The Interaction of Diabetes Duration and Tear Film Osmolarity in the Estimation of Tear Film Instability: A Cross-Sectional Study of 60 Patients with Type 2 Diabetes

Background/Objectives: Dry eye disease is more common in diabetes than in non-diabetic populations (pooled odds ratio 2.30), and its risk rises with diabetes duration. We aim to examine if diabetes duration potentiates the association between tear osmolarity and tear film stability. Methods: Cross-sectional study of 60 consecutive adult patients with type 2 diabetes mellitus on stable therapy for ≥3 months, who discontinued artificial tears 1 week before assessment. Worse-eye Non-Invasive-Break-Up-Time (NIBUT) was modelled by Tobit regression with left-censoring at the 1 s instrument floor. We adjusted for body mass index, age, diabetes duration and maximum osmolarity. Results: The duration × osmolarity interaction was significantly associated with NIBUT (β = 0.023, 95% CI 0.005–0.040; Wald p = 0.010; likelihood-ratio p = 0.002). At a normal osmolarity of 290 mOsm/L, expected NIBUT declined from 3.19 s at 5 years to 1.10 s at 20 years; at the 308 mOsm/L cut-point from 2.15 s to 1.35 s; and at an elevated 320 mOsm/L it was essentially unchanged (1.70 s to 1.73 s). Conclusions: The association between a longer diabetes duration and a shorter NIBUT is confined to patients whose osmolarity remains normal and is not detectable once hyperosmolarity is established. Instability is already severe in hyperosmolar patients; what attenuates as osmolarity rises is the association with additional duration, not the instability itself. Osmolarity assessment may therefore be most informative in longstanding diabetes with still-normal osmolarity. Given our limited sample size, our results should be interpreted with caution and are hypothesis-generating for future external validation.