Phenol red thread test

The phenol red thread test is also a method to quantitatively evaluate aqueous tear production.This test is similar to the Schirmer test, but instead of a paper filter strip a thin thread is inserted into the lower cul-de-sac about one-third of the distance from the outer canthus. The small caliber of the thread diminishes basal tear contribution as compared with the Schirmer I test.A color change in the thread from yellow to red makes the visibility of the wet versus dry portions of the thread readily visible. Interpretation of this test is also the subject of some debate, but it is generally agreed that after 15 seconds less than 10 mm of wetting is abnormal, whereas 20 mm or greater indicates normal tear production. This test is done one eye at a time, because measurement is made in seconds versus minutes. 

It should be noted that both the Schirmer and phenol red thread tests should be performed before the instillation of ophthalmic dyes. A summary of these tests and interpretation of the results is provided in Table 24-2. 

Saleh T, McDermott B, Bates A, et al. Phenol red thread test vs Schirmer s test a comparative smdy Eye 2005 ePub ahead of print. 

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