Multilayer test elements have been extensively used in laboratory environments to determine an analyte of interest. Multilayer reagent strips are shown in U.S. Pat. No. 3,672,845. U.S. Pat. Nos. 3,993,451 and 4,438,067 disclose the use of particulated reagents, and U.S. Pat. No. 3,723,064 discloses the use of a membrane having regions of differing permeability. U.S. Pat. No. 3,783,105 demonstrates the use of freeze-dried reagents. Other multilayer test designs are shown by U.S. Pat. Nos. 3,980,437, 4,042,335, 4,144,306, 4,160,008, 4,178,153, 4,292,272, 4,318,985, 4,387,990, 4,427,632, 4,452,887, 4,532,107, 4,604,264, and 4,668,472. Another alternative to the multilayer test device approach is shown in U.S. Pat. No. 3,476,515, which discloses rupturable sample and reagent pouches. U.S. Pat. No. 3,158,532 teaches the manufacture of a filter which has pores of graduated size and filtration capacity.
Specific reagents have been found useful in assays for the presence of glucose, cholesterol, alcohol, and other analytes (see, for example, U.S. Pat. Nos. 2,912,309 and 2,981,606). Buffers, stabilizers, dyes, and other detectable moieties have been incorporated into multilayer devices and test strips.
One difficulty in the prior art has been accurate dosing of the test sample into or onto the assay device. Quantitative evaluation for an analyte has generally involved careful control of sample volume. Reagents in the test device are allowed to react completely with the sample, and the extent of reaction determined. Analyses which do not involve sample control have generally been semi-quantitative or qualitative, rather than quantitative tests.
The presence of red blood cells in whole blood samples may cause interference in the detection of color changes. Cellular components in blood have been blocked or filtered from the determining layer by use of cellulose (U.S. Pat. No. 3,092,465), amino acids (U.S. Pat. No. 3,552,928), glass fibers (U.S. Pat. No. 4,477,575), or carbohydrate (U.S. Pat. No. 4,678,757). Fluid metering with the concurrent removal of cellular components of blood is addressed in U.S. Pat. Nos. 4,250,257 and 4,260,392.
Barrier or blocking layers have been used to segregate cellular components from the serum portion of a whole blood sample. Such barriers used in prior art elements generally are required to be permeable to the ligand, the reagents of the reagent layer, or products of their interaction. In such devices, determination of the detectable species is made from the detecting layer surface away from the reagent layer. Alternatively, a barrier may be included on the element to confine an applied sample to a predetermined region of the element surface. Barrier layers in multilayer elements are shown in U.S. Pat. Nos. 3,992,158, 4,166,093, 4,255,384, 4,256,693, 4,363,874, 4,390,343, 4,478,944, 4,631,174, and 4,066,403 (U.S. Pat. No. Re. 30,267).