A corneal scraping is a diagnostic procedure used to investigate severe inflammation or infection of the cornea, the clear, dome-shaped front surface of the eye. It involves the gentle collection of cells and material from the infected area using a specialized, sterile instrument. The goal is to obtain a sample of the microorganism causing the infection, known as microbial keratitis. This identifies the source of the problem before a definitive treatment plan can be established. The procedure is performed under topical anesthesia and typically takes only a few moments to complete.
Identifying Corneal Infections
The purpose of a corneal scraping is to identify the infectious agent responsible for the corneal ulcer. Accurate diagnosis is necessary because treatment depends entirely on the type of microbe involved. Without identification, doctors must rely on broad-spectrum medications, which may be ineffective and contribute to antibiotic resistance. The scraping helps differentiate between the four major categories of pathogens: bacteria, fungi, viruses, and parasites.
Pathogen Identification
Bacterial infections require antibiotic eye drops, while fungal infections necessitate antifungal medication, often requiring a longer treatment duration. Fungal keratitis often presents with visual clues like feathery edges, prompting a scraping. Parasitic infections, particularly those caused by Acanthamoeba, demand specialized anti-amoebic agents. These infections are often extremely painful and can mimic other types of keratitis, making laboratory confirmation necessary. Viral infections, such as those caused by Herpes Simplex, require antiviral medications, as antibiotics and antifungals are ineffective. Guessing the cause can lead to treatment failure, potentially resulting in vision loss or the need for a corneal transplant. Obtaining a sample allows for the focused application of the correct medication, increasing the likelihood of a successful outcome and faster healing.
Analyzing the Sample
Once the sample is collected, it is immediately sent to a microbiology laboratory for analysis, involving two distinct stages: direct examination and culturing.
Direct Examination
Direct examination provides the fastest preliminary results, often within hours. A portion of the scraped material is smeared onto a glass slide and stained using dyes, such as Gram stain or Giemsa stain. The stained slide is viewed under a microscope, allowing rapid observation of any present organisms. Gram staining quickly classifies bacteria into Gram-positive or Gram-negative groups, which helps the doctor start targeted antibiotic treatment immediately. This rapid initial result guides the first steps of therapy while waiting for more definitive data.
Culturing and Sensitivity Testing
Culturing is the second stage, where the remaining sample is spread onto specialized agar plates designed to support the growth of different microbes. These plates are incubated, allowing slow-growing organisms, especially fungi, to multiply until they form visible colonies. Fungi may take several days or weeks to grow, but their development confirms the diagnosis. After the organism is identified, the laboratory performs sensitivity testing, also called an antibiogram. This involves exposing the grown colonies to a panel of different medications to determine which effectively stop their growth. This testing customizes the treatment, ensuring the patient receives the most potent and effective medication for their specific infection.
When the Procedure is Necessary
A corneal scraping is not routine; it is reserved for specific, concerning clinical scenarios that pose a significant threat to vision. The procedure is typically performed when a serious infection is suspected. One indication is a corneal ulcer that is large, centrally located, or deep, as these cases carry a higher risk of complications.
The procedure is also necessary when an infection does not respond to initial empirical treatment. If the ulcer worsens after two to three days of broad-spectrum antibiotic drops, a scraping is needed for definitive identification. This lack of response suggests the presence of a resistant bacterium or a non-bacterial organism like a fungus or Acanthamoeba.
Atypical clinical presentation is another trigger. Ulcers with irregular or feathery borders often suggest a fungal cause, while ring-shaped infiltrates can point toward Acanthamoeba keratitis. In these situations, the visual presentation justifies the scraping to confirm the suspected pathogen and begin specialized treatment immediately. The scraping moves management from a generalized approach to a precise, organism-specific therapy.