Dear CLL experts can someone please tell me what the FISH test is? I do understand that it is the main test to determine CLL. Since I have only had PET scans and MRI I'm wondering if the FISH test is a blood test? Or is the FISH test a bone marrow test? Thanks I know this is a very basic question but I really want to understand this before my next CLL appointment.



12 Replies

  • Hi Kathy,

    The Flow Cytometry test is what's used to determine if you have CLL, rather than another Leukaemia. The FISH - (Fluorescent In Situ Hybridisation) test is a subsequent test that checks for known genetic abnormalities in CLL cells. Both are blood tests and can also be done on a bone marrow or lymph node sample. Note that those without CLL cells in their blood may need to have a node biopsy to recover CLL cells for testing. It is possible to have different results from FISH tests done on blood, bone marrow or a node, because there could be different mixes of clones in the different locations.

    There are a number of different FISH testing panels available, which check for different numbers of abnormalities and we are learning of new ones regularly. Having a FISH test is only really important when you come to needing treatment, because it can identify if your CLL clones have genetic malformations that would prevent a treatment (specifically the older chemo treatments like FCR) from being effective. It can also be used to provide some prognostic indications on whether you are likely to ever need treatment and how long until that's needed. Because of the varying interactions between the different types of genetic damage (plus we are still discovering and learning of new types of damage and how these all interact), many of us prefer not to have a FISH test for that purpose. The only real way to be sure what's going to happen with your CLL is to monitor its progression over time...

    In summary:

    * Flow Cytometry looks at the 'fingerprint' of protein markers on the lymphocyte cell surface, the percentage of CDs or Clusters of Differentiation

    * FISH tries to identify markers within the CLL B-lymphocyte's DNA from within the cell's nucleus

    So an investigation into what ails us first looks at the outside of the lymphocytes we have in excess to figure out what particular blood cancer we have (Flow Cytometry), then may look at the nuclei of those now identified CLL cells to find out where they have gone wrong (FISH).


  • Dear Neil,

    Thanks so much for this remarkable answer. I have watched many of the wonderful videos that were provided in links on this website and read many discussions on CLL topics too but never could quite figure these tests out! The Flow Cytometry is used to determine if you have CLL. I get it. The FISH is used to find out genetic abnormalities in CLL cells. OK. I have listened to many discussions about why you want mutated genes. Also which genes tend to be less or more of a problem. I am taking baby steps I am sure but I think we all need to understand this. I hope watch and wait lasts forever but if it doesn't I want to be ready. I have to thank you again. (It seems like I am always thanking you!) I appreciate your kindness and the way you always answer the question so completely and so well. I plan to start charting my blood tests over time as you suggest and of course keep reading the posts on this website.

    I'm very grateful for your help and hope you are enjoying your spring time in Australia and getting some great pictures!


  • Kathy, perhaps I misunderstood your comment in response to Neil's explanation of FISH. You say: "I have listened to many discussions about why you want mutated genes." I am no expert, but it is not mutated genes you want but rather the IgVH mutation which renders you B cells more effective.

  • I'm sure that's what Kathy meant :) , but you are clarifying a very important point, given the relationship between DNA mutations and cancers. Being pedantic, CLL cells that have IgVH mutation status are more mature than those without that status and having that status correlates to a longer time before treatment is necessary (if it is needed at all) and a longer overall survival. 'Mutated' in this context relates to purposeful rearrangement of protein chains, not genetic damage. IvGH mutated and unmutated CLL cells are both 'rogue' and don't work as intended.


  • Yes .......sorry. But I'm glad you made the point because it is complicated and I am very new at this.

  • Thank you for such a full and clear explanation. Posts support those writing but also those reading. Thanks AussieNeil and kathypawpaw.


  • Really disappointed

    ..thought it was going to be a fish food diet to kill cancer!!!....wishful thinking! :)

  • Hi Kathy,

    In addition to the excellent explanation from Neil, you may find this description of many of the prognostic testing procedures in CLL and the methods useful. I certainly did.

    Hope your next appointment goes well.


  • Dear Newdawn,

    I am deeply grateful for this answer. If I didn't have this website I would not know any of this. I know there are many CLL people out there especially the new folks that need this excellent information. I finally know what all the tests are and what they are measuring. I have asked my doctor for the paperwork from every checkup so that I can become better educated. I also know I am not yet with a doctor that only treats CLL. I will look for a CLL specialist in my area. Thanks again I will refer to this again I am sure.


  • Thanks again Newdawn. Informative website.


  • As a comment... PET scans are poor in CLL and are usually only used by experts to determine possible transformation...

    The better scan in CLL is the CTscan, but early use and annual scan surveillance is being discouraged now, due to radiation exposure to the patient and the lack of benefit derived from surveillance CTscanning in CLL.

    CTscans are of great value, but they should be used judiciaously....


    More on scans by Dr. Sharman

  • Thanks Chris,

    I read Dr. Sharman's explanation about the difference between a PET and a CT scan. Very interesting. Turns out I have had both kinds and I always wondered why both? I must admit I was a little concerned about the radiation risk and his article is quite reassuring. I am heading back to the link to read a few more posts. Thanks again you "guys" always know who to plug us into when we need information. I hope you are enjoying the beautiful fall weather up there in Canada. The south has finally cooled down some and I'm loving it.

    Many thanks,


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