Dry eye disease is usually a symptomatic disorder that ranges in severity from mildly irritating to severely disabling. The main aim of diagnosis is to distinguish dry eye disease from other ocular surface diseases with similar symptoms, such as ocular allergy, surface toxicity and chronic, low-grade infection. Tests should also be performed to establish the impact of the disease on a patient's quality of life and to quantify disease severity in order to guide therapeutic choices.¹ However, its diagnosis has always been difficult, especially in the early stages of the disease, because of the lack of availability of an agreed standard diagnostic test.

Tear hyperosmolarity is central to the dry eye disease process and the value of measuring tear osmolarity in the diagnosis of dry eye disease is well established. However, tear osmolarity measurement has been restricted to research settings because conventional testing systems are complex, costly, time-consuming, or need larger tear samples than are obtainable in some forms of dry eye.

With the development of new instruments that accurately and easily measure tear osmolarity, we believe that this test should become the 'gold standard' for diagnosing dry eye disease in both the routine clinical and research settings.

Aren't current tests good enough?

Figure 1: The central role of tear hyperosmolarity in the dry eye process.

Current dry eye disease tests have a number of shortcomings. Firstly, most tests look at physical endpoints or parameters that are signs of late-stage disease. This makes it difficult for eye care practitioners to definitively diagnose the majority of patients, because most have mild to moderate forms of the disease. Secondly, commonly used tests such as the Schirmer test, ocular surface staining and tear film break-up time, are only semi-quantitative, subjective and have poor specificity. Thirdly, existing tests are time-consuming and can only be performed by an experienced doctor or technician.

Some eye practitioners use symptoms as the criterion for diagnosing dry eye. The administration of a structured questionnaire to patients is certainly a valuable tool for screening patients with dry eye disease.¹ However, symptoms alone are inadequate for the differential diagnosis of dry eye because the same symptoms can be experienced with a range of ocular surface conditions and tear film disorders.^2,3 In fact, the 2007 International Dry Eye Workshop (DEWS), in which we both participated, recommended that symptomatology questionnaires should be used in combination with objective clinical measures of dry eye status.¹

The role of tear hyperosmolarity

The TearLab System determines osmolarity through measurement of electrical impedance of tear samples. It uses a lab-on-a-chip approach: the disposable tip not only takes the tiny tear film samples (0.05 μL) from the eye but also contains electrodes that rapidly measure tear osmolarity.

The ocular surface requires a complete tear film to maintain its health and function, and this involves adequate production, retention and balanced elimination of tears. Any imbalance of these components can lead to the condition of dry eye.

A number of theories have been put forward to explain the mechanism(s) underlying the break-up of the tear film. We considered these various theories during the 2007 DEWS meeting and concluded that tear hyperosmolarity is the central mechanism causing ocular surface inflammation, damage and symptoms.¹ Acknowledging that our understanding of the etiopathology of dry eye is rapidly advancing, current concepts relevant to tear hyperosmolarity can be summarized as follows:

• Tear hyperosmolarity arises when there is excessive water evaporation from the surface of the eye. This is caused by diseases or conditions that either lead to tear film break-up and consequential excessive evaporation (evaporative dry eye), and/or lead to low aqueous tear flow (aqueous- deficient dry eye) (Figure 1).¹
• Tear hyperosmolarity initiates a cascade of inflammatory events that damage the surface epithelium of the eye. Damaging effects include epithelial cell death, a loss of goblet cells and a reduction in mucus secretion. Epithelial damage leads to tear film break-up, which further exacerbates tear hyperosmolarity, thereby creating a vicious circle.¹