THE EFFECT OF REDUCING THE NUMBER OF CELLS SCORED ON THE PERFORMANCE OF THE IN-VIVO RAT-LIVER UDS ASSAY

The most labour-intensive feature of the in vivo rat liver UDS assay i s the scoring of hepatocyte autoradiograms by microscope. Even with im age analyser and computer equipment the scoring phase of a full study might require half of the technical effort applied. Practice recommend ed by guidelines has been to score 50 cells/slide and two slides per a nimal. Now sufficient data have been accumulated, an evaluation was ma de to observe whether this procedure was necessary. An analysis of the accumulated UDS database in our laboratory was made to determine the sources of variability of mean net nuclear grain count, [N - C]. It wa s observed that the two largest components of variation in negative co ntrol animal mean [N - C] were between-day and interanimal variability . The contribution from sampling error during slide scoring was relati vely small. Theoretical calculations showed that the greater sampling error derived from scoring 30 rather than 50 cells/slide would result in only a marginal increase in total assay variation. To test this, 30 cells/slide were randomly selected from the 50 cells scored originall y in negative control animals in each of 18 studies over an 18-month p eriod. It was confirmed that reducing the number of cells had a neglig ible effect on the variation of negative control animal mean [N - C] v alues. Furthermore, analysis of data from 10 more studies confirmed th at within-study variation would be essentially unaffected by scoring 3 0 cells/slide. The use of 30 rather than 50 cells per slide (a total o f 60 cells per animal) has therefore been adopted for al current studi es and scoring procedures modified to avoid operator bias during the s election of a smaller number of cells.