Dry eyes can be more than just an annoyance. In severe cases, dry eyes can cause damage to the cornea--the curved membrane that covers the iris and pupil--and vision loss. Dry eye can be caused by one of two problems: an inability to produce adequate tears, or an inability for the tear film to remain on the eye. There are several tests commonly used to check for dry eyes.
Shirmer's Test
Shirmer's test measures the eye's ability to produce tears. Shirmer's test measures tear production by collecting tears produced over a five-minute period. If few tears are produced, the problem is production. If adequate tears are produced, but the symptoms of dry eyes are present, the problem is evaporation or break up of the tear film.
According to the University of Illinois Eye and Ear Infirmary, the test is done by placing a piece of filter paper on the lower eyelid. Patients blink normally; at the end of the test, the amount of wetting of the filter paper is measured; a negative test, which is more than 10mm of wetting, means tear production is adequate. Dry eyes are diagnosed if the amount is less than 5mm. the National Eye Institute recommends not using a numbing drop before the test, because it changes the type of tear secretion.
Tear Film Dye Studies
If your doctor suspects that your eyes are producing enough tears, but that the tear film isn't staying on the eye, she may do a dye test to test tear film break up time (BUT). Break up time refers to the amount of time it take the tear film on the eye to break up and stimulate a blink reflex. The Eye and Ear Infirmary states that the tear film breaks up more quickly in dry eyes. To assess break up time, fluorescein, a dye, is applied to the lower eyelid through a strip of paper, which is then removed. The break up time of the tear film is examined under a slit lamp microscope; time is measured from the last blink to the BUT as seen under cobalt blue light. The dye filter paper may feel scratchy, according to Medline Plus, especially if the eyes are dry.
Rose Bengal is another type of dye that may be used to diagnose dry eye; this dye stains the cells so that the spots where the tear film is inadequate can be seen, according to DocShop.com.
Phenol Red Thread Dye Test
In this test, a thread stained with phenol, a dye that changes colors in response to pH changes, is placed in the lower corner of the eye for 15 minutes. At the end of the time, the length of dye thread that has changed from red to yellow is measured. Normal results are between 9 and 20mm; less than 9mm indicates dry eye, according to the Eye and Ear Infirmary.
