Absolutely. No one symptom can diagnose or rule out B12 deficiency. And fingernails are a good thing to pay attention to, as many correctible problems can be pointed to by their condition.

Here's a couple of helpful sites:

http://www.mayoclinic.com/health/nai...=false&ISAGG=0

http://www.aafp.org/afp/20040315/1417.html

rose