Assessment of current test procedures.

The belief that screening tests for teratogenicity are of low predictive value has many supporters who point to the inconsistency with which malformations are induced. However, to fault the test systems when such inconsistency is predictable from both the inherently unstable nature of a malformation and from fundamental principles of teratology, is unrealistic, and, as is shown, perhaps the greater faults lie with the critics. It is suggested by examples that, if attention was concentrated not on the inconsistent malformations but on more consistent embryopathic effects which in one form or another are always associated with malformations, the predictive value of the screening tests would appear in a more favorable light. Thus, even if malformations are not demonstrated, the range of conditions (dosages) in which they might occur can be determined. Such information, used in conjunction with that obtained from other preclinical studies, can then form a reasonably sound basis for extrapolation to man.