To understand PCR results, you need to understand the actual procedure. DNA is taken from cells and "encouraged" to replicate (make copies of itself). Depending on the amount of time that it is given, you end end up with either 1,000,000 to 1,000,000,000 copies of the DNA. This increased number is then tested for the presence of a specific piece of DNA.
So - 1. This is extraordinarily sensitive. As a consequence, you might have only 1 cell in your entire body that is positive and it might show up here.
2. The test does not discriminate between live cells, damaged cells, or dead cells. So it is possible that the positive is a result of the dead cell's DNA or the cell that cannot multiply because the rest of the cell is so damaged or a cell to be concerned about.
3. As with all tests but especially highly sensitive ones, there is always the possibility of false positives. Exactly how trusting are you that your DNA can copy itself a billion times under stressed conditions (shortened time) with absolute perfection? And even if a mistake happens only once, it will result in a positive just as if the DNA was really there.
4. In remissions, the first signs are seen in the blood stream, followed by the bone marrow aspirate/biopsy, then by the cytospin in the fluids, then by chromosomal analysis, then FISH and finally PCR. So the last place to "become" negative will be the PCR . Commonly, the term "complete remission" refers to the absence of the disease in the blood and bone marrow. Chromosomal remission means nothing is found in the karyotype (if that is a possible test for that disease) or FISH. Genetic remission means that all tests, including the PCR, are negative.
Dennis, PCR does frequently oscillate between positive and negative. Part of this reason is the extremely high sensitivity of the procedure. It is so sensitive that even one almost dead or dead cell can be picked up. Another reason is that, because of the sensitivity, occasional false positive results are possible. And a third part of this is that we still do not fully (or even marginally) understand the concept of "minimum residual disease".
When you were first diagnosed, you probably had somewhere in the area of 100,000,000,000,000 malignant cells. We know what that is and we know the outcome of that large a tumor burden. Remission occurs around 1,000,000 cells. PCR remission is around 10,000 cells. We do know that most people have some malignant cells in them every day. If you have 1 cell, is that cancer? Not as we have always known it, it isn't. So what do we say about a person who has 10 bad cells - so far we know that their quality of life is excellent. - we know that they appear to be no t at risk for anything troublesome - we know that all other functions appear to be within reference range
We believe them to be cancer survivors - who just need to take care of themselves. But we cannot predict the future.