MSI Results

POSITIVE RESULT

Once a tumor is classified as MSI + (or MSI-H or MSI-high), the next step is to sequence the gene(s) most likely to be mutated, using a blood sample from a relative who had/has colorectal cancer. 

Identifying the gene(s) responsible for a family cancer by gene sequencing is a complex task, only possible in individuals who have/had cancer and have higher chances of the mutation.

A mutation will serve as a "flag" to find the gene.

If available, a blood sample from another relative may also be tested as long as he/she had colorectal cancer.

If a hereditary mutation is detected, the test can then be offered to healthy family members to assess their potential risk of cancer.

NEGATIVE RESULTS

The microsatellite instability (MSI) test is particularly relevant if the result is positive supporting suspicions of HNPCC. A negative result is much less informative.

If there is a significant family history, a negative result does not rule out HNPCC.
Although it might indicate that the familial cancer is the result of another type of colorectal cancer syndrome/mutations.

For this reason, the microsatellite instability test (MSI) should always be interpreted in the context of that family history of colorectal cancer.

If there are two or more negative MSI results in a family, HNPCC is unlikely.

The advice, in case of a negative MSI result, is to see a geneticist or a medical specialist who can advise you on the measures suited to your level of risk. Even with a negative MSI result, additional screening measures might be necessary.

INCONCLUSIVE RESULTS

A result of MSI- (= MSI-L or MSI-low) or an undetermined MSI means that the result is not informative/is inconclusive.

It is a situation similar to when there are contradictory results (e.g., one relative has an MSI result - and another MSI +).

In these cases, testing other family members who have developed cancer may add vital information to understand the family genetics.