Which anatomical feature can interfere with TFBUT measurements in dogs?

• A. The nictitating membrane can interfere with the test even when eyelids held open.
• B. Eyelid color
• C. Corneal thickness
• D. Lacrimal gland function

Answer: (B)

TFBUT hinges on clearly seeing the fluorescein-coated tear film on the corneal surface under blue light. Any factor that reduces visual contrast or obscures the cornea makes it hard to detect the moment the tear film breaks up. In dogs with pigmented eyelids, the dark coloration around the lid margins lowers the contrast and can mask the fluorescein signal, making breakup harder to observe accurately. So eyelid color directly affects the observer’s ability to time the breakup, making it the most influential anatomical feature in this context. The other factors either affect tear dynamics or may occasionally obscure view, but they don’t consistently interfere with visualization as eyelid pigmentation does.