Eye Culturing for Drainage
Obtain corneal cultures before initiating antimicrobial therapy if the eye drainage is associated with a central or large corneal infiltrate (≥2 mm), significant stromal involvement, anterior chamber cells, or any sight-threatening features. 1
When to Culture Eye Drainage
Mandatory culturing indications:
- Central corneal infiltrate within 3 mm of corneal center AND ≥2 mm in size 1
- Presence of ≥1+ cells in the anterior chamber 1
- Stromal melting or significant stromal involvement 1
- Multiple corneal infiltrates or ≥2 adjacent lesions 1
- History of corneal surgery 1
- Atypical features suggesting fungal, amoebic, or mycobacterial infection 1
Additional situations warranting culture:
- Chronic infection or poor response to broad-spectrum antibiotics 1
- Trauma with organic matter 1
- Contact lens wear in hot tub 1
- Post-operative infiltrates in surgical wounds 1
Culture is optional for:
- Small, peripheral infiltrates without stromal melting 1
- Simple bacterial conjunctivitis in adults (unless neonates, who require aggressive workup for Neisseria gonorrhoeae) 1
Proper Culture Technique
Anesthetic selection:
- Use proparacaine 0.5% as topical anesthetic 1
- Avoid tetracaine due to antimicrobial effects that interfere with culture results 1, 2
- Avoid anesthetics with preservatives to improve culture yield 1
Specimen collection:
- Obtain scrapings from the base or periphery (advancing borders) of the infected area using sterile blade, platinum spatula, or jeweler's forceps 1
- Do not collect only purulent material—this yields inadequate results 1
- Alternative: use nylon-tipped flocked swab, which has similar culture-positivity rates and is more cost-effective 1
- Perform under slit-lamp biomicroscope magnification 1
Culture media handling:
- Inoculate directly onto appropriate culture media to maximize yield 1
- If direct inoculation not feasible, place in transport media (modified Amies medium or thioglycolate broth-moistened swab) 1
- Immediately incubate or promptly deliver to laboratory 1
- Adding liquid culture media increases bacterial isolation compared to solid media alone 1
Additional cultures:
- For contact lens wearers: culture the lens, lens case, and solution 1
- For deep stromal infections: consider corneal biopsy if poor response to treatment or repeated negative cultures 1
Staining Recommendations
Concurrent with cultures:
- Gram stain and Giemsa stain for large, central, or sight-threatening infiltrates 1
- Add potassium hydroxide, Calcofluor white, and acid-fast stains for post-surgical infiltrates 1
- Stains are optional for small, peripheral infiltrates 1
Critical Pitfalls to Avoid
Do not perform aqueous or vitreous taps for hypopyon in bacterial keratitis unless high suspicion of endophthalmitis (following intraocular surgery, perforating trauma, or sepsis), as hypopyon is usually sterile 1, 2
For previously treated cases with poor response:
- Cultures may still be obtained but pathogen recovery may be delayed 1
- Consider stopping antibiotics for 12-24 hours then reculturing if initial cultures negative 1
Recognize that cornea specialists correctly differentiate bacterial from fungal keratitis in fewer than 70% of cases by clinical appearance alone, highlighting the critical importance of cultures for proper pathogen identification 1