Culturing the Cornea
BY MARJORIE J. RAH, OD, PHD
When a patient presents in your office with the signs and symptoms of an infectious keratitis, what do you do? Do you begin treatment immediately or do you culture the conjunctiva and cornea first? According to the Ocular Therapeutics Handbook, A Clinical Manual, you should obtain a culture of the lids, conjunctiva and cornea "any time a significant bacterial infection of the cornea is suspected." Cultures of the eyelid margin aren't routine because of low percentages of positive cultures and the costs involved; however, practitioners typically perform cultures of the conjunctiva in addition to corneal cultures. Approximately 47 percent of cultures are positive.
Getting to the Source
Culturing of the cornea in the case of microbial infection is important in selecting the appropriate pharmacological management for each case. The treatment regimen for a bacterial infection is vastly different from that for a fungal infection. The drug you initially select is based on your judgement of the source of the infection at the initial visit. You should obtain the culture before initiating this treatment, otherwise it may be difficult to isolate the microbe responsible.
The results you obtain from analysis of the culture will help refine your treatment. It will indicate the susceptibility of the causative agent to specific pharmacological interventions. This process, while costly, time consuming and somewhat uncomfortable for the patient, is the quickest route to a cure. Because microbiological laboratory testing is a costly procedure, it's best to specify the microbes to test for. The laboratory can provide a checklist to include when submitting the culture for analysis.
Performing the Culture
Use a culture tube (a cotton-tipped applicator in transport media) to obtain cultures from the conjunctiva. Individually wrapped sterile culture tubes are available that can be kept in office for use in this procedure. Be aware that they have expiration dates. Using an expired culture tube is not recommended for specimen collection. If you don't have the facilities to store the culture medium, contact your area laboratory to establish a protocol for referring patients for cultures.
To obtain a conjunctival culture, pull the lower eyelid away from the globe and wipe the cotton swab along the lower cul-de-sac. Then place the cotton-tipped applicator back in the culture tube and tightly seal it for transportation. Obtain conjunctival cultures without anesthetic if possible to help increase the chances of obtaining a positive culture. If anesthetic is necessary, use nonpreserved topical anesthetics whenever possible. Preservative in the anesthetic can interfere with the collection of the offending microbes.
When culturing the cornea, collect specimens from both the center and the periphery of the ulcerative area. Obtain the culture with either a culturette tube or a sterile spatula. Instill topical anesthetic (nonpreserved if available) before collecting the sample. Then either wipe the ulcerative area with the cotton-tipped applicator or scrape it with the spatula and seal the specimen in the culture tube for transportation.
Transporting the Specimen
Once you have obtained the specimen, you must deliver it to the laboratory for analysis as soon as possible. Store the specimen at room temperature. Make sure you include the patient's name on every container you send to the lab. You can deliver the specimen in person or send it via a courier or the U.S. Postal Service. Be sure to check the protocol for sending human specimens through the mail as there are specific guidelines that you must follow.
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #129.
Dr. Rah is an assistant professor at the New England College of Optometry where she works primarily in the Cornea and Contact Lens Service in patient care, teaching and research.