Polyethylene evaporation covers

Abstract: There have been problems with the reading of different types of test elements at the same read station due to contamination of certain elements by a by-product produced during the reaction of a element previously occupying the station. Disclosed herein is a evaporation cover (30) and a method of analysis which uniquely solve the problem of contamination of CKMB test elements when phosphorous test elements are incubated in the same incubator (10). The solution is to have the surface of the evaporation cover which contacts the test element (E) be made of a non-porous polyethylene material.

Description: This invention relates to incubators used in clinical analyzers and specifically to the evaporation covers for such incubators. Analyzers using the so-called dried test elements in which the reagents are pre-incorporated in a dried format allow a large number of different tests to be conducted. These include slide-like test elements which react colorimetrically to assay, for example, for phosphorous and CKMB (Creatine Kinase Muscle Brain). In the past, phosphorous tests have been incubated in end-point incubators, along with other end-point assays, while CKMB has been incubated in a separate rate-assay incubator. To simplify the analyzer, there has been a need to make one incubator, hereinafter "single incubator", function for both rate and end-point assays. However, there has been a problem prior to this invention when phosphorous test elements have been incubated together with CKMB test elements. For reasons which were not clear, if phosphorous had been tested at an incubator station prior to a CKMB test element, the latter produced a higher reading than it should have. Although the operator could endeavor to run CKMB tests at a totally different time than phosphorous tests, this is an unacceptable limitation for most laboratories, particularly those involved in STAT (immediate) operations, that is, those which must be run immediately. Yet another problem encountered by certain test elements is which their detected levels of analytes have been routinely higher than expected, when tested on this single incubator, particularly when compared to the reading obtained on a dedicated colorimetric analyzer. These are elements in which the spreading layer first contacted by the sample is a beaded spreading layer of the type described in US-A-4258001, particularly useful in assays such as total protein and albumin. Again, the cause of this positive bias has not been clear prior to this invention. It will be understood that such bias could be acceptable if the math modeling were adjusted to correct for the bias (when providing the conversion of reflectance to concentration). However, in switching to a single incubator for both rate and colorimetric assays, it is preferred that as much as possible of the software used in the conventional colorimetric analyzers and rate analyzers be adapted or transferred to the single incubator. Since the bias present, e.g., in total protein takes the values completely out of the "normal" reading domain heretofore used, such transfer heretofore has not been possible. Japanese Kokai 63/47661 published February 29, 1988 teaches the use of a wide variety of non-porous, non-absorbing polymers for evaporation covers at incubator stations featuring dried test elements. The polymers listed as being useful are polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate, butylene terephthalate and polyethylene-polybutylene terephthalate. The purpose is stated to be to reduce the absorption of any reactive gas produced by a test element. However, the only reactive gas which is specifically discussed is ammonia from BUN (Blood Urine Nitrogen) test elements. There is no appreciation of the cause of the problem created when one incubator has to be used for both phosphorous test elements and CKMB test elements. There is, furthermore, no discussion of a colorant to be used in the cover, or of problems associated with test elements constructed using beaded spreading layers. It has been discovered the causes of the above-noted problems. In the case of the CKMB bias, SO ₂ gas produced by the phosphorous test elements as a by-product is an interferant in the CKMB reaction and will carry-over in amounts that can affect a subsequent CKMB test element. The above problems have been addressed by determining that only polyethylene has superior resistance to the SO₂ carry-over. As to the bias occurring with beaded spreading layer elements, this is caused by their translucency, and the passage of detecting light out of the elements so as to reflect off a heretofore non-white evaporation cover used in the single incubator. More particularly, in accordance with one aspect of the present invention, there is provided a method for preventing the biasing of a CKMB test element incubated in the same incubator station previously occupied by a phosphorous test element, the station comprising an evaporation cover which covers a porous top of either of the CKMB or the phosphorous test elements, the method comprising the steps of: inserting a phosphorous test element into an incubator station with patient sample thereon so that SO₂ gas is generated as a by-product at the station, reading the phosphorous test element, removing the phosphorous test element from the station, and thereafter inserting a CKMB test element into the same station as was occupied by the phosphorous element and reading it, characterized in that the evaporation cover is made from a polyethylene material and is effective to prevent SO₂ gas produced by the inserted phosphorous test element from being absorbed by the cover, and carried over to the inserted CKMB test element. It is an advantageous feature of the invention that a single incubator can be used with both phosphorous and CKMB test elements without regard to the timing of those elements with respect to each other, and without the phosphorous test element contaminating the CKMB test element. It is another advantageous feature of the invention that such an incubator can be used with test elements featuring beaded spreading layers without the development of a bias due to unwanted light absorption. The present invention will now be described by way of example only with reference to the accompanying drawing, the single figure of which is a fragmentary sectioned elevational view of an incubator constructed in accordance with the present invention. The invention is discussed hereinafter in connection with preferred embodiments, that is, with an incubator of a particular construction, and with test elements such as those made under the trademark "Ektachem" by Eastman Kodak. In addition, the invention is useful in any incubator, and with dried test elements of other formats, provided that the incubator has both phosphorous and CKMB test elements run in it. As will be seen by the following data, the presence of SO₂ gas, a by-product of the phosphorous test element, will reduce the reading which is the correct reading in a CKMB test element. As a result, unacceptable standard deviations of three or more, for an expected reading of 50, can occur. This is corrected by using a polyethylene material as the evaporation cover, or at least the surface in contact with the porous top of the test elements that have received sample liquid. The polyethylene can be clear, or colored with any color or colored with a white pigment or dye for the reasons noted hereinafter, of sufficient density as to be non-porous to gas, for example, high-density polyethylene. The entire cover can be such polyethylene, or it can comprise a support material such as metal, bearing polyethylene on the exterior surface which contacts the test element. In some cases, the test elements can have a translucent, beaded spreading layer as described in US-A-4258001. Alternatively, a spreading layer can be used which is opaque, that is, incorporates a light-blocking agent. If the spreading layer is not opaque, the scanning light of the reflectometer at the reading station tends to penetrate beyond the test element, and is reflected off the evaporation cover. If, such covers are of a material other than white, as has been customary in incubators other than those dedicated to colorimetric assays, light is reflected back with a spurious decrease in intensity because of its partial absorption by the cover, even though there may be little or no dye produced by the assay in the element. When using such translucent spreading layers, the polyethylene cover preferably includes a white colorant, that is, a pigment, to prevent spurious absorption. Figure 1 illustrates a useful incubator 10, in which a plurality of stations 12 are mounted on a ring 14 journaled at 16 to a platform 18. (Only one station is shown.) Ring 14 can have gear teeth at 20 to engage a driving gear 22 operated by motor 24 to rotate the stations about axis 25. Each station 12 has a support plate 26 attached to ring 14, a spring 28 pressing down towards plate 26, and an evaporation cover 30 with a boss 32 projecting through the spring. A test element with a porous top surface 34, pre-wetted with liquid sample, is held between cover 30 and plate 26. Preferably, these height locating buttons 40 are spaced about stationary surface 42, one of which is adjacent read station 50, to ensure that the proper vertical distance is maintained between element E and a light source 52 which forms part of the read station. (A photodetector (not shown) is also present at the read station to receive reflected light as indicated by arrow 54.) Aperture 56 is provided in plate 26 to allow the reading of the element. This single incubator provides for the reading of both colorimetric and rate test elements at read station 50. In accordance with the invention, all, or at least the surface part 60 of cover 30 which contacts element E is made from a polyethylene material as described above. The polyethylene used preferably includes a white colorant if the test elements E to be incubated and read are the type with a translucent beaded spreading layer. Examples 1 to 4 To demonstrate the significance of the selection of polyethylene, and that it is unique as a material, four different batches of pooled human serum, all having a known CKMB activity of 50, were tested on "Ektachem" CKMB test elements using an incubator which had just prior to the CKMB test element, assayed for phosphorous using an "Ektachem" brand phosphorous test element. (The phosphorous test also used pooled human serum having the same amount of phosphorous in each case.) There were six incubator stations, and each was filled with the same kind of test element for each "fill". The results of the six stations were averaged and the standard deviation determined for n=6, for each "fill". The "fill" number refers to the nth test element to be placed in that incubator station after the phosphorous test. In particular, fill #1 was the first test element placed there, #2 the second, and so on. The results are listed in Table I. Two factors are important in this Table, namely, the consistency from fill to fill in Part A, and the standard deviations of Part B. In the first of these, polyethylene was more consistent in its reading, ranging in the case of natural polyethylene from 46.391 to 50.736. Polypropylene on the other hand had a much bigger, and therefore unacceptable, range. This is supported by the second factor, the standard deviations. A standard deviation for polypropylene which is as high as 7.9 is totally unacceptable. On the whole, it was clear that polyethylene, with or without a colorant, was more likely to minimize error than any of the other materials (polypropylene, teflon or metal).

Claims: A method for preventing the biasing of a CKMB test element incubated in the same incubator station previously occupied by a phosphorous test element, the station comprising an evaporation cover which covers a porous top of either of the CKMB or the phosphorous test elements, the method comprising the steps of: inserting a phosphorous test element into an incubator station with patient sample thereon so that SO₂ gas is generated as a by-product at the station, reading the phosphorous test element, removing the phosphorous test element from the station, and thereafter inserting a CKMB test element into the same station as was occupied by the phosphorous element and reading it, characterized in that the evaporation cover is made from a polyethylene material and is effective to prevent SO₂ gas produced by the inserted phosphorous test element from being absorbed by the cover, and carried over to the inserted CKMB test element. A method according to claim 1, wherein the polyethylene material includes a white colorant, to reflect any light which passes through a test element in the station without adding to the detected density of the test element.

Inventors: JACOBS MERRIT NYLES C O EASTMA, PRZYBYLOWICZ CATHERINE S C O E, JACOBS, MERRIT NYLES, C/O EASTMAN KODAK CO., PRZYBYLOWICZ, CATHERINE S., C/O EASTMAN KODAK CO.
Applicants: EASTMAN KODAK CO, CLINICAL DIAGNOSTIC SYSTEMS, INC.
Application year: 1990
Main classification: C12Q1
IPC codes: G01N33, G01N31, G01N35, C12Q1