Why is measuring blood glucose levels an indicator of someone's lactase activity?

The milk contain sugar in form of lactose. When milk is taken in food, the digestive enzyme lactase will break down the lactose sugar in milk and convert it to glucose. This will shows in the blood glucose level as an increase in glucose.
Thus, to determine the lactase activity of a person, milk can be given to that person and the glucose that are made from the milk[ [which are product of lactase activity] can then be measured.