I don't test. If it looks/smells/tastes then I treat it like fungus. I also ask the patients any questions about trauma, chemo/radiation. If the nail is thick green and black I usually just remove it. I don't have time to waste ****ing around with topicals and promises that maybe the nail will get better or whatever.
If it's not fully destroyed and no trauma/radio history, try lamisil
If it's partially destroyed with trauma, temp removal with the caveat it may not grow normal
If it's fully destroyed I remove it, most times permanently