The Determination of Sugar in Blood and in Normal Urine

In a recent paper, S. R. Benedict? describes a calorimetric copper method for the determination of the blood sugar which is based upon the idea that his well known copper citrate reagent should prove as superior for quantitative work on blood as it has been for qualitative tests on urine. This expectation seems to have been fulfilled to a certain extent, for by a series of paralleL blood sugar determinations he shows that the new method does give materially lower values than are obtained by the method of Folin and Wu. Benedict seems to believe that the only essential feature of his qualitative sugar reagent is the fact that its alkali is a carbonate instead of a hydroxide. This concept I think is not quite correct. When Benedict, in 1908; described his citrate reagent the most important new feature was the use of citrate instead of a tartrate, and the purpose, as stated by Benedict, was to obtain a more stable reagent than had been obtained by the use of Rochelle salt. Increased stability was, however, not the only result, or indeed the most important result achieved by substituting citrate for tartrate. The citrate exerts a powerful depressive action on the oxidative properties of dissolved copper compounds and it is because of this inhibiting effect that the original citrate reagent proved so suitable as a qualitative reagent for sugar in urine. The inhibiting effect of the citrate includes the reaction with reducing sugars, and it is because of this factor that citrate reagents are qualitatively less sensitive and quantitatively give much less cuprous oxide from a given amount of sugar (0.2 to 0.4 mg.)