Why Schiff s reagent can t distinguish between glucose and fructose???

Schiff's reagent can be used for detection of many organic aldehydes that has also found use in the staining of biological tissues. Such method is known as Schiff's test.

Although fructose is a ketose, it can give a positive result in the Schiff's test. This is because fructose is readily isomerized (such isomerization is known as tautomerism) to a mixture of glucose and mannose under basic conditions. Both glucose and mannose are aldoses and thus give positive result in Schiff's test. The equation of such isomerization is shown below.