Which tear film assessment is most useful to distinguish aqueous deficiency from evaporative deficiency?

• A. Schirmer Tear Test
• B. Interferometry
• C. Tear osmolarity
• D. TBUT

Answer: (A)

Distinguishing aqueous deficiency from evaporative deficiency hinges on whether the tear production itself is reduced. The Schirmer tear test directly measures how much aqueous tear is produced by the lacrimal glands. By placing a small paper strip in the conjunctival sac for about one minute, you quantify the amount of tear that wicks up the strip. A low value indicates aqueous-deficient dry eye, meaning the glands aren’t making enough fluid. If the tear production is normal or only mildly reduced, yet the eye remains dry or shows signs of instability, evaporative deficiency is more likely due to rapid evaporation or poor tear film quality rather than a lack of production.

Other tests look at different aspects of the tear film but don’t as clearly separate the mechanisms. Interferometry assesses the lipid layer, which is often implicated in evaporative disease. Tear break-up time measures how long the tear film stays intact, and it tends to be shorter with both types of dry eye but is particularly sensitive to evaporative issues. Tear osmolarity reflects overall tear film dryness but doesn’t specify whether the problem is production or evaporation.

So, the test that best distinguishes aqueous deficiency from evaporative deficiency is the Schirmer test, because it provides a direct measure of tear production and helps identify true aqueous deficiency.