Polymerase Chain Reaction (PCR)
It is a scientific technique from the branch of molecular biology that allows us to amplify DNA segments. We think it is much easier if you think of it as the specialized “photocopier” of your DNA.
Polymerase – it is an enzyme. It’s primary function is to polymerize new DNA or RNA taken from a template of existing DNA or RNA that are in the process of transcription and replication. Polymerization is a technical term that relates to a process of causing monomer molecules to react in a chemical reaction to form three dimensional networks or polymer chains.
Using a sample of your blood, lab technicans isolate the RNA, extract it and create a chemical reaction through a process of heating and cooling in a special thermal cycler – PCR machine. This heating and cooling causes the RNA to replicate itself, making thousands and even hundreds of thousands of copies of itself.
This test allows the doctor to assess how well you are responding to treatment. Forcing your own cells to replicate themselves in this lab environment tells us the level of disease in our DNA. The only other way to detect the actual number of BCR ABL genes in your body, would be to take all the blood out of your body and count it. But that wouldn’t help much, you cannot live without your blood!
At diagnosis, a typical CML patient in the chronic phase would have a result of 100% – that would be your baseline. As you continue to take your specific treatment for CML (Gleevec, Sprycel, Tasigna or newer drugs in clinical trials) You would see your levels of detectable disease go down.
There is a program to standardize the results, but for now, they can be reported in two ways; either as a log reduction or as a percentage.
Each time you go down a log, lets say -1 log to -2 logs, this means you have 10 times fewer cells containg the BCR ABL gene. The same is true with the International Standard (IS) which reports in percentages. If you go from 100% to 10% you have ten times fewer cells containing the BCR ABL gene.
Here is a table that may help you compare your results:
| Log Reduction | Response achieved | International Scale (IS) % of cells with BCR ABL |
| Baseline | 100% | |
| -1 log | Complete Hematologic Response | 10% |
| -2 log | Complete Cytogenetic Response | 1.0% |
| -3 log | Major Molecular Response | 0.1% |
| -4 log | Major Molecular Response | 0.01% |
| -4.5 log | Complete Molecular Response-undetectable amount of BCR ABL cells | 0.001% |
| -5 log | Complete Molecular Response-undetectable amount of BCR ABL cells | 0.0001% |
In 2009, The CML Society of Canada collaborated with our Medical Advisory team, Dr. Suzan Kamel-Reid at the University Health Network in Toronto, Ontario, Canada, and with an unrestricted educational grant from Bristol-Myers Squibb – we put together an educational video explaing PCR.
The video features Dr. Pierre Laneuville from the McGill University Health Centre. The animation was done by award winning animator Tim Linklater from Vancouver British Colombia.
Please click on the link below and watch the video, it may help you to better understand what we have written here.
