Background: The NCCN guidelines recommend BCR-ABL kinase domain mutation testing when CML patients show a loss of response to treatment. About 20% of patients with CML started on a first-line TKI will develop treatment resistance due to mutation in the BCR-ABL kinase domain. A “rule of thumb” is to conduct mutation testing when BCR-ABL results rise 10X or higher. We studied a group of CML patients who had mutation testing after treatment started to determine if the 10X rule of thumb is being followed. Methods: We studied data from 183 CML patients in the nationwide Medivo lab test database who were tested for BCR-ABL between 2011 - 2014. This group of patients had at least 1 mutation test after a positive slope in their previous BCR-ABL tests. We performed a simple descriptive analysis to identify the proportion of these patients who were tested for mutation when previous BCR-ABL test results had risen < 10X. Results: We found 33 instances (18%) where patients were tested for BCR-ABL mutation soon after a 10X or higher rise in BCR-ABL results, and 150 instances (82%) where patients were tested for BCR-ABL mutation when previous BCR-ABL results had risen < 10X. We are 95% confident that the true proportion of patients tested for the BCR-ABL mutation when previous BCR-ABL results had risen < 10X is between 76% and 88%. The median value for the ratio of current BCR-ABL/previous BCR-ABL for all 183 patients before mutation testing is 2.4 [IQR: 1.4 - 6.2]. Conclusions: Current NCCN guidelines recommend mutation testing in patients with CML showing lack of response, loss of response or disease progression. Our results show that mutation testing is occurring more often prior to a 10X rise in BCR-ABL test results than after a 10X rise, indicating that the “rule of thumb” is not being followed in most cases. Further study is needed to determine the underlying reasons for the differences between the groups of patients tested prior to a 10X rise and the group tested after a 10X rise in BCR-ABL test results.