What is the preferred specimen for suspected bacterial keratitis?

• A. Corneal scraping for Gram stain and culture
• B. Conjunctival swab
• C. Anterior chamber paracentesis
• D. Corneal biopsy

Answer: (A)

Sampling the infected cornea with a corneal scraping for Gram stain and culture is the preferred method. This directly targets the corneal tissue where the infection sits, giving quick preliminary information from the Gram stain (helping distinguish Gram-positive versus Gram-negative organisms) and, importantly, definitive culture and susceptibility results to guide targeted antibiotic therapy. Sampling the conjunctiva is less reliable because it often reflects normal surface flora rather than the pathogen causing keratitis. More invasive procedures like anterior chamber paracentesis are reserved for intraocular infections such as endophthalmitis and are not first-line for a surface keratitis. Corneal biopsy is an option only in complex, chronic, or culture-negative cases due to its greater risk of complications. If possible, obtain the specimen before starting antibiotics to maximize diagnostic yield.