If you’re concerned about eye pain, redness, or vision changes, understanding the diagnosis of keratitis could be life-changing. As a leading cause of preventable blindness worldwide, keratitis demands swift, accurate diagnosis and treatment. This article will guide you through the gold-standard diagnostic approaches—slit-lamp examination and corneal scrapings—explaining how they work, why they matter, and what to expect if you or a loved one faces this eye emergency.

Direct answer: How is keratitis diagnosed?

Keratitis is diagnosed primarily through clinical evaluation with a slit-lamp biomicroscope and laboratory analysis of corneal scrapings. The slit-lamp allows ophthalmologists to visualise characteristic features of corneal inflammation and ulceration. In contrast, corneal scrapings provide samples for microbiological testing, which is crucial for identifying the causative organism and guiding targeted therapy. Together, these methods form the cornerstone of modern keratitis diagnosis, enabling rapid, precise, and effective management.

Introduction to keratitis

Keratitis refers to inflammation of the cornea—the clear, dome-shaped window at the front of the eye. It can be caused by various infectious agents (bacteria, viruses, fungi, protozoa) or non-infectious factors like trauma, dry eye, or autoimmune disease. Infectious keratitis is an ocular emergency; without prompt diagnosis and treatment, it can lead to permanent vision loss due to scarring or perforation.

Why accurate diagnosis matters

The stakes in keratitis are high. The cornea is responsible for focusing light and protecting the eye’s interior. Damage from infection or inflammation can result in:

• Corneal scarring
• Perforation
• Endophthalmitis (infection inside the eye)
• Permanent vision loss

A precise diagnosis is critical because different pathogens require different treatments, and some therapies (like steroids) can worsen certain infections. Misdiagnosis or delayed intervention can have devastating consequences.

The slit-lamp examination

What is a slit-lamp?

A slit-lamp is a specialised microscope with a bright, narrow light beam. It allows ophthalmologists to examine the eye’s structures in detail, especially the cornea, conjunctiva, anterior chamber, and lens.

How is it performed?

The patient sits at the slit-lamp with their chin and forehead stabilised.

The clinician uses various lighting angles and magnifications to inspect the cornea for:

• Infiltrates (white or grey opacities)
• Ulceration
• Epithelial defects
• Stromal thinning or perforation
• Hypopyon (pus in the anterior chamber)
• Satellite lesions or feathery edges (suggestive of fungal infection)
• Dendritic lesions (suggestive of herpes simplex virus)

What do clinicians look for?

The slit-lamp reveals subtle differences in lesion appearance and distribution, helping to distinguish between bacterial, viral, fungal, and protozoal keratitis. For example:

• Bacterial keratitis often presents as a solitary, well-demarcated ulcer with surrounding oedema
• Fungal keratitis may show feathery borders and satellite lesions
• Viral keratitis (especially herpes simplex) can produce dendritic or geographic ulcers
• Acanthamoeba keratitis may present with ring infiltrates and severe pain

However, clinical features can overlap, and atypical presentations are common, especially if the patient has used topical steroids or antibiotics.

Corneal scrapings

When and why are they done?

Corneal scrapings are indicated whenever a significant microbial keratitis is suspected, especially if:

• The ulcer is central, large, or involves the stroma
• The infection is chronic or is not responding to treatment
• There is a history of corneal surgery
• The clinical features are atypical
• There are multifocal infiltrates

The procedure

• After applying topical anaesthesia, the ophthalmologist gently scrapes the base and edges of the ulcer with a sterile spatula or blade
• Samples are immediately transferred to slides and culture media for laboratory analysis

Laboratory analysis

Microscopy and staining

Gram stain, Giemsa, or calcofluor white can rapidly identify bacteria, fungi, or protozoa.

Culture and sensitivity

Samples are plated on blood, chocolate, and Sabouraud dextrose agar to grow bacteria and fungi. Sensitivity testing guides antibiotic or antifungal selection.

Polymerase chain reaction (PCR)

Especially useful for detecting viral DNA (e.g., herpes simplex, varicella zoster) or Acanthamoeba, PCR can provide rapid, specific results34.

Corneal scrapings should be performed before starting antibiotics, as prior treatment can reduce the likelihood of isolating the causative organism3. Add correct reference format eg superscript numbers¹   

Integrating clinical and laboratory findings

No single test is perfect. The diagnosis of keratitis relies on integrating:

• Patient history (risk factors, symptoms, prior treatments)
• Slit-lamp findings (appearance, location, severity)
• Laboratory results (microscopy, culture, PCR)

For example, a contact lens wearer with a painful, central ulcer and ring infiltrate might be suspected of Acanthamoeba keratitis, but confirmation requires PCR or culture. Similarly, a patient with feathery-edged infiltrates and satellite lesions raises suspicion for fungal keratitis, but only laboratory testing can confirm the diagnosis and guide therapy.

Challenges in diagnosis

Overlapping symptoms

Redness, pain, photophobia, and decreased vision are common to all keratitis, making clinical differentiation challenging.

Limitations of clinical assessment

Studies show that even experienced clinicians can misclassify the type of keratitis based on appearance alone. For instance, one study found that cornea specialists could only distinguish bacterial from fungal keratitis in 66% of cases based on photographs. Consider changing to “bacterial keratitis

Importance of microbiology

Because of these challenges, microbiological confirmation is essential, especially in severe, atypical, or non-responsive cases. Culture and sensitivity testing identify the pathogen and detect antimicrobial resistance, ensuring the most effective treatment.

Advances in diagnostic technology

Imaging modalities

While the slit-lamp remains the cornerstone, other imaging tools are increasingly valuable:

Confocal microscopy

Provides in vivo, high-resolution images of the cornea, which are useful for diagnosing Acanthamoeba and fungal keratitis.

Optical coherence tomography (OCT)

Offers cross-sectional images, helping to assess the depth and extent of corneal involvement5.fix format of reference number 

Artificial intelligence and deep learning

Recent research has explored deep learning systems that analyse slit-lamp images to classify types of keratitis. Some models have achieved diagnostic accuracy comparable to or better than that of experienced ophthalmologists, potentially reducing misdiagnosis and improving access to expert care. These technologies are not yet standard in clinical practice but hold promise for the future.

Case study: A typical diagnostic journey

A 28-year-old contact lens wearer presents with eye pain, redness, and blurred vision.

History

The patient reports swimming with contact lenses and notes that symptoms began two days ago.

Slit-lamp exam

Reveals a central corneal ulcer with a ring infiltrate and stromal oedema.

Corneal scraping

Performed at the slit lamp, with samples sent for Gram stain, culture, and PCR.

Laboratory findings

PCR confirms Acanthamoeba DNA; cultures are negative for bacteria and fungi.

Treatment

Initiating anti-amoebic therapy, with close follow-up and adjustment based on clinical response.

This case illustrates the importance of combining clinical suspicion, slit-lamp findings, and laboratory confirmation to ensure accurate diagnosis and effective treatment.

Summary

Keratitis is a sight-threatening condition that requires rapid, accurate diagnosis. The combination of slit-lamp examination and corneal scrapings represents the gold standard, enabling clinicians to visualise characteristic features, obtain microbiological confirmation, and tailor therapy to the specific pathogen. While advances in imaging and artificial intelligence offer exciting possibilities, the fundamentals of careful history, detailed examination, and laboratory analysis remain irreplaceable in the fight against blindness from keratitis.

Frequently asked questions (FAQ)

Q: What are the main symptoms of keratitis?

A: Redness, pain, photophobia (light sensitivity), tearing, discharge, and decreased vision are common symptoms. Severe cases may show a visible white spot or ulcer on the cornea.

Q: Why is a slit-lamp exam necessary?

A: The slit-lamp provides a magnified, illuminated view of the cornea, allowing detection of subtle lesions, infiltrates, and other signs that are not visible to the naked eye.

Q: Is corneal scraping painful?

A: The procedure is performed after applying topical anaesthetic drops, so discomfort is minimal. Patients may feel slight pressure but not pain.

Q: How long do culture results take?

A: Preliminary results from stains may be available within hours, but cultures for bacteria and fungi can take several days. PCR results are often faster for viruses and protozoa34.

Q: Can keratitis be diagnosed without laboratory tests?

A: In mild, peripheral cases, experienced clinicians may treat empirically. However, laboratory confirmation is crucial for central, severe, or non-responsive cases to ensure the correct diagnosis and treatment.

Q: What are the risks if keratitis is left untreated?

A: Untreated keratitis can lead to corneal scarring, perforation, endophthalmitis, and permanent vision loss.