Which of the following is NOT an appropriate use of a cobalt blue filter during a slit lamp exam?

• A. Performing Goldmann IOP with fluorescein
• B. TBUT testing
• C. Evaluating herpetic dendrites with rose bengal
• D. Evaluating fleischer rings in keratoconus without fluorescein

Answer: (C)

Cobalt blue illumination is used to excite fluorescein, making fluorescein-stained corneal defects and tear film patterns visible during slit lamp exams. That glow helps you assess tear breakup time and the corneal surface when fluorescein is used, such as during Goldmann IOP measurements and TBUT.

Evaluating herpetic dendrites with Rose Bengal isn’t appropriate under this illumination because Rose Bengal is a different stain that is best viewed with white light (not blue excitation). Dendritic ulcers are classically imaged with fluorescein under cobalt blue light, which highlights the characteristic branching pattern. Rose Bengal under cobalt blue doesn’t provide the same diagnostic view for dendrites.

Fleischer rings can be seen with standard illumination and don’t require fluorescein, so using cobalt blue for that purpose isn’t necessary.