Hi, I am an American Board Certified Neurologist and I am very glad to help you with your health concerns.
In these circumstances doing just one test may not be sufficient rather you would need a group of neurological tests to find out what kind of brain injury is causing your symptoms.
You need tests which help in finding out the cause for your memory loss and the following are the various tests that can help.
1) Certain blood tests (like vitamin b12, folate, lyme, thyroid etc)
2) A Brain scan (MRI scan of the brain)
3) A PET scan of the brain especially Amyloid PET scan of the brain
4) An EEG (Study of the brain waves)
5) A Spinal tap or Lumbar puncture to test the CSF (Cerebrospinal fluid).
If any of the above tests have already been done then there is no need to repeat it. If you see any of the above tests have not already been conducted then please ask your neurologist to arrange for these tests and these tests can help in finding out the precise cause for your brain related symptoms and also help in instituting the appropriate treatments.
Please let me know if you have any further questions; and if you feel you have received the answer to your question please rate me so that I can receive credit for the time and effort put by me in answering your question.