Legal's exam
Legal's test is used to detect ketone bodies in the urine.
Ketones in urine:
3-Hydroxybutyric acid (β-hydroxybutyric acid, hydroxybutyrate), which is often classified as a ketone body, is not actually a ketone body and therefore cannot be demonstrated by the Legal reaction.
The principle of the test
The principle is the reaction of ketone bodies (acetic acid and acetone) with sodium nitroprusside in a strongly alkaline environment of sodium hydroxide. The result is a purple colored product.
This principle has a number of modifications, mainly when securing an alkaline environment, and all work procedures use it, both in the form of diagnostic strips and in the form of a powder reagent.[1]
False negativity:
3-hydroxybutyric acid is the most abundant in ketoacidosis, which means that even a negative result does not completely rule out ketoacidosis.
False positivity:
Urine containing pigments or metabolites of levodopa (dihydroxyphenylalanine) may give false positive results.[2]
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