You don't really need to prove that you have XYZ disease per se. You only need to prove that you can't work at the sedentary level - in other words, you can't sit at a desk and work consistently 8 hours a day, 5 days a week.
Sedentary is the lowest level of work activity. The other two are light and heavy.
This is what I did:
Go through the SSRP letter for CFS and provide evidence for whatever abnormal tests you have that are mentioned in the letter. (EBV titers, etc.).
Provide any other abnormal labs or testing you have.
Get a Functional Capacity Evaluation. This shows how pain and fatigue limit your ability to work.
Get a Neuropsych exam. This shows how cognitive impairment affects your ability to work.*
If you can get a letter from your doctor pulling all this info together for the judge, great. It looks like MassCFIDS has some info on this in their disability handbook:
https://www.masscfids.org/images/pdf/handbook/Disability_Handbook.pdf
Then (and this is the part my judge liked the best) - get a record of continuing disability. This is accomplished by going to your doctor every 3 months and reporting that remain disabled. My lawyer had a nifty form for this. By the time you get to the judge (a couple of years), it will be obvious you're not going to recover any time soon.
Get a lawyer or representative either for the initial application if you can, or as soon as you get denied.
Don't use a lawyer from your Long Term Disability company if you also have ERISA LTD insurance from work.
Make photocopies or scans of your initial application. You can use this later after you get approved and you get the long form review, which is the same questions all over again. You will thank yourself later!
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*The only caveat on the neuropsych exam is if you're also applying for ERISA LTD disability through your work. They're notorious for denying your claim or limiting it to 2 years for mental disorders - which they will try to make it look like you have.
https://www.masscfids.org/disabilit...nsurance-and-social-security?showall=&start=5
There is lots of other great disability info on the MassCFIDS site.
https://www.masscfids.org/disability-home-page