A method of treating a patient with a microneedle array having an active agent disposed on at least a portion of one or more of the microneedles, wherein the array with active agent is characterized by a first optical response when probed with a selected incident light spectrum. The array with active agent is applied to a skin surface, optionally allowed to remain on the skin surface for a specified time, and removed. The used array is probed with the selected incident light spectrum, a second optical response is sensed, and the difference between the first optical response and the second optical response is determined and compared to a predetermined threshold value.