15% CML cells is always 15%.
You need to know the number of cells tested and the number of CML cells found in order to calculate the %.
A FISH test usually looks at 200 cells. If 30 of these cells are CML cells then you have 15% leukemic cells in your body.
A BMA usually looks at 25 cells. If 4 of these were CML cells then you would have 16% CML cells in your body.
Now for the PCR calculation. Big problem. They never tell you how many cells they look at nor do they tell you how many of these cells are CML cells. So you cannot do the calculation.
The reason they do the PCR test is because it is a very sensitive test and once you have attained 0 by FISH test you need some other test to see how well you are doing.
There are 2 types of PCR tests (qualitative and quantitative) that are done. The qualitative PCR test simply reports YES or NO. A % is not reported. This is a handy test if they tell you how many cells they looked at. Some labs look at 10,000 cells and other labs will look at 1,000,000 cells.
The quantitative PCR test reports in 2 ways. The first way will tell you how well you have done compared to where you were when you were first diagnosed with CML. Since they normally don't do a PCR when you are first diagnosed, they guess at what that starting number is. (This is yet a further comlication for us mortals.) The results are then reported as log reduction from the value at your initial diagnosis. An excellent result to have will be a 3 log reduction. This means that the number of CML cells in your body are 1000 times less than you had when first diagnosed. (The assumption is that you were 100% CML positive at diagnosis.
The second way that the PCR test is reported is as a %. This is the part that is very confusing. They don't tell you the number of cells that they looked at, nor do they tell you how many were CML cells so you can't do the calculation. In fact, it is not cells that they look at but parts of a cell. It is a ratio of the bcr-abl gene to a housekeeping gene ... blah blah blah.
All this is to tell you that 15% via a PCR test is not what us ordinary mortals think it is or should be.