In a sputum specimen from suspected lobar pneumonia showing many white blood cells and gram-positive cocci primarily in pairs, which test should be performed on the culture isolate?

• A. Perform a PYR test on the culture isolate.
• B. Perform an Elek test on the culture isolate.
• C. Perform an optochin test on the culture isolate.
• D. Perform a hippurate hydrolysis test on the culture isolate.

Answer: (C)

When sputum from a patient with suspected lobar pneumonia shows many white blood cells and gram‑positive cocci arranged mostly in pairs, the most likely organism is Streptococcus pneumoniae. To confirm this on the culture isolate, perform an optochin susceptibility test. Optochin inhibits the growth of S. pneumoniae, so colonies near the optochin disk show a zone of inhibition, indicating sensitivity. Viridans streptococci, which can resemble pneumococcus on Gram stain and are often alpha-hemolytic, are resistant to optochin and will not show inhibition. This makes the optochin test the best and fastest way to distinguish pneumococcus from other alpha-hemolytic streptococci in this clinical context.

Other tests mentioned serve different purposes: PYR is used for certain streptococci like some Group A or Enterococcus; the Elek test detects diphtheria toxin production; hippurate hydrolysis differentiates organisms such as S. agalactiae from other streptococci. They’re not as directly informative for confirming pneumococcus in a pneumonia setting.