Hello everyone,
I was just wondering if anyone knew how sensitive a PCR test is?
What I mean specifically is what is the smallest value it can detect (ie how many decimel places), and thus anything below this lowest value i guess would be flagged as "undetected".
Thanks
David
Hi David
Unfortunately there's not a single answer to this. The FAQ page of this site gives an answer
"It can detect 1 Ph+ CML cell out of 100,000 cells, so it can detect very low numbers of remaining Ph+ CML cells.",
indicating three decimal places of %, and some centres do this. Some other centres only quote results to two decimals of % (1 cell in 10,000). There are discussions about trying to standardise better between centres, but this is far from resolved. Results from different centres may not be directly comparable, and you do see upward blips in sets of results at the same centre, which may arise from process variability. The key indication that things are going well is a long term downward trend in PCR results towards zero from your testing centre.
Personally for the last two years my result (from samples tested in Newcastle) has been given as 0.00%, which I don't think anyone would argue with as PCR undetectable.
Hope this helps
PCR sensitivities vary from centre to centre, and there is a difference in the kinds of PCR that can be done. The basic test is RQ-PCR. My own tests, which are done at the Hammersmith so fairly sensitive, are reported to 3 decimal places (which is 4.5/5 log reduction: 100 percent down to below 0.001 percent), so when those three places are all zero it is identified as undetectable. I don't believe there is a clear definition worldwide on what is undetectable because sensitivites do vary but generally I understand anything below 4.5 log reduction is classed as undetectable. Where exactly anyone actually is below that number we don't generally know but there are more sensitive PCR tests than RQ-PCR which are being used in the context of some "stop" studies. These can potentially measure to 6 or 7 log. They are basically personalised tests. But that's about as far as the PCR can get as I understand it.
The Lab that does my tests in South Africa reports my results down to 5 decimal places.
But what is important is to get a Zero reading. That is PCRU and that is what it is all about.
So whether it is 3 decimal or 5 decimal places, Zero is Zero.
I believe there are moves afoot to standardize RQ-PCR testing and reporting world wide. Not sure though if that will ever be realized.
Edit: My Last test was reported as "not detected"
The actual results:
BCR/ABL 0.00025
BCR/ABL Ratio 0.00000
Don't as me how the ratio calculation is done, I really do not know. From past results it seems that my test is "detectable" when the BCR/ABL test gets to 0.00085. But at that point it is not an issue for me as long as following tests do not reveal an alarming upward trend.
5 log reduction is 0.001% which represents five decimal places if not expressed as a percentage.. Hammersmith is the same as the South Africa quote.
John
