A recent study demonstrated that dry eye signs and symptoms could work together to determine a single dry eye severity variable that is measurable on an interval scale. Researchers combined clinical observations with the symptom questions to estimate a single severity measure for dry eye disease using a Rasch model, which estimates the most likely value of the patient's dry eye severity from the pattern of observed values of the patient's signs and symptoms.

This study included 203 dry eye patients and 51 controls. Administered tests included the Ocular Surface Disease Index (OSDI), tear osmolarity, Schirmer's test, noninvasive break-up time, and ocular surface staining. Corneal and conjunctival staining were the most informative, while NIBUT was the least informative. However, no single test included >10% of total information about dry eye severity —the 18 indicators working together provided 10 times the information of the single most informative indicator.

Researchers believe including more tests, such as tests evaluating lipid layer of tear film, meibomian glands, or neurosensory abnormalities, into their method of using Rasch analysis may yield more information on severity of dry eye. The study concluded that an objective single severity measure for dry eye disease would be simple and very useful in clinical trials, particularly to determine eligibility and evaluate the effectiveness of the therapy. 

Karakus S, Akpek EK, Agrawal D, Massof RW. Validation of an objective measure of dry eye severity. Transl Vis Sci Technol. 2018;7(5):26.