Abstract: The present invention provides a method of determining the concentration of a selected drug in the body of a subject comprising the steps of holding a liquid collecting means comprising an absorbent substantially inert member 1 in a position in close proximity to an eye 4 of the patient for collecting the fluid 7 therefrom; allowing the tear fluid collected to come into contact with reagent means 3 for reacting with said selected drug so as to produce, after a predetermined time interval, a physically detectable change dependent, at least semi-quantitatively, upon the concentration of said drug; and determining the physical change in said reactive portion after a predetermined interval of time.