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Appendix A.8

Indications

Small (<1 mm) infiltrates may be treated empirically with intensive commercially available broad-spectrum antibiotics without prior scraping. We routinely culture infiltrates larger than 1 to 2 mm, in the visual axis, unresponsive to initial treatment, or if we suspect an unusual organism based on history or examination. See 4.12, Bacterial Keratitis.

Equipment

Slit lamp; sterile Kimura spatula, knife blade, or dry calcium alginate swab (note: swab can be moistened with nonpreserved sterile saline, or thioglycolate or trypticase soy broth); culture media; microscopy slides; and an alcohol lamp if a Kimura spatula is used.

Procedure

  1. Anesthetize the cornea with topical drops. Proparacaine is best because it appears to be less bactericidal than others.

  2. At the slit lamp, scrape the ulcer base (unless significant corneal thinning has occurred) and the leading edge of the infiltrate firmly with the spatula, blade, or swab. Place the specimens on the slides first and then on the culture media. Sterilize the spatula over the flame of the alcohol lamp between each separate culture or slide. Be certain that the spatula tip temperature has returned to normal before touching the cornea again. See Video: Corneal Culture Procedure.

Media

Routine

  1. Blood agar (most bacteria).

  2. Sabouraud dextrose agar without cycloheximide; place at room temperature (fungi).

  3. Thioglycolate broth (aerobic and anaerobic bacteria).

  4. Chocolate agar; lab will place into a CO2 jar (Haemophilus species, Neisseria gonorrhoeae).

Optional

  1. Löwenstein–Jensen medium (mycobacteria, Nocardia species) should be included in patients with a history of LASIK or an atypical ulcer appearance.

  2. Nonnutrient agar with Escherichia coli overlay if available (Acanthamoeba).

Slides

Routine

  1. Gram stain (bacteria and fungi).

  2. Calcofluor white; a fluorescent microscope is needed (fungi and Acanthamoeba).

Optional

  1. Giemsa stain (bacteria, fungi, and Acanthamoeba).

  2. Acid-fast stain (Mycobacterium species and Nocardia species).

  3. Gomori methenamine silver stain and periodic acid-Schiff (PAS) stain (fungi and Acanthamoeba).

  4. KOH wet mount (fungi, Nocardia species, and Acanthamoeba).

  5. Extra slide to send to pathology at a local institution.

NOTE

When a fungal infection is suspected, deep scrapings into the base of the ulcer are essential. Sometimes a corneal biopsy is necessary to obtain diagnostic information for fungal, atypical mycobacterial, and Acanthamoeba infections.