Guillain-Barré Syndrome is diagnosed by its symptoms and tests. Symptoms are bilateral progressive weakness, tingling, numbness, paralysis, and pain that usually starts in the feet and works its way towards the head. Loss of deep tendon reflexes (DTR), which are your reflexes such as knee kick or the elbow. Organ paralysis such as the lungs, which require being hooked up to a breathing machine.
Spinal tap is usually done to see if there are elevated proteins in the spinal fluid. Elevated proteins do not always show up in the first week. A Nerve Conduction Velocity (NCV) test may be done, which can show whether it is demyelinating, axonal, or a mix.