some other compounds, such as allantoin, citrulline, alloxan, and even proteins if present in sufficient quantities, give rise to a color similar to that produced by urea (3), the specificity of the Archibald method when applied to rat blood was not assured. When it was found that under certain conditions the urea concentration in the blood of very young rats, as measured by this method, rose to extremely high levels, further studies on the specificity of this method were clearly indicated. Such studies might also reveal extensive variations in the concentration of chromogens other than urea in the blood of rats. In this paper data are reported on (a) a comparison of the colorimetric method and a urease method for determination of urea in rat blood and (b) the extent of destruction by urease of substances which are chromogenic in the calorimetric method.