In differentiating viral pharyngitis from streptococcal pharyngitis, which test is definitive?

• A. CBC with differential
• B. ASO titer
• C. Cold agglutinin antibody test
• D. Throat culture

Answer: (D)

The key idea is that to definitively distinguish streptococcal pharyngitis from viral illness, you must directly identify the bacteria in the throat. A throat culture does exactly that by collecting a pharyngeal swab and growing any bacteria present; if Group A Streptococcus grows, you have definite evidence of strep pharyngitis. This direct identification makes it the most reliable test for guiding antibiotic treatment and avoiding unnecessary use for viral infections.

Other options don’t provide this definitive identification. A CBC with differential is nonspecific and can be influenced by many illnesses; it doesn’t confirm the causative organism. An ASO titer measures antibodies to streptolysin O and reflects past or remote infection rather than an acute diagnosis, so it isn’t useful for differentiating the current illness. Cold agglutinin antibody testing is associated with other infections (like Mycoplasma) or immune phenomena and is not used to diagnose streptococcal pharyngitis.