Which of the following is a limitation of TFBUT when using fluorescein dye?

• A. The test requires no specialized equipment.
• B. It does not depend on technique.
• C. The presence of dye always improves test accuracy.
• D. Dye-Induced Instability: The addition of fluorescein dye can actually decrease tear film stability.

Answer: (D)

TFBUT with fluorescein can artificially destabilize the tear film, so the measured breakup time may be shorter than the true baseline stability. Fluorescein can alter surface properties and interact with the tear film’s lipid and mucin layers, and the dye can change how the tear film behaves under blinking and spreading. This means the test outcome may reflect dye-induced changes as much as the natural tear film stability, making that limitation a real concern. Clinicians recognize this and interpret TBUT with fluorescein cautiously, sometimes opting for non-invasive methods when possible to avoid dye-related artifacts.