Hi Mike, Here's some information I found that may offer some help understanding diagnosing ALS vs MS but doesn't mention SFN.

"ALS is characterized by alterations in the patterns seen on electromyography (EMG), while MS patients have a normal study. A lumbar puncture (spinal tap) will show a characteristic pattern with MS that is not seen with ALS."
--- MS vs. ALS: Similarities and Differences - Symptoms: https://www.verywellhealth.com/als-and-ms-symptoms-2440783

SFN vs ALS:
"Can ALS be confused with neuropathy?
Notably, the percentage of misdiagnosis in degenerative LMNS has been reported to be as high as 19%; moreover, up to 10% of patients initially diagnosed as ALS are ultimately re-diagnosed as having another disease, including peripheral neuropathy"
--- The Peripheral Nervous System in Amyotrophic Lateral Sclerosis: Opportunities for Translational Research
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6603245/