First of all, consider this from the NIH: “…What is the prognosis?
Guillain-Barré syndrome can be a devastating disorder because of its sudden and unexpected onset. Most people reach the stage of greatest weakness within the first 2 weeks after symptoms appear, and by the third week of the illness 90 percent of all patients are at their weakest. The recovery period may be as little as a few weeks or as long as a few years. About 30 percent of those with Guillain-Barré still have a residual weakness after 3 years. About 3 percent may suffer a relapse of muscle weakness and tingling sensations many years after the initial attack.
emphasis on ‘3% may suffer a relapse…many years after…’
Secondly, a less scientific, yet generally accepted cutoff goes something like this: “…The nadir of clinical symptoms had to be reached within 8 weeks….”
Meaning, that for GBS, you should reach your absolute worst within 8 weeks.
Thirdly, there are generally accepted diagnostic criteria for CIDP. They are locatable by search on the ‘net or this website. But, in my experience include a high Spinal fluid tap protein but, not always; an abnormal EMG/NCV to include both motor (muscle) and sensory (feelings) abnormalities with specific conduction blocks with conduction blocks in multiple extremities (but not always), f-wave time lags and so on.
Finally, Nerve and or muscle biopsy are sometimes used looking for problem axons, naked axons (axons with no covering left) and onion bulbs (evidence of de and re-myelination and certain other anti-body markers discovered after biopsy slide preparation and staining.
Ya’ know what? It really matters not between gbs and cidp. The treatment is the same.
By the way, there are multiple specific tests that must be run to exclude ‘other’ problems.
My final diagnosis was made by nerve biopsy (not sural).