A simple turbidimetric method of estimating blood urea.

THE CLASSIC ESTIMATION OF UREA IN THE BLOOD, while not technically difficult, is time consuming, and a quicker method was sought for an emergency procedure. It has been known for many years that a solution of xanthydrol with urea forms an insoluble precipitate, dixanthylurea. Also that this precipitate dissolves in 50% sulfuric acid to give an intense yellow-colored solution which obeys Beer’s law up to certain concentrations. Unfortunately xanthydrol itself gives the same yellow color in sulfuric acid, so the precipitate of dixanthylurea has to be washed free of the xanthydrol reagent before preparing it for the colorimeter. Beattie (1), in 1928, described a method which involved the repeated washing of the precipitate and the final evaluation of the yellow solution in sulfuric acid. Later, in 1937, Lee and Widdowson (2) elaborated Beattie’s work and devised a micro-method which, they claimed, was accurate. This method was tedious and unsuitable for our purpose. Ten years later, in 1947, Engel and Engel (3) reconsidered the whole matter and produced an elegant method, but again it involved repeated washing of a small precipitate and the colonmetric estimation of the sulfuric acid solution. Caraway (4), in 1955, proposed a turbimetric method using the same reaction of xanthydrol and urea but taking the turbidity of the mixture, after five minutes, as a measure of the urea present. A disadvantage of this method is the continuing development of the turbidity after the stipulated 5 minutes. Furthermore the estimation of turbidity by eye, without any standard for comparison, is very difficult and introduces the per-