Fig. 2. A conceptual representation explaining the presence of false positives in an in vitro assay. The circle outlined in red shows the entire collection of chemicals that cause developmental toxicity in vivo caused by any mechanism. The orange circle represents only those chemicals that cause developmental toxicity in vivo by the principal mechanisms in the mechanism-based in vitro assays; these represent a small part of the whole. The light blue circle represents chemicals classified as positive in the in vitro assay. A considerable number of chemicals cause developmental toxicity by mechanisms other than the principal mechanisms (overlap of the circle outlined in red and the light blue circle, exclusive of the orange circle). In a strict sense, these are incorrectly detected as positive. Nevertheless, such in vitro assays may have been evaluated as good models (single tests) for detecting many developmentally toxic chemicals, possibly leading to the high false positive rates of in vitro assays.
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