Qualitative Sugar Tests.
(1) Fehling's Test:—Boil about 4 c.c. of Fehling's[1] solution in a test tube, and add to the hot Fehling's an equal amount of urine, a few drops at a time, boiling after each addition.
A yellow or red precipitate indicates sugar.
For practical purposes in the following of a diabetic's daily urine, this is a valuable test, and the one which we always use.
(2) Benedict's Test:—To 5 c.c. of Benedict's[2] reagent add 8 drops of the urine to be examined. The fluid is boiled from 1 to 2 minutes and then allowed to cool of itself. If dextrose is present there results a red, yellow, or green precipitate, depending upon the amount of sugar present. If no sugar is present the solution may remain perfectly clear or be slightly turbid, due to precipitated urates.
This is a more delicate test than Fehling's.
[1] Fehling's solution is prepared as follows:
(a) Copper sulphate solution: 84.65 gm. of copper sulphate dissolved in water and made up to 500 c.c.
(b) Alkaline tartrate solution: 125 gm. of potassium hydroxide and 178 gm. of Rochelle salt dissolved in water and made up to 500 c.c.
These solutions are kept in separate bottles and mixed in equal volumes when ready for use.