Ki-67 is a tumor marker that indicates cell proliferation or how quickly a cancer cell copies its DNA and divides into two (2) cells. The more rapidly the cells divide, the faster the cancer is growing or the more aggressive it is. Ki-67 testing was added to the AJCC 8th Edition Staging Manual and may be used for treatment planning purposes or to estimate treatment outcomes. It is important to note that test results may vary depending on the lab doing the testing, testing method(s) or what part of the tumor is assessed.
Ki-67 is a relatively new data item (NAACCR Item #3863) for breast cancer cases diagnosed 1/1/2018 or later (AJCC Chapter 48). Coding is site-specific and instructions are found in the NAACCR Site Specific Data Items (SSDI) manual on the NAACCR website. Of note, Ki-67 is also reported for neuroendocrine (NET) tumors of the GI tract (AJCC Chapters 29-34) for cases diagnosed 1/1/2021 and after. Refer to the SSDI Manual for GI tract specific coding instructions for neuroendocrine tumors (NET).
General Coding Instructions
- A physician statement of Ki-67 may be used for coding if the actual test results are not in the medical record.
- If there is no evidence of a primary tumor, code results from nodal or metastatic tissue.
- Ki-67 test results are only reported as a percentage using a single decimal point ranging 0.0-100.0. See also clarification below for results reported as “less than 1%.”
- Ki-67 results are reported as a percentage of the cell nuclei that stain positive. Going back to as early as 2017 there are no established standards for interpretation of the results or cutoffs for positive or negative values. As noted above, there are a number of variables including the testing lab, its test methods or what part of the tumor is submitted for testing.
- For example:
- Ki-67 reported as 14% would be coded in the abstract as 14.0,
- Ki-67 reported as 8.6% would be coded as 8.6.
- For example:
NAACCR Version 2.1 SSDI Codes and Descriptions
| Code | Description |
| 0.0-100.0 | Enter the percent positive as 0.0-100.0 |
| xxx.7 | Denotes the test was done but the actual percentage was not stated in the report or physician consult or note |
| xxx.8 | Test information was not collected for this case, or it is not applicable |
| xxx.9 | Ki-67 (MIB-1) was not assessed, or it is unknown if assessed; not documented in the medical record |
Coding Clarifications
There are coding clarifications found in the SSDI Manual or the Commission on Cancer (CoC) CAnswer Forum.
- Edit Error: Some standard setting agencies may require coding for Ki-67 so CTRs should be aware of their facility’s specific requirements. If Ki-67 is required, coding xxx.8 for NA or test results not collected may result in an edit error. (SSDI Manual, version 2.1)
- Test Results Less Than 1%: for test results less than 1% use the decimal points and code one less than the value. For example, “less than 1%” would be coded as 0.9. (CoC CAnswer Forum).
- Test Results from both in situ (DCIS) and invasive (IDC) components: in the 2022 SSDI manual update, clarification was added that addresses using the Ki-67 from both an in situ (DCIS) or invasive (IDC) tumors as described below. (CoC CAnswer Forum)
- If there are both an invasive and in situ components in the primary tumor and the Ki-67 is reported for both tumors, ignore the in-situ results and code the value from the invasive component.
- If in situ and invasive components are present and Ki-67 is reported only on the in-situ component, code as unknown.
In summary, the cancer registrar should know if their facility, state, or accreditation program requires coding of the Ki-67 data item. If it is required, then review of the SSDI manual and the clarifications noted above should provide the background needed to correctly code this data item and avoid any edit errors.
