This week's “Ask Dr. Durie” comes from a patient who wants to know: “What is minimal residual disease (MRD)?” And more particularly, what does it mean when someone says that it's at ten to the minus five or ten to the minus six? This has become an increasingly common question as we focus more on achieving deep responses with the best currently available therapies.
We are trying to push to achieve the maximum response and have the minimal amount of residual disease remaining in the bone marrow. And so, to test for this, a sample is obtained of the bone marrow and the bone marrow sample is processed and put through a flow cytometer to see if any myeloma cells are remaining in the bone marrow.
Now, you can put through different numbers of cells. And so, it's ideal to have as many cells as possible. And so, one option is to have 1 million cells processed through the flow cytometer—that is ten to the sixth cells. And so, if ten to the six cells, a million cells are put through the flow machine and no myeloma cells are found—zero myeloma, this is called negative at ten to the minus six.
A million cells were checked, and no myeloma cells were found. If you put through 100,000 cells and no myeloma is found, this is called negative at ten to the minus five. Ten to the minus five cells were checked, and no myeloma was found. And so ,this is how we look at the outcomes when we are trying our best to achieve the best results. And so ,we focus on this because if the bone marrow is negative at this ten to the minus six or ten to the minus five-level, then this correlates with really improved outcomes.
And so, this is a key yardstick that we use to assess the impact of different new therapies so that we can compare one therapy against another. And so BOTTOM LINE, MRD means minimal residual disease. Ten to the minus six means negative after checking a million cells, and ten to the minus five means negative after checking 100,000 cells.
And so, these are things to keep in mind as you look at those numbers, comparing one treatment versus another.