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
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
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
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
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
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.
Contact Lens Spectrum, Issue: August 2006