Process and Composition for Separating Plasma or Serum From Whole Blood

This invention relates a method and a composition for separating plasma or serum from whole blood.

In clinical chemistry, the separation of serum or plasma from blood is of outstanding importance since, in practice, it is required to carry out the analysis of dissolved blood components without disturbance.

The normal and most conventional manner of separating serum or plasma from erythrocytes is centrifuging. However, especially in the case of using small amounts of sample, this gives rise to problems and the separation of supernatant and blood cake is not always simple so that, for this purpose, a whole series of adjuvants are to be found in the literature, for example Federal Republic of Germany Patent Specification No. 25 59 242.

The use of whole blood in the case of rapid diagnostic agents gives rise to special problems. Rapid diagnostics are reagent-containing, absorbent or swellable carriers, preferably filter paper, upon which a small amount, for example a droplet, of the liquid to be investigated is applied and in which, because of the preponderantly short period of reaction, a colour change takes place which is either evaluated visually or is measured remission photometrically. Since turbid or coloured solutions, such as blood, disturb the readings, attempts have, therefore, been made to make rapid diagnostics available for the direct use of whole blood. Thus, for example, mention may be made of the coating of test papers with semi-permeable membranes (see U.S. Pat. No. 3,092,465) and the use of swellable films into which only the dissolved components of the blood can penetrate but not the erythrocytes (see Federal Republic of Germany Patent Specification No. 15 98 153). These two methods are per se usable but only for tests for low molecular weight components of blood, for example glucose or urea; higher molecular weight components of the blood, for example lipids or substrates bound to serum protein, for example bilirubin, cannot be determined in this way because they are not able to penetrate into the film or to pass through the semipermeable membrane. Furthermore, suggestions have been made for covering diagnostic agents with membrane filters for separating off the blood cells (see Federal Republic of Germany Patent Specifications Nos. 22 22 951 and 29 22 958). A disadvantage of these diagnostic agents is that the blood can only penetrate through the membrane filter very slowly and in small amounts because the membrane is very easily blocked up and the reaction takes a correspondingly longer time. In contradistinction to the previously-mentioned diagnostic agents, which are already commercially available, "rapid tests" of the last-mentioned type have not yet been marketed.

Furthermore, from Federal Republic of Germany Patent Specifications Nos. 29 08 721 and 29 08 722, it is known that lymphocytes and leukocytes can be separated from blood when blood is filtered through a layer of synthetic resin fibres with an average fibre diameter of 5 to 20.mu. in the case of separating lymphocytes and of 3 to 10.mu. in the case of separating leukocytes. However, since the erythrocytes preponderantly pass through the filter with the plasma, these filters are not suitable for obtaining plasma. Furthermore, carbon fibres, glass fibres and metal fibres are mentioned purely speculatively.

Therefore, it is an object of the present invention to provide a simple means for separating plasma or serum from whole blood which separates small amounts of blood quickly and certainly without centrifuging and is especially suitable for preparing samples for diagnostic purposes.

We have now found that the separation of plasma or serum from whole blood takes place quickly and simply and in sufficient amount when the blood is allowed to run through a packing of glass fibres alone or in admixture with other fibres. This fact must be regarded as being the more surprising since, in the above-mentioned Federal Republic of Germany Patent Specification No. 22 22 951, the use of glass fibre mats has already been described for the separation of white corpuscles but, for the separation of erythrocytes, the additional use of membrane filters is stated to be absolutely necessary.

Thus, according to the present invention, there is provided an agent for the separation of plasma or serum from whole blood, which comprises a layer of glass fibres with a diameter of 0.2 to 5.mu., preferably of 0.5 to 2.5.mu., and especially of 0.5 to 1.5.mu. and with a density of 0.1 to 0.5 g./cm.sup.3, the volume of the plasma or serum to be separated being at most 50% and preferably less than 30% of the absorption volume of the glass fibre layer.

The glass fibres can be in loosely stapled form, or can also be in the form of papers, fleece or felts or can also be formed in any desired shape when held by a forming body.

The so formed glass fibres can, when used as a covering for one of the above-described rapid diagnostics, allow these diagnostics, which hitherto required previously obtained serum or plasma, to now be used directly with whole blood.

Furthermore, columns, filter funnels or other suitable vessels packed with glass fibres can also be used for obtaining serum or plasma by simply allowing blood to run through, without centrifuging, and to make the serum or plasma available in an appropriate manner for diagnostic agents since the serum or plasma passes through such a layer more quickly than the erythrocytes and leukocytes.

The above-mentioned glass fibres can consist of fibres of differing diameters. The glass material can consist of alkali-containing or alkaline-free borosilicate glass or pure quartz glass. Fibre of other technical glasses, for example boron-free alkali glasses, crystal glass, lead glass and the like, are not commercially available with the necessary dimensions and could not be investigated. However, it is to be assumed that they are also suitable. The average diameter of the glass fibres can be from 0.2.mu. to about 5.mu., preferably from 0.5.mu. to 2.5.mu. and especially from 0.5.mu. to 1.5.mu.. The diameters of the fibres can, depending upon their mode of production, vary very considerably but an upper limit of about 10.mu. should only be exceeded in exceptional circumstances. The length thereof is only limited by the nature of the packing but otherwise has no influence. Depending upon the nature of the packing, densities of 0.1 to 0.5 and usually of 0.2 to 0.4 g./cm.sup.3 are found.

Furthermore, the glass fibres can be mixed with one another and also with other materials, the internal holding together of the fibres thereby being improved. Thus, for example, use can be made of synthetic fibres, such as of polyester, polyamide and the like, as well as fibres of polyethylene, polypropylene and other thermoplastic synthetic resins. These additional fibres can also have a larger diameter (e.g. 10 to 20.mu.), so long as the amount thereof is not so large that it adversely affects the separating ability of the finer glass fibres used according to the present invention.

Furthermore, the glass fibres can be consolidated by the addition of inorganic binding agents, for example waterglass, or of organic binding agents, for example polyvinyl acetate, polyvinyl propionate, polyacrylic acid esters and the like, by means of which the glass fibres are stuck at the points of contact.

The combination of the glass fibre layers according to the present invention with diagnostic agents is especially preferred and is also the subject of the present invention.

In these diagnostic agents, the glass fibres can also contain reagents which prevent the haemolysis of erythrocytes, as well as reagents which inhibit or promote coagulation and also reagents which are required in the indicator layer but are incompatible with other reagents present therein. These latter reagents can, of course, also be placed in layers which are between the glass fibres and the indicator layer.

Preferred embodiments of the invention will now be described with reference to drawings, in which:

FIG. 1 is a schematic elevation of a first preferred embodiment;

FIG. 2 is an elevation of a second preferred embodiment;

FIG. 3 is an elevation of a third preferred embodiment;

FIG. 4 is an elevation of a fourth preferred embodiment;

FIG. 5 is a top view of the fourth preferred embodiment shown in FIG. 4;

FIG. 6 is an elevation of a fifth preferred embodiment;

FIG. 7 is a top view of the preferred embodiment shown in FIG. 6;

FIG. 8 is an elevation of a sixth preferred embodiment;

FIG. 9 is an elevation of the seventh preferred embodiment;

FIG. 10 is an elevation of an eighth preferred embodiment;

FIG. 11 is an elevation of a ninth preferred embodiment;

FIG. 12 is an elevation of a tenth preferred embodiment;

FIG. 13 is an elevation of an eleventh preferred embodiment;

FIG. 14 is an elevation, partly in section, of a twelth preferred embodiment;

FIG. 15 is an elevation, partly in section, of a thirteenth preferred embodiment;

FIG. 16 is an elevation, partly in section, of a fourteenth preferred embodiment;

FIG. 17 is a top view of the fourteenth preferred embodiment shown in FIG. 16;

FIG. 18 is an elevation, partly in section, of a fifteenth preferred embodiment;

FIG. 19 is an elevation, partly in section, of a sixteenth preferred embodiment;

FIG. 20 is an elevation, partly in section, of a seventeenth preferred embodiment;

FIG. 21 is an elevation, partly in section, of a eighteenth preferred embodiment;

FIG. 22 is an elevation, partly in section, of a nineteenth preferred embodiment;

FIG. 23 is an elevation, partly in section, of a twentieth preferred embodiment;

FIG. 24 is an elevation, partly in section, of a twenty-first preferred embodiment;

FIG. 25 is a top view of the embodiment shown in FIG. 24;

FIG. 26 is an elevation, partly in section, of a twenty-second preferred embodiment;

FIG. 27 is a top view of the embodiment shown in FIG. 26;

FIG. 28 is an elevation, partly in section, of a twenty-third preferred embodiment;

FIG. 29 is a top view of a twenty-fourth preferred embodiment;

FIG. 30 is an elevation, partly in section, of a twenty-fifth preferred embodiment; and

FIG. 31 is an elevation, partly in section, of a twenty-sixth preferred embodiment.

The construction of one such diagnostic agent according to the present invention is illustrated in FIG. 1 of the accompanying drawings. On to a stiff substrate 2 is struck the reaction layer 1 of the rapid diagnostic agent. Closely above the reaction layer is applied a thin and inherently stable separating layer 4 which is permeable to liquids, which consists of a meshwork of woven or felted synthetic resin fibres, and which is stuck, on both sides of the reaction layer to the substrate in an easily removable manner and in such a way that an easily gripped end 4a on a longer end of the substrate remains free. Above the reaction and separating layers, there is applied a glass fibre paper 3. This is fixed in position by a further meshwork 5 which, like the separating layer 4, is stuck down on either side of the reaction layer.

The reaction layer 1 can consist of an impregnated absorbent carrier or of a swellable or porous synthetic resin film. The substrate 2 is preferably a comparatively thick synthetic resin film, a rigid cardboard, a glass plate or some other stable carrier material.

After the application of a drop of blood 8 to the upper side of the rapid diagnostic agent, the plasma is separated from erythrocytes and leukocytes in the glass fibre paper 3. The plasma separated in this manner passes via the separating layer 4 into the reaction zone or layer 1 of the diagnostic agent. After an appropriate time, in which the plasma has penetrated into the reaction zone, the free end 4a of the separating layer is gripped and the layer 4 removed, together with the glass fibre paper 3 and the meshwork 5. Subsequently, the reaction layer in which the reaction now takes place can be evaluated visually or remission photometrically.

Another possible form of construction of the rapid diagnostic agent according to the present invention is illustrated in FIGS. 2 and 3. In addition to the above-described construction, they have one or more layers 6 which are permeable to all kinds of liquids above (FIG. 2) or below (FIG. 3) the glass fibre paper 3. These additional layers can be impregnated with reagents which are either readily soluble and, together with the plasma, pass into the reaction zone or are less soluble and allow one or more preliminary stages of the detection reaction to take place outside of the final reaction zone 1.

FIGS. 4 and 5 illustrate another construction of the rapid diagnostic agent according to the present invention in which the glass fibre paper 3 is stuck directly on to the substrate 2. The reaction layer 1 is applied to one part of the glass fibre paper. Blood 8 is applied to the other part of the glass fibre paper which remains free. The plasma separating from the erythrocytes in the glass fibre paper diffuses in the glass fibre paper towards the reaction layer and passes into it. The reaction colours resulting due to the reaction can be observed from the upper side of the rapid diagnostic agent and can be evaluated. The reaction layer can be applied directly to the glass fibre mat by "printing" or coating. However, it can also be stuck on to the glass fibre paper in the form of a wholly or partly impregnated absorbent carrier.

Furthermore, as illustrated in FIGS. 9 and 10 of the accompanying drawings, the diagnostic agent according to the present invention can be constructed in such a manner that on to the substrate 2 there is first stuck an absorbent material 9, for example cellulose paper or a synthetic fibre fleece, and above this material the glass fibre paper 3 and the reaction layer 1. The absorbent material 9 can thereby have the same surface area as the reaction layer (FIG. 10) or can have a larger surface area so that the material 9 has an uncovered area (FIG. 9). The blood is dropped on to the uncovered surface area of the absorbent material (FIG. 9) or directly next to the absorbent material (FIG. 10) and rapidly taken up by this and sucked under the glass fibre paper. Subsequently, due to the absorbency of the glass fibre paper, blood is sucked upwardly through the glass fibre paper, separation of the erythrocytes thereby taking place, and plasma passes into the reaction layer 1. The reaction is, as in FIG. 4, observed from the upper side of the rapid diagnostic agent.

FIG. 11 of the accompanying drawings illustrates yet another construction of a rapid diagnostic agent suitable for the direct use of whole blood. This is constructed in such a manner that, on a stiff substrate 2, there are applied, side-by-side, an absorbent layer 9 which consists, for example, of a cellulose paper or of a cellulose-containing synthetic fibre fleece, and a glass fibre layer 3. The two layers should be in close contact with one another. On the surface of the absorbent layer 9, there are present in reaction layer 1 the detection reagents needed for the rapid diagnostic, which can be applied, for example, by coating with an open film as described in Federal Republic of Germany Patent Specification No. 29 10 134. Upon applying a drop of blood on to the side of the glass fibre layer remote from the reaction zone, the separation of plasma from erythrocytes takes place in such a manner that the plasma front first reaches the point of separation from the absorbent layer 9 and is immediately sucked up by this. Due to capillary forces, the plasma then passes into the actual reaction layer 1 where the detection reaction becomes visible, for example, by a colour change, which can be seen from above.

Furthermore, the diagnostic agent according to the present invention can be produced in the manner illustrated in FIG. 14 of the accompanying drawings. There glass fibre paper 3 is stuck on to a substrate 2, the substrate having one or more holes 11 at the point of contact. On the other side, a reaction layer 1 is applied to the glass fibre paper either directly or by sticking on. Blood 8 is applied to the diagnostic agent in such a manner that it can pass through the hole or holes to the glass fibre paper 3. The plasma separated in the glass fibre paper now passes to the reaction layer 1 and gives rise to a reaction which can be evaluated on the surface thereof visually or remission photometrically. The reaction layer can also be protected by a transparent covering layer 7 as shown in FIG. 15.

The reaction layer 1 can consist not only of, for example, a layer "printed" on to the glass fibre mat but can also consist of a multi-layer element in which the layers can contain various reagents and/or can fulfil several functions, as is described, for example, in Federal Republic of Germany Patent Specification No. 29 22 958.

FIG. 16 of the accompanying drawings shows a side view and FIG. 17 a top view of a further construction of a diagnostic agent according to the present invention in which the layer 3 consisting of glass fibres, as well as one or more layers 6 necessary for the reaction can be held together by a previously produced formed body 12. The blood 8 is dropped on to the glass fibre filter side of the diagnostic agent. The separated plasma subsequently passes into the reaction layer 1 and the indicator reaction on the side opposite to the glass fibre filter can be evaluated visually or remission photometrically.

For reasons of clarity of the drawings, in some Figures (for example FIGS. 1 and 16) the various layers are, in part, shown with intermediate spaces. In the practical construction, however, the layers lie on one another in such a manner that liquids can pass unhindered from one layer to another. Furthermore, in FIG. 16 the reaction layers 1 and 6 are illustrated as divided horizontally in order to indicate that they can also consist of several layers lying on top of one another.

The blood separation with the agents according to FIGS. 3, 5, 9 and 11 can be substantially improved when a hydrophobic barrier is applied on the glass fibres 3 beside or in the reaction layer 1 which barrier 15 partly projects into the glass fibres 3. FIGS. 6, 7, 12 and 13 show these barriers 15 but otherwise correspond in their construction to that of FIGS. 4, 9 and 11. This barrier not only prevents blood 8 on the surface of the glass fibres from spreading out and contaminating the indicator zone, but it also decisively improves the separating action of the glass fibres. Therefore, it is possible to operate with relatively small glass fibre papers 3, the result of which is that substantially smaller amounts of blood are required. The barrier can be be applied in a conventional manner, for example from nozzles or with rollers. The hydrophobic material for the barrier can be, for example, a melt adhesive, a conventional solvent adhesive or wax.

Furthermore, FIG. 18 of the accompanying drawings illustrates the construction of a formed body according to the present invention which separates plasma from the erythrocytes of whole blood without contrifuging and makes it available for diagnostic purposes. For this purpose, a tube or a vessel 13, which has, for example, the illustrated shape, is packed with the glass fibres 14. Blood 8 is introduced into the vessel from above. On the downward path, the plasma is separated from the erythrocytes of the blood by means of the glass fibres. The plasma collecting in the lower part of the vessel 13 can now, for example, be withdrawn or sucked off through an "end-to-end" capillary 13a and used directly for diagnostic purposes.

Another construction according to the present invention is illustrated in FIG. 19 of the accompanying drawings. It has vessel 13' appropriate for the separation of the erythrocytes having the form of a piston syringe, the lower part of which is tightly packed with glass fibres 14. The blood 8 is introduced into the open top of the vessel. After separation of plasma and erythrocytes has taken place, the plasma thereby collecting in the lower part of the vessel can be forced out of the syringe by the insertion and careful pressing of a piston 17.

The process according to the present invention for obtaining plasma can, as illustrated in FIG. 20 of the accompanying drawings, also be carried out with the use of a vessel 13". It is divided into two parts by a valve 16 which operates to permit passage in one direction, the vessel 13" being packed with the above-described glass fibres. The blood 8 is introduced into the top of the vessel. After separation from the erythrocytes, the plasma collects in the lower part of the vessel and can then be removed by pressing together the lower part of the vessel, the valve 16 preventing the plasma from flowing back into the upper part of the vessel containing the blood cells.

Furthermore, the process according to the present invention can be carried out with an arrangement as illustrated in FIG. 1, the reaction layer 1 stuck on to the substrate 2 thereby consisting of a definite absorbent material so that, upon applying blood, a definite volume of plasma passes into the layer 1. After separation of the layers 3, 4 and 5, the plasma, i.e. the substance to be analysed, can be eluted by a solvent. The elution of the plasma and the analysis can be carried out immediately but also, depending upon the substance to be analysed, at some later time in another place. If the analysis is only to take place subsequently, it can be advantageous first to dry in the plasma, for example with warm air or by freeze drying. Furthermore, it is possible, separately from the zone for the application of the sample, to provide one or more zones which contain reagents so that, when eluting with a solvent, the whole reaction mixture is simultaneously eluted.

If the reaction colour is to be evaluated not only visually but also in a remission photometer, it is preferable to cover the reaction layer 1 with a covering layer 7 e.g. FIGS. 8, 15, and 21) in order to avoid a contamination of the measurement arrangement. If the reaction layer 1 is a film, for example as described in Federal Republic of Germany Patent Specification Nos. 29 10 134 or 15 98 153, then it is preferable to coat this directly on to the covering layer 7 and then assemble the two together to the substrate. One possible embodiment is illustrated in FIG. 8 of the accompanying drawings. Of course, the covering layer 7 can also be used in the case of the other embodiments, such as is illustrated in FIG. 15 which otherwise corresponds to the construction of FIG. 14.

It has proved to be advantageous to choose arrangements in which the reaction layer 1 is not brought into fluid-conveying contact with the glass fibre layer 3 or the absorbent layer 9 until said layers 3, 9 are completely filled with plasma or serum. The advantages of these arrangements are that the plasma or serum can be brought into contact with the reaction layer 1 at a previously determined and exact time. Furthermore, this contacting takes place over the whole surface area so that chromatographic effects, such as can possibly occur in the case of the above arrangements, are prevented. The fact that, between the application of the blood 8 and the commencement of the reaction in the reaction layer 1, there can be a previously determined time is of great importance in the case of reactions which are to take place under specially controlled conditions. Thus, in the case of the determination of enzymes, which must take place at especially constant temperature, the reaction can first be commenced when the diagnostic agent has a sufficiently constant temperature. Furthermore, the layers 3 and 9 in which the plasma collects can be provided with reagents which bring the substances to be detected into a particular state by a time-dependent reaction, i.e. permit a preliminary reaction to take place to completion. One example of this is the activation of creatine kinase with N-acetylcysteine.

FIGS. 21, 22 and 23 of the accompanying drawings illustrate various possible arrangements. In FIG. 22, the hydrophobic barrier 15 simultaneously serves to secure layers 1 and 7. The description and composition of the other layers correspond to FIGS. 4 to 13.

FIG. 23 illustrates the use of a hydrophobic mesh 18 between the reaction layer 1 and glass fibres 3 or the absorbent layer 9. This hydrophobic mesh protects the arrangement against unintentional slight contact and only permits liquid contact to take place upon applying pressure. This results in an improved practicability.

Of course, it is possible that the reaction layer 1 consists of two or more different zones (e.g. FIGS. 1 to 3 and 16). These can contain either the same substance in different concentration ranges or different substances when the formulations are appropriately selected. Furthermore, it is also possible to make constructions in which various reaction layers are simultaneously moistened by the plasma emanating from a single point of application (e.g. FIGS. 24 et seq.). In this case, a large variety of shapes can be used, such as longitudinal, circular or the like.

FIGS. 24 to 29 of the accompanying drawings illustrate some of these possible arrangements, the various test zones being indicated by 1a to 1d (appropriately primed), the other components corresponding to those of the previously described Figures.

The blood separation can also be improved when, on the glass fibres 3 at the point of dropping on the blood, a further glass fibre zone 3a is applied as in FIGS. 30 and 19. In this case, layers 3a and 3 can consist of the same material but for 3a a material can be chosen with a different thickness or with a different fibre diameter.

FIGS. 30 and 31 of the accompanying drawings illustrate possible arrangements which otherwise generally correspond to FIGS. 8 and 21.

The following Examples are given for the purpose of illustrating the present invention:

0.117 g. methyl hydroxpropylcellulose (Culminal MHPC 8000)

7.000 g. titanium dioxide

0.138 g. monopotassium dihydrogen phosphate

0.479 g. disodium monohydrogen phosphate hydrate

3400 U cholesterol esterase

5000 U cholesterol oxidase

7.times.10.sup.4 U peroxidase

0.476 g. sodium dioctyl sulphosuccinate

are dissolved in 70 ml. water. There are then successively homogeneously incorporated

14.0 g. cellulose

8.4 g. polyvinyl propionate dispersion.

Finally, there is added

0.66 g. 3,3',5,5'-tetramethylbenzidine, dissolved in 1.6 ml. acetone.

This batch is coated in an approximately 300.mu. thick layer on to a smooth synthetic resin film and, after drying at 60.degree.-70.degree. C., cut up into 6 mm. wide strips. Subsequently, these strips, together with a 60.mu. thick meshwork of nylon and a glass fibre paper (glass fibre filter No. 3362 of Schleicher & Schull; paper thickness 0.47 mm., density 0.27 g./cm.sup.3, average fibre diameter about 2.5.mu.), also cut up into 6 mm. wide strips, are stuck on to a polyester film. Subsequently, it is cut up into 6 mm. wide strips.

If 40 .mu.l. of blood are applied to the upper side of the test strip and, after 1 minute, the glass fibre with the residual blood is removed, together with the meshwork, by tearing off, then, within 3 minutes, a reaction colour forms on the test zone which corresponds to that which is obtained when, instead of the blood, use is made of plasma centrifuged off from the same blood.

0.45 g. monopotassium dihydrogen phosphate

1.55 g. disodium monohydrogen phosphate hydrate

1.5.times.10.sup.4 U cholesterol esterase

1.times.10.sup.4 U cholesterol oxidase

3.times.10.sup.5 U peroxidase

2.0 g. sodium dioctyl sulphosuccinate

6.9 g. sodium alginate (Algipon)

are dissolved in 250 ml. water and then 2.0 g. 3,3',5,5'-tetramethylbenzidine, dissolved in 15 ml. acetone, are added thereto, whereafter 20.0 g. kieselguhr are homogeneously distributed therein. This reaction mass is applied in 6 mm. wide strips with a silk screen printing machine (fabric: 190.mu.) on to a glass fibre paper (for example glass fibre filter No. 85/90 of Machery, Nagel & Co.) in the manner described in Example 1. The "printed" glass fibre paper is dried at 60.degree. to 80.degree. C. and then cut up into 12 mm. wide strips in such a manner that the "printed" reaction zone accounts for one half of the strip. This strip is stuck on to the end of the polyester film and this then cut up into 6 mm. wide strips at right-angles to the glass fibre papers. When 40 .mu.l. of blood are now dropped on to the edge of the uncoated glass fibre paper remote from the reagent layer, the plasma diffuses under the reaction zone. Depending upon the cholesterol concentration of the blood, this now assumes a blue reaction colour of differingly strong colour. The intensity of the reaction colour corresponds to that which is obtained when, instead of the blood, there is used a serum or plasma obtained from the same blood.

The papers described in the following Table 1 can be used in the same way:

A reagent mass consisting of

16.0 g. cellulose M+N Ac 10

86.0 g. of a 0.2% solution of methylhydroxypropyl cellulose (Culminal MHCP 8000)

0.32 g. wetting agent (Marlon)

0.68 g. wetting agent (sodium dioctylsulphosuccinate)

12.0 g. polyvinyl propionate dispersion (Propiofan 70 D)

0.48 g. 3,3',5,5'-tetramethylbenzidine

10.0 g. titanium dioxide

9600 U cholesterol esterase

7200 U cholesterol oxidase

1.04.times.10.sup.4 U peroxidase

0.01 g. gallic acid is coated in a thickness of 0.2 mm. on to a hydrophobic polyester fleece (Reemay 2033, du Pont) and dried at 60.degree. C. Subsequently, a 6 mm. wide strip of this coating and a 12 mm. wide strip of glass fibre filter (e.g. Filter 3362 of Schleicher & Schull) are stuck side by side on a firm plastics strip in such a manner that the glass fibre filter is very close to the coated fleece. When strips of 6 mm. width are cut transversely from this plastics strip, test strips are obtained with which, after dropping about 50 .mu.l. whole blood on to the side of the glass fibre filter remote from the reagent fleece, after a short time only pure plasma passes over to the reagent fleece and results in the formation of a blue reaction colour, the intensity of which increases with the concentration of the cholesterol in the blood.

A synthetic resin vessel which downwardly narrows conically (e.g. a synthetic resin tip with a piston pipette, length 5 cm., thickness 0.5 cm.) is loosely filled two thirds full with glass fibres according to the following Table 2, packing densities of 0.1 to 0.4 g./cm.sup.3 being obtained. After the upper free part has been filled with blood, the serum diffuses into the tip of the vessel. From there, an "end-to-end" capillary of 15 .mu.l. capacity can be filled by attachment to the opening of the pipette tip. The plasma obtained in this manner can now be used directly for any desired analytical process.

For demonstrating the effects of a hydrophobic barrier 15, an arrangement according to FIG. 14 was assembled comprising a transparent polycarbonate film 2of 6 mm. width and 0.3 mm. thickness, a 9.times.6 mm. absorbent layer 9 of 0.09 mm. thick glass fibre paper, a hydrophobic nylon meshwork 18 of 0.075 mm. thickness and a transparent covering film 7. A glass fibre paper 3 is fixed in absorbent contact on to the carrier, which paper 3 consists of a glass fibre paper (Schleicher & Schull, No.3362, thickness 0.47 mm.) of 6 mm. width and with the length given in the following Table 3. In each case, one half of these devices is provided on the point of contact between the layers 3, 9 and 18 with an approximately 2 mm. wide and 0.1 mm. thick barrier 15 of paraffin wax.

For the determination of the amount of plasma obtained, on the middle of the glass fibre paper 3 there are applied the amounts of blood indicated in the following Table 3 and, after 30 seconds, the wetting of the absorbent layer 9 is determined, as well as any possible supersaturation (depth of penetration of the erythrocytes into layer 9). The results given in Table 3 show that, due to the hydrophobic barrier, the separation is improved and a complete saturation is achieved so that such a test can be carried out even in the case of very small volumes of blood, for example with capillary blood from a finger tip. In each case, the experiments were carried out five times and the results determined.

It will be understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.