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Six cytogenetics laboratories joined in a collaborative study of rat chromosome aberrations to measure interlaboratory variation in results of standardized procedures and to devise methods to minimize interlaboratory differences. A preliminary workshop was held to resolve scoring differences, to develop a joint protocol and common glossary, and to reach agreement on uniform reporting methods. Osborne-Mendel rats from a common source were sent to each laboratory. Triethylenemelamine (TEM) was used at doses of 100, 200, 300 and 400 microgram/kg to induce clastogenic effects; results were compared to those of a control group of untreated animals. Femoral bone marrow cells were evaluated with the scorers unaware of the dosage. Final results showed highly significant dose effects with the test compound, and most laboratories showed a similar pattern of dose response. This study illustrates that rat cytogenetic analysis can be an effective test system for evaluation of a compound for mutagenic potential, particularly for the index reflecting the proportion of abnormal cells, but that results should be interpreted cautiously when arbitrary values are assigned for some of the categories being analyzed, as was done in this project for the category of severely damaged cells.

Cytogenetics data resulting from one laboratory in a multiple-laboratory study were analyzed to determine if 5 well-trained scorers produced significantly different results using metaphase scoring procedures. Although the scorers reached the same general conclusion, results show that scorer differences exists (p less than 0.01). Consequently, all participating scorers in a laboratory should be used equally in all treatment groups and the results should be analyzed accordingly to account for scorer variations. This is easily accomplished in controlled prospective experiments; however, it is often difficult in retrospective studies using data which exists. In such studies, every effort should be made to analyze and interpret the data so that scorer differences are taken into account. For severely damaged cells not only were there scorer differences but the difference were greater at higher doses. This phenomenon may be related to the operational definition of a severely damaged cell, since scorers who identify more damage than other scorers would logically tend to classify more cells as severely damaged both overall and at lower doses.

In a large multilaboratory cytogenetic study of interlaboratory variations using 4 dose levels of triethylenemelamine and a control, a severely damaged cell was operationally defined as a cell which contained at least 10 aberrations of any type. A review of this study suggested that the use of this definition introduced a bias in the measurement and interpretation of results for the other cytogenetic categories studied. As a result, the original severely damaged cells were carefully reanalyzed to investigate the characteristics of this bias and to seek procedures to minimize or eliminate it. Results characterize this bias and demonstrate that when a severely damaged cell is defined as one containing at least 20 aberrations and those aberrations in the remaining non-severely damaged cells are classified by specific type, the bias is significantly reduced and chromosome analysis can be improved as a test system.