Ambiguity of the thymidine index.

The observed thymidine indices of seven experimental tumor lines are compared as a function of duration of emulsion exposure. The effects of dose level of tritiated thymidine and background threshold are also evaluated. The results indicate that an arbitrary high background threshold discriminates against "lightly" labeled cells at short periods of exposure but that the chosen threshold becomes less critical with longer exposure. The observed thymidine index increases with increasing duration of emulsion exposure but appears to approach a plateau for all tumor systems. The "thymidine index curves" are significantly different for each tumor. There is an inverse relationship between the dose of tritiated thymidine and the duration of exposure required to recognize the same fraction of cells as labeled in a given tumor. Similar experimental conditions do not necessarily guarantee a valid basis for comparison of observed thymidine indices among tumors.