the contact lens exam

Practical Guidelines for Culturing Corneal Infiltrates

the contact lens exam
Practical Guidelines for Culturing Corneal Infiltrates

We often encounter corneal infiltrates associated with lens wear, and we must first differentiate an infectious corneal ulcer from a sterile ulcer. A concurrent question is, "When do I culture?"

Knowing When to Culture

Contact lens-induced peripheral ulcers (CLPU) or infiltrates are usually small (< 2mm) and have only confined injection. CLPU or infiltrates occur in the peripheral cornea and don't need culturing unless they persist after discontinuing contact lens wear.

Infectious ulcers, or microbial keratitis, are often 2mm or larger and generate a large area of severe conjunctival injection and, possibly, lid edema. They're usually located at or near the central cornea, although they may also occur in the periphery. Infectious ulcers will produce a significant mucopurulent discharge and an anterior chamber reaction.

Consider any ulcer located in the central cornea and/or that's 2mm or larger infectious until proven otherwise. In these cases, culturing the ulcer and testing for antibiotic sensitivity are certainly warranted.

Figure 1. Culturing the cornea with the BBL Culture Swab Plus.

Getting the Culture

First anesthetize the eye with topical proparacaine. Gently scrape the ulcerated area with a sterilized Kimura spatula starting at the edge of the wound and advancing to the center. Use both sides of the spatula while debriding and hold the top eyelid to avoid contamination from the lid or lashes.

Smear the spatula onto an agar plate in multiple "C" shapes. Use one side of the spatula to inoculate a blood agar plate and smear the other side on chocolate agar. If you suspect a fungal or Acanthamoeba infection, then take additional scrapings with another sterile Kimura spatula. (Swab these onto a Sabouraud's agar plate [fungal] or a non-nutritive agar [Acanthamoeba].)

Figure 2. A sterile BBL Culture Swab Plus.

You can also use a transport media such as the BBL Culture Swab Plus (Troy Biologicals, Inc.), shown in Figure 1, which comes pre-sterilized (Figure 2) and provides an environment for both aerobic and anaerobic microorganisms. Simply obtain the culture as described previously, then place the swab in the transport media and send it to the lab.

Culturing Tips

Along with the specimen, provide the source of the culture, diagnosis and any pertinent patient history (such as being struck in the eye with vegetative matter). Request antibiotic sensitivity testing for any antibiotics you may prescribe for the patient. Inform the lab if you suspect a fungal or other atypical infection.

Use the diagnosis code for corneal ulcers (370.03 for a central corneal ulcer and 370.01 for a peripheral ulcer) when coding for the appropriate evaluation and management level. You may also bill for the corneal scraping using CPT code 65430 ("scraping of cornea, diagnostic, for smear and/or culture").

After six years of private practice, Dr. Miller joined the faculty at NSUCO in 2002, where he currently teaches medical microbiology and has clinical duties.