PCR Explained In Layman Terms
When a patient tests negative in the 20 cell cytogenetics test for the Ph chromosome as well as the 500 cell FISH
test for the BCR-ABL fusion gene, there is still residual CML disease left in the body, sometimes a million
leukemic cells or more. Polymerase chain reaction (PCR) test measures this low level disease by being able to
detect 1 leukemic cell in a million or more normal cells by amplifying one leukemic cell into many copies. The
PCR test is a molecular test and the results is the molecular response of a patient.
• Qualitative PCR gives a positive or negative value for the presence of disease.
• Quantitative PCR (Q-PCR) gives the level of disease positivity in numbers.
• Nested PCR can sometimes be more sensitive than the above as it uses double primers and repeats the
experiment so that errors are corrected.
Most doctors will recommend a quantitative PCR to measure residual disease. This can be recommended right
from diagnosis or after other tests show negative, it really depends on the doctor. However, it is the most
important tool, nowadays, to follow low disease load. You follow the disease by PCR numbers in the same way you
tracked the Ph chromosome when the disease load was high by other tests. Do remember, that absolute PCR
values cannot be compared to FISH and Cytogenetics values and also not between different PCR labs as it will be
different.
A PCR report looks like the one above. Quant BCR-ABL means a quantitative PCR was done to look for BCR-
ABL transcripts. In layman terms, it is looking for leukemic cells carrying the BCR-ABL signature. In the Q-
PCR experiment, a control is used and in most labs, that is ABL. A control correlates to the number of normal
cells. Therefore, when you see the result of 0.6%BCR-ABL/ABL, it means the percentage ratio of bad cells to
total cells.
• One PCR test, by itself, has little meaning. The PCR trend is important. Ideally, the PCR numbers should
be going down with time as the disease is controlled by therapy.
• There are two types of molecular response. A major molecular response (MMR) when the PCR value of a
patient has decreased 1000-fold or 3-log from diagnosis and complete molecular response (CMR) when the patient
tests negative for leukemia from the PCR tests.
• If there is a 10-fold or 1-log increase in PCR numbers, usually, doctors will first recommend a re-test and
many patients have reported a re-test showing the rise to be a fluke. If the re-test and subsequent tests show a
rising PCR trend, most doctors will recommend ABL point mutations testing to see if resistance to therapy is
being developed. These are special tests done only in a handful of laboratories around the world and it is usually
done from a blood draw.
• The goal of Gleevec therapy is to reach a 1000-fold or 3-log reduction from diagnosis PCR values as this
gives only a 0.5% risk of relapse per year. Many patients do not have diagnosis PCR values so a PCR lab has a
diagnosis PCR baseline from newly diagnosed patients and you can correlate your PCR value to that.
• A PCR test can be done from either the blood or marrow. However, recent medical studies have shown that
the two may not show absolute correlation so doctors recommend PCR testing from the blood only, to keep the
source constant and because recommendations are to do a PCR every 3 months and a bone marrow aspiration
every 3 months is considered invasive.
So, what are the questions to ask the doctor when you get a report as above?
• CML experts recommend a PCR sensitivity of 1 in 100,000 cells as the minimum so ask the doctor if the
PCR lab is able to detect 1 leukemic cell in 100,000 cells.
• If you do not have a diagnosis PCR value, ask the doctor what log reduction is 0.6% from the lab baseline.
If it is 3-log and below, you are in good molecular response.
• Watch the PCR trend over time and if you find that with your current dose of Gleevec, your PCR is not
going down any further after some time and you do not have a 3-log reduction, discuss with the doctor options of
raising Gleevec dose or his advice on what to do next.
For a schedule of recommended PCR monitoring, see the article below from ASH 2005 from Dr. Deininger of
OHSU.
http://www.asheducationbook.org/cgi/content/full/2005/1/174