Which test is best used for determining the quality of a patient's tears?

The test that is best used for determining the quality of a patient's tears is the tear break-up time (BUT) test. This test evaluates the stability of the tear film, which is crucial in assessing tear quality. It measures the time taken for dry spots to appear on the cornea after a blink, indicating how long the tear film remains intact before it begins to break down.

A longer tear break-up time generally suggests better tear quality and stability, while a shorter time indicates potential issues like dry eye disease or evaporative dry eye. Understanding tear quality is essential for diagnosing and managing conditions related to tear film abnormalities.

In contrast, other tests like tear count and related evaluations focus more on measuring volume rather than quality. Thus, while these tests can provide useful information about tear production and other factors, they do not directly assess the quality of the tear film as effectively as the BUT test does.