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Questions submitted by readers and answered by the CLL Society Medical Advisory Board
By Susan Leclair, PhD, CLS (NCA)
I was diagnosed with 17p deletion FISH test about 6 months ago. Recently, I had another FISH test the results no 17p deletion. Both tests were done at different labs. What are some reasons for this?
First option – You have two populations of lymphocytes – some are normal and some are not. If you were to test every single cell, then you would get a mixed pattern of positive and negative cells. But you don’t test every single cell. Approximately 5 millilters (not very much really, somewhere in the vicinity of 1/6 of a shot glass of liquid) is withdrawn from your body. Then (for the sake of discussion I will use a large, more than adequate sample size, 50 microliters from that 5mL is used for testing. So only 0.5mL of that original 5mL is actually used for testing. (why take so much- in case of multiple repeats or different additional follow up tests.)
total volume of blood – around 5 Liters or 5,000,000 microliters
total volume of specimen collected from you – around 5mL or 1/1,000,000 (one in a million cells)
total volume of sample used in test – around 50µL or 0.05mL or 1/ 100,000,000 (one in one hundred million cells)
So the likelihood that there may be sufficient malignant cells to show up on a consistent basis is not guaranteed.
Second option – By some strange and not yet understood alchemy, malignant cells can be much more fragile than normal cells so the collection, preparation to transport, transportation, storage, preparation for testing and final testing can do quite a number of on them. It is possible that cell death occurred and if it does, then it will preferentially involve many more malignant cells than normal ones.
Third option – Genetic testing is still rather “new” and it is possible that the equipment and/or reagents used by laboratory #1 are different than laboratory #2. These changes in the procedure might be causing the difference.
Fourth option – You did not mention if you had been treated previously. It is possible that the treatment killed off sufficient numbers of malignant cells such that there are not enough of these cells present in the sample to create a positive result.
As you can see, none of these are mutually exclusive. that is, you could have only one option occurring, or you could have two or more working together.
Does this help at all to understand what is going on?
Susan Leclair, PhD, CLS (NCA) is Chancellor Projessor Emerita at the University of Massachusetts Dartmouth; Senior Scientist, at Forensic DNA Associates; and Moderator and Speaker, PatientPower.info – an electronic resource for patients and health care providers.
Originally published in The CLL Tribune Q1 2019.