Liquid purification or separation – Processes – Separating
Reexamination Certificate
1998-09-01
2001-06-12
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
Processes
Separating
C210S406000, C210S435000, C210S505000, C210S645000, C210S767000, C422S051000, C422S105000, C436S177000
Reexamination Certificate
active
06245244
ABSTRACT:
The invention relates to a method for separating plasma from whole blood. in which:
a) a mixture of whole blood, a blood anticoagulant and a diluent is prepared in a container provided with a sealing cap, and
b) the mixture obtained under a) is passed through a filter containing glass fibres.
Belgian Patent Application 849,898 describes a device for diluting and filtering a sample having a very small volume, such as, for example, a blood sample. Said device comprises a container which is made of a flexible plastic material and which contains a liquid diluent for the sample to be treated. The container is furthermore provided with a hollow cylindrical neck whose extremity is sealed with a removable stopper, and a cylindrical tube made of a flexible plastic material. The internal diameter of the cylindrical tube is essentially equal to the external diameter of the neck. The extremity of the cylindrical tube is inserted into a nozzle or fixed on a filter, the nozzle being sealed by a cannula. The container may optionally contain a capillary which contains a measured quantity of the sample, for example a blood sample. To dilute and filter, for example, a blood sample, the procedure is as follows. First the stopper is removed and the blood sample is transferred to the container either by means of the capillary or directly with the aid of an hypodermic syringe. The cylindrical tube is then mounted on the neck of the container and the device is shaken in such a way that a homogeneous mixture of the blood sample and the diluent is obtained, care being taken to ensure that the filter is not moistened. The homogeneous mixture is then filtered by squeezing the container, in which process the homogeneous mixture passes through the filter and a filtrate is obtained which has the desired dilution. A disadvantage of said device is that it is unsuitable for separating plasma from quantities of whole blood of, for example, less than 100 &mgr;l because the yield of pure plasma is low when such a small quantity of whole blood is separated. A further disadvantage of the device according to Belgian Patent Application 848,898 is that a large dilution has to be used in order to obtain an adequate volume of the homogeneous mixture: the volumetric ratio of the diluent and the blood sample should be between 10:1 and 1000:1. After dilution and filtering, therefore, a very dilute plasma will be obtained, as a result of which only diagnostic tests having a low sensitivity can be carried out.
In medical diagnostics, the separation of plasma from whole blood is extremely important for analysing constituents present in the blood. Such analyses often take place with the aid of rapid diagnostic means. Rapid diagnostic means are, for example, substrates which comprise a separating matrix for separating plasma from whole blood and a test reagent. In this procedure, a drop of whole blood is applied to the substrate, clear plasma passing through the separating matrix and the blood corpuscles, such as erythrocytes and leucocytes, remaining behind in the matrix. The plasma then reacts with the test reagent, in which process a colour change occurs which is then evaluated visually or spectrophotometrically. A disadvantage of said rapid diagnostic means is that haemolysis of erythrocytes may occur, in which process haemoglobin is entrained with the plasma and may interfere with the colour reaction.
These problems do not occur, for example, in the case of rapid diagnostic means in which plasma containing no haemolysis products is applied instead of whole blood. In this case, the plasma first has to be separated from the whole blood by means of centrifuging, after which the plasma can be separated from the blood corpuscles deposited by means of pipetting. The separation of plasma from whole blood by this method is not easy if it has to be carried out on a small scale. This is primarily of importance in the case of, for example. children, where the taking of a small amount of blood by means of a heel or finger prick results in far fewer problems with the patients than if blood is collected with the small vacuum tubes normally used. A small vacuum tube is a small tube which has a capacity of approximately 7 to 10 ml, which is provided with a stopper made, for example, of rubber and which has to be fitted on an injection needle after the injection needle has been introduced into a vein, for example a vein in the arm (venepuncture). Because a lower pressure prevails in the small vacuum tube than in the vein, blood will flow out of the vein into the small vacuum tube.
Another disadvantage of the method described above is that the separation of the plasma has to be carried out by expert staff or by staff specially trained for the purpose and that special equipment is required. It is therefore not possible to collect whole blood and to separate the plasma therefrom at the same operational site. This is of importance, for example, under circumstances in which urgent help has to be offered and one or more diagnostic tests have to be carried out and special equipment is not available. Thus, for example, a general practitioner or a medical specialist will possibly consider it necessary, when visiting patients at home or in cases where acute help is needed, respectively, to carry out a certain test but will not be in a position to extract the required amount of plasma rapidly and expediently. Another example is the evaluation of new diagnostic blood parameters for which no specific whole blood tests are generally available for a long time. Only small amounts of blood are also necessary for many diagnostic tests, for example tests for the determination of the concentration of a metabolite, a hormone or an antibody. Furthermore, for example, veterinary surgeons cannot obtain an amount of plasma for a certain diagnostic test from an animal in its own living environment, for example a cow in its shed.
U.S. Pat. No. 4,477,575 describes a method for separating plasma from whole blood in which whole blood is applied to a layer of glass fibres. A device is also described for separating plasma from whole blood, the device comprising a conical plastic vessel, the top and bottom of the vessel being open. The lowermost part of the vessel is filled with glass fibres. The blood is then introduced into the free section of the vessel, after which plasma flows through the layer comprising the glass fibres and can be removed at the bottom with the aid of a capillary. U.S. Pat. No. 5,262,067 describes a device for separating plasma from whole blood, in which little haemolysis occurs and in which the device comprises a layer comprising glass fibres coated with polyvinyl alcohol or polyvinyl alcohol/polyvinyl acetate, and a means which causes erythrocytes to aggregate. If an amount of whole blood is applied to the top of the layer, erythrocytes are aggregated and plasma containing little or no haemolysis products passes through the layer. This method and device have, however, the disadvantage that they are unsuitable for expediently separating and isolating plasma from small amounts of whole blood so that said plasma can be used for other diagnostic tests.
The object of the invention is to solve the abovementioned problems and it therefore relates to a method as stated in the preamble, in which a pressure which is lower than the pressure prevailing at ambient temperature prevails in the container. The invention also relates to a kit for separating plasma from whole blood.
Advantages of the method according to the invention are that the method is particularly suitable for separating plasma from small amounts of whole blood, for example amounts of less than 100 &mgr;l . Such amounts can be collected by means of a heel or finger prick in a particularly patient-friendly manner. In addition, such amounts are in many cases quite sufficient for many diagnostic tests, for example tests for determining the concentration of a metabolite, a hormone or an antibody in blood.
For the heel or finger prick, a capillary is advantageously used which comprises a containe
Drodge Joseph W.
Micro Diagnostic Innovations Nederland B.V.
Young & Thompson
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