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

When lactase activity of a person needs to be measured, the individual will be given milk to drink and the level of glucose in the blood will be measured after sometime in order to determine the effectiveness of the person's lactase activity.
Lactose is a sugar that is found in milk. When milk is taken, the lactase enzyme will act on the lactose to break it down to glucose. Glucose is the major fuel that the body system use and the level of glucose in the blood can be measure directly using simple means.
Thus, if the glucose level increase as expected after drinking milk, then that means the lactase enzyme activity is optimal. But if the glucose level in the blood remain the same after taking milk, then the lactase enzyme is not working fine.