Final answer:
The addition of a concentrated caffeine solution or methyl alcohol in bilirubin tests is to facilitate the reaction of indirect bilirubin with the color reagent, distinguishing it in cases like hemolytic and infective jaundice.
Step-by-step explanation:
In bilirubin determinations, the purpose of adding a concentrated caffeine solution or methyl alcohol is to allow indirect bilirubin to react with the color reagent. The indirect Van Den Bergh test demonstrates this by showing that unconjugated bilirubin, which does not normally give color with Diazo reagent, will produce a purple-red color when alcohol is present. This test is commonly seen in cases of hemolytic and infective jaundice. On the contrary, the Direct Van Den Bergh test indicates that conjugated bilirubin reacts with Diazo reagent without the need for alcohol, and this reaction is typically associated with obstructive jaundice.