Post-LASIK Corneal Scraping Location with Negative Fluorescein Stain
When the flap interface is involved but no surface ulceration is visible (negative fluorescein stain), you must elevate the LASIK flap to obtain scrapings directly from the interface and stromal bed. 1, 2
Why Flap Elevation is Mandatory
Post-LASIK microbial keratitis characteristically presents with focal corneal infiltrates that extend beyond the lamellar interface into deeper or superficial stroma, often without surface epithelial defects. 1, 2
The absence of fluorescein uptake does not exclude infection—it simply indicates the epithelium is intact while the infection involves the interface or deeper layers. 2
Scrapings must be obtained from the involved area, which requires direct access to the flap interface when surface ulceration is absent. 1, 2
Specific Scraping Technique
Use topical anesthetic (proparacaine 0.5%, not tetracaine due to antimicrobial effects that interfere with culture results). 3
Elevate the LASIK flap to expose the interface and stromal bed. 1, 2
Obtain scrapings from both the undersurface of the flap and the residual stromal bed using a sterile blade, platinum spatula, or jeweler's forceps. 1, 4
Scrape from the base and periphery of the infiltrate—avoid collecting only purulent material as this yields inadequate results. 1, 3
Critical Microbiologic Work-Up
Send specimens for bacterial, fungal, atypical mycobacterial, and Nocardia cultures immediately. 2
Apply material to slides for Gram stain, Giemsa stain, potassium hydroxide, Calcofluor white, and acid-fast stains. 3
The microbiology of LASIK-associated infections differs from other corneal infections and includes atypical mycobacteria, methicillin-resistant S. aureus, Nocardia, fungi (including Aspergillus, Acremonium, Fusarium), and HSV. 1, 2, 4, 5
Immediate Management After Scraping
Initiate intensive broad-spectrum topical antibiotics immediately after obtaining cultures. 1, 2
Apply antibiotics directly onto the flap interface if the infiltrate involves this space. 1, 2
Consider voriconazole interface wash (100 μg/mL) if fungal infection is suspected based on clinical presentation or early stain results. 4
Common Pitfalls to Avoid
Never assume a negative fluorescein stain rules out infection—interface infections frequently present without surface epithelial defects. 2
Never delay flap elevation when interface involvement is suspected, as this is the only way to access the infected tissue for adequate sampling. 1, 2
Never rely on clinical appearance alone to differentiate bacterial from fungal keratitis—cornea specialists correctly identify the pathogen in fewer than 70% of cases without cultures. 3
Be aware that prior intensive topical corticosteroid use can mask infection severity and modify clinical presentation. 1, 2