A simple method for incubation of tissue specimens with tritiated thymidine under 3 atm oxygen tension gave an in vitro labeling index (LI) that corresponded closely with LI’s determined by injection of tritiated thymidine in vivo. The LI of tumors was not affected by storage at room temperature for up to 135 minutes. The mean LI for 39 human mammary carcinomas was 3.8, but the distribution was skewed to the right, and the most frequently observed LI’s were between 2 and 3. Larger carcinomas had significantly larger LI’s than smaller tumors, which suggests that the larger tumors were faster-growing at the time of observation than the smaller tumors. The capacity of the larger tumors to maintain a relatively rapid growth rate may account for their large size. The data suggested, but did not prove, that a high LI was associated with more frequent metastasis to axillary lymph nodes.

Meyer, Bauer, , , , , , , (1975). In vitro determination of tritiated thymidine labeling index (LI). Evaluation of a method utilizing hyperbaric oxygen and observations on the LI of human mammary carcinoma. Cancer, 1975 Oct;36(4):1374-80. https://www.ncbi.nlm.nih.gov/pubmed/1100222