The significance of matrix effects on the measurement of lactate dehydrogenase (LD) activity using Kodak dry slide technology in the Ontario Laboratory Proficiency Testing Program

Impact of reference materials on accuracy in clinical chemistry

The analytical accuracy of the results of routine clinical chemistry measurements is contributed by a two-steps mechanism, involving transferring trueness from a higher metrological and monitoring the time-stability of trueness itself. In both operations, different materials are used: however, accuracy in the routine assay of genuine patient samples has to be the end product of this overall process. To such an aim, the materials must show an intermethod behavior similar to that of patient sera, i.e., they have to show commutability. Definitions of commutability and methods for assessing such a property are mentioned. The following aspects of lack of commutability of materials are then discussed: frequency; effects on the measured interlaboratory variability; and effects on the recalibration of analytical systems. The causes giving rise to lack of commutability are neither clear or easy to be shown. Matrix effect is one of the main causes; also, differences in the characteristics of the component being measured are often responsible for noncommutability of materials for enzyme activity measurements. Examples of these two different situations are given. It is concluded that, for an efficacious overall quality assurance process, either a set of minimally processed patient sera or commutable reference materials are to be used in the operations concerned with the control of trueness. An additional alternative approach is based on the use of materials with system-specific assigned values.

Changes in lactate dehydrogenase isoenzymes associated with relapse of childhood acute lymphocytic leukemia

Twenty-eight children with high-risk acute lymphocytic leukemia underwent monthly serum lactate dehydrogenase (LDH) and LDH isoenzyme fraction determinations to examine whether LDH isoenzyme fractions change with an increase in the body burden of tumor cells. The 9 patients who relapsed and 5 patients who presented with leukemia during the study period had a slightly lower mean LDH-1 isoenzyme fraction. When the period from 3 months before to 3 months after relapse was examined, significant increases in the LDH-3 isoenzyme fraction and decreases in the LDH-1 and LDH-2 isoenzyme fractions were seen at the time of relapse. These results were highly significant when patients with non-T-cell and T-cell leukemia were combined and when bone marrow and central nervous system relapse was included. The changes at relapse appeared to revert with intensification of chemotherapy. The changes at relapse were not different in magnitude from random variation occurring in patients who remained in remission throughout the study. Although changes in LDH isoenzymes appeared to occur at the time of relapse compared with the periods immediately before and after relapse, these changes were not specific for relapse. LDH isoenzymes do not appear to be useful in predicting relapse in children with leukemia.

A programme for assigning target values for external quality assessment schemes in countries with no authorized reference laboratories. Annex. Experiences with deviating results on Ektachem 700 XR

The use of consensus values in external quality assessment schemes (EQAS) involves several problems and should preferably be replaced with target values obtained by methods of high metrological level. However, such values are difficult to obtain. In the present study we transferred values from the NIST (former NBS) certified reference serum SRM 909 to lyophilized and frozen test sera for various inorganic components using flame absorption or flame emission spectrometry. Enzyme values were assigned by laboratories of members of the former Scandinavian Enzyme Committee. The assignment was based on 2-4 determinations each day through 3 days of experiment. A total of 10 laboratories participated in the work. The results were utilized in a Danish EQAS. One practical concern is the fairly long time (9 months) which was needed for production, collection and compiling all data. To get an impression of how much dry chemistry analysers, e.g, could influence consensus values a Kodak Ektachem 700 XR was studied using lyophilized and frozen sera. The results are reported in the annex. On NIST SRM 909 the values found for sodium(I) were 6% too high even though the findings on frozen human sera were accurate. For aspartate aminotransferase a result three times the target values was found on a human lyophilized serum, while the values on the frozen sera only were slightly too high.