Which statement best describes using STT to differentiate ADDE from EDED?

• A. STT is always abnormal in EDED
• B. STT can differentiate ADDE from EDED because ADDE shows abnormal STT and EDED is within normal limits
• C. STT has no role in differentiation
• D. STT measures lipid layer quality

Answer: (B)

The key idea is that the Schirmer Tear Test measures how much aqueous tear fluid the lacrimal glands produce. In aqueous-deficient dry eye (ADDE), tear production is reduced, so the STT result is abnormal (low). In evaporative dry eye (EDED), tear production is typically normal; the problem lies in rapid evaporation rather than production, so the STT tends to be within normal limits. Because of this, STT can help distinguish ADDE from EDED: an abnormal result points toward ADDE, while a normal result points toward EDED. Of course, other tests for tear film stability and ocular surface health can add context, but the fundamental distinction rests on the difference in tear production vs evaporation.