After 6 months of enduring a sinus infection, i am wondering why my wife's reg. dr or her ent has not taken a sample for accurate diagnosis when muliple antibiotics have not worked? wouldn't it help?

I don't know.: If sinus infection is persistent after 6 months of antibiotics and she still has same problem, you need a second opinion, may be a ct scan of sinuses will help.She may have polyps in the sinuses or an allergy . Try to get a second or third opinion.

Absolutely: It is clearly reasonable to get cultures from under the middle turbinate where the frontal, maxillary, and anterior ethmoid sinuses drain when there has been ineffective antibiotic response. It is best done endoscopically. Reasons for treatment failure would be antibiotic resistance, secondary yeast infection, or noninfectious inflammation. Collecting cells by nasal lavage can also point to cause.