Division 1.3.4 Calculation of results

Results are expressed as the Stimulation Index (SI). When using the pooled approach, the SI is obtained by dividing the pooled radioactive incorporation for each treatment group by the incorporation of the pooled vehicle control group; this yields a mean SI. When using the individual approach, the SI is derived by dividing the mean DPM/animal within each test substance group and the positive control group by the mean DPM/animal for the solvent/vehicle control group. The average SI for vehicle treated controls is then 1.

Use of the individual approach to calculate the SI will enable the performance of a statistical analysis of the data. In choosing an appropriate method of statistical analysis the investigator should maintain an awareness of possible inequalities of variances and other related problems that may necessitate a data transformation or a non-parametric statistical analysis. An adequate approach for interpreting the data is to evaluate all individual data of treated and vehicle controls, and derive from these the best fitting dose response curve, taking confidence limits into account (8)(12)(13). However, the investigator should be alert to possible “outlier” responses for individual animals within a group that may necessitate the use of an alternative measure of response (e.g., median rather than mean) or elimination of the outlier.

The decision process with regard to a positive response includes a stimulation index ≥ 3 together with consideration of dose-response and, where appropriate, statistical significance (3)(6)(8)(12)(14).

If it is necessary to clarify the results obtained, consideration should be given to various properties of the test substance, including whether it has a structural relationship to known skin sensitisers, whether it causes excessive skin irritation and the nature of the dose response seen. These and other considerations are discussed in detail elsewhere (7).