An article and a method for monitoring the concentration of glucose in blood. In one aspect, the invention involves an article comprising a multiple-layer element utilizing reagents capable of reacting with an analyte of interest. In a preferred embodiment, the element comprises:(a) a core layer having two major surfaces, an optical reading chamber extending from a first opening in one of the two major surfaces to a second opening in the other of the two major surfaces, the core layer further having a third opening therein and a flow channel, one end of which flow channel communicates with the third opening and the other end of which flow channel communicates with the optical reading chamber;(b) a base layer in face-to-face contact with one major surface of the core layer; and(c) a cover layer in face-to-face contact with the other major surface of the core layer, the cover layer having an opening therein to vent the element.In another aspect, the invention involves a method comprising the steps of:(a) obtaining a sample of biological fluid, e.g., interstitial fluid, from the body of a patient;(b) introducing the sample to an article comprising a multiple-layer element having an optical reading chamber;(c) allowing reagents to react with an analyte of interest in the sample; and(d) measuring the concentration of analyte in the sample by means of an optical instrument.