Diagnostic assay for a sample of biological fluid

An article and a method for monitoring the concentration of an analyte, e. g., glucose, in blood. In one aspect, the invention involves an article comprising a multiple-layer element. In one embodiment, the article comprises:a multiple-layer element comprising:(a) a base layer(b) a cover layer, the cover layer having a first opening for venting the multiple-layer element and a second opening for receiving a biological sample; and(c) a core layer having a first major surface and a second major surface, the core layer disposed between the base layer and the cover layer, the core layer comprising a sample introduction chamber and a optical reading chamber, the first major surface of the core layer in face-to-face contact with the base layer, the second major surface of the core layer in face-to-face contact with the cover layer.The multiple-layer element of this invention allows the use of samples of blood or other biological fluids, such as interstitial fluid, to provide extremely sensitive assay results. The device has been shown to provide accurate and reproducible results with samples having volumes ranging from about 5 muL to about 20 muL. In another aspect, the invention involves a method for using the aforementioned article.