The separation of serum or plasma from whole blood is of overriding importance in clinical chemistry. In particular, many diagnostic detection reactions of blood components proceed without impairment only after the red blood corpuscles have been separated off. This particularly applies to color reactions that are evaluated either by reflectometry or visually, or also electrochemically.

The use of special erythrocyte retention substrates in the field of whole blood analysis with the aid of test strips, such as blood sugar monitoring under home user conditions, is of particular importance.

Graphite Fiber Test Strips require lower whole blood volumes, and provide higher mechanical strength and aesthetic and technical advantages because of their retention of the red color of the blood.

A multi-layer test system comprises a reagent layer and an erythrocyte separation zone with at least one or more glass fiber layers, which may be different. The whole blood is applied to the glass fiber layer, the erythrocytes being adsorbed into this layer as a consequence of agglutination, while plasma and serum diffuse into the reagent layer where the detection reaction can proceed without interference by erythrocytes. The glass fiber layer can comprise auxiliary reagents such as certain polar dyestuffs, which have the effect of coagulation or agglutination of the erythrocytes.

A disadvantage of the diagnostic test systems with erythrocyte retention substrates of glass fiber nonwovens is their relatively high requirement of whole blood volumes, which are about 10 μl for the known Reflotron® glucose test systems. However, smaller amounts of blood, for example, 5 μl or less, are of great advantage, in particular in respect to obtaining the blood as painlessly as possible.

Another important disadvantage of the glass fiber nonwovens is their low mechanical strength, which is even significantly below the known values for thin blotting papers. Mechanical working such as cutting or impregnation using conventional machines that require certain tear strengths is made exceptionally difficult as a result.

Nonwovens of graphite fibers can meet the requirements imposed on the erythrocyte separation function in an outstanding manner without the above mentioned disadvantages occurring. In this technology, the blood is applied via an opening and on the opposite side, a blue color reaction that is unimpaired by erythrocytes and correlated with the glucose concentration of the whole blood is observed after a few seconds.

Such graphite nonwovens are produced by SGL Carbon Group, the type Sigrafil SPC 7011 having proved to be particularly suitable for the erythrocyte separation layers. These are black nonwovens of high tear strength comprising graphite fibers with an average fiber diameter of 7 μm, a weight per unit area of 30 g/m2, a thickness of 0.5 mm, and a binder system of cross-linked polyvinyl alcohol, the content of which is about 20 to 24% by weight. Woven fabrics that can also be produced from graphite fibers — and are marketed under the name Sigratex® — are also suitable for preparation of the erythrocyte retention layers.

As a consequence of this very hydrophilic polymeric binder, the Sigrafil® graphite nonwovens are distinguished by an excellent wettability. Similarly to glass fiber nonwovens, a very rapid transportation of liquid is observed both in the horizontal and vertical directions.

Two further important advantages of the graphite nonwoven erythrocyte retention substance result from its black color. After application of the blood onto a two-layer test system of the retention layer and reagent membrane, the red erythrocytes retained in the graphite nonwoven can scarcely still be detected visually, which is to be evaluated as an aesthetic advantage. This advantage also manifests itself in the reflectometric evaluation of the color reaction in the reagent membrane, because in contrast to the conventional systems (red-colored, erythrocyte-containing glass fiber layers as a background), no adverse reflectometric interferences can result.

The graphite nonwovens impregnated with known agglutinating agents, such as lectins, can be used as single- or multi-layer systems. In the case of multi-layer retention substrates, all or only individual graphite nonwoven layers can be impregnated with one or various agglutinating agents. Multilayer retention substances can also comprise other porous layers, such as polyvinyl alcohol nonwovens or monoor multi-filament woven fabrics, as elemental components, in addition to graphite nonwovens. It is essential only that the main content of the agglutinated erythrocytes is retained in a graphite nonwoven layer, and that preferably the top layer (application of blood) and the layer closest to the reagent membrane are made of graphite nonwoven.

Another typical property of the graphite nonwovens is their electrical conductivity, which is in the region of a few ohms typical of carbon fibers. As a result of the combination of electrical conductivity with the outstanding property of horizontal transportation of liquid, layers with bifunctional functions can be built up in respect to electrochemical sensor systems. In an amperometric test format, a graphite nonwoven layer can simultaneously perform the function of liquid-drawing microcapillaries, and simultaneously function as a reference electrode.

The worldwide market for blood-handling diagnostic test strips is huge. Diabetes testing represents only one type of strip. Test strips, by their nature, can be used only once. Many clinical tests require separating the serum or plasma from the whole blood; that is, filtering out the red blood cells (erythrocytes) from the remainder of the blood constituents. In particular, many diagnostic detection reactions cannot proceed accurately without first separating out and capturing the red blood cells so that the remaining constituents can be exposed to reagents deeper within the test strip.

This technology is offered by Bayer MaterialScience LLC. For more information, view the TechPak at .