*3.3.1.1 Schirmer test*

It is a test to quantify the tear production.

It is done with a blotting paper strip of 5 X 35 mm which is popularly known as Whatman filter paper number 41.

Method of application- It is folded 5 mm from the inner end which is rounded and placed in the lower fornix at the junction of middle and outer one-third and kept for 5 minutes. Touching the cornea or lashes should be avoided. Eyes should be gently closed during the procedures [23].

The normal tear production varies between 0.5 to 0.67 ml of tears/day and that wets more than 15 mm of the strip.

The Schirmer test are basically of 3 types but the most important amongst them are the first two.


The cut-off values for diagnosis have been proposed as ≤5 mm/5 min to ≤10 mm/5 min with 77–85% sensitivity and 70–83% specificity [23].

This test lacks repeatability and shows variable results, so a single test should not be used to diagnosis rather a series of abnormal results in Schirmer's test raise a suspicion of DED. However, low cost of this test makes it one of the most commonly used tests clinically.

• Variations

#### *3.3.1.1.1 Strip meniscometry*

A variation of the above test is available now-a-days, which is done by dipping a strip into tear meniscus for 5 seconds. 25 mm polyethylene terephthalate is used to make the strip and it is covered with a urethane-based material [24]. The value ≤ 4 mm raises suspicion of DED. When used alone it has a sensitivity and specificity of 84% and 58% respectively and when combined with Tear film break-up time test the sensitivity is reduced to 81% but the specificity increases to 99% [25].

### *3.3.1.1.2 1-minute Schirmer test*

It was proposed by Nelson to decrease the ocular discomfort and save time by decreasing the time of performing the test from 5 minutes to 1 minute [26]. The cut-off value was set to be 6 mm. Bawazeer and Hodge et al. in 2003 concluded that the 1-minute Schirmer test with anesthesia highly correlates with the 5-minute Schirmer test with anesthesia [27]. In cases of severe dry eye, a value of ≤ 5.5 mm in a 5-minute Schirmer test highly correlates with 2 mm in a 1-minute Schirmer test while in cases of mild to moderate dry eye a value of 5–10 mm in a 5- minute Schirmer test corresponds with 3–6 mm in a 1-minute Schirmer test.

### *3.3.1.2 Phenol red thread test (PRT)*

It uses a thin cotton thread impregnated with pH-sensitive dye "phenol red". When the dye is dry, the thread is of yellow color, but when the dry is moistened by tears, the thread turns red (as the tears has slightly alkaline pH between 7 and 8) [28].

Method of application- the folded end of the thread is hooked over the lower eyelid margin in the temporal one-third if the eyelid for 15 seconds.

It has few advantages, as it is small in dimension so less chance of reflex tearing and the minimal amount of dye on thread decreases the chances of reflex tearing [29, 30]. It suggests that the reading of PRT is indirect and realistic measure of the tear volume in resting phase [31, 32]. But despite these potential advantages it is rarely used in clinical practice as it is manufactured only in few countries which makes their supply costly.

A cut-off value of 20 mm is used for differentiation of DED with and without aqueous deficiency [33]. Sensitivity and specificity of a cut-off value of 10 mm are 25% and 93% respectively [34].

Doughty et al. concluded that there is no statistically significant difference in the PRT performed with open or closed eyes [35].

#### *3.3.2 To test the tear clearance*

#### *3.3.2.1 Fluorescein clearance test*

Method- 5 μl of 2% fluorescein dye is instilled in the eyes. One set of Schirmer papers are inserted for each 10-minute interval for 30 minutes. The amount of strip becoming wet and the disappearance of the dye were recorded. Nasal stimulation is done using a cotton tip along with the last strip to induce reflex tear secretion.

It is used to measure basal tears, reflex tears and tear clearance all at the same time.

A cut-off value of ≥ 3 mm at the 10-minute interval suggests normal tear secretion. If the dye cannot be detected at the 20-minute interval, it is known as "Clearance" [36].

It has many advantages such as it is inexpensive, easy to perform, availability of the materials used is adequate. The disadvantages are same as in standard Schirmer test. Jordan and Baum et al. in 1980 reported that the above disadvantages cannot be suppressed by use of topical anesthesia [37].

#### *3.3.2.2 Tear function index (TFI)*

Method- The procedure is similar to the Schirmer test with anesthesia, but it uses 10-μl of 0.5% fluorescein.

Fluorescein is instilled into the lower conjunctival fornix and after 5 minutes of instillation the length of the wetted portion of the strip is measured and the intensity of the staining of dye is compared to the standard strips. The rate at which the color of the fluorescein dye fades is used to determine the tear clearance rate (TCR). It is graded as 1, 1/2, 1/4, 1/8, 1/16 1/32, 1/64, 1/128 and 1/256.

$$TFI = \frac{\text{Values of Schirner test with anestheisia}}{TCR} \tag{2}$$

Kaye et al. proposed a variation of TFI by suggesting use of prepared strip containing 1.3 μl of 0.5% fluorescein. They reported that 10 μl of fluorescein use increases the volume of tear and it may also act as a stimulant. This in turn limits the applicability of the TFI test [38].

#### *3.3.2.3 Fluorophotometry*

Fluorophotometry is useful clinical tool, because an increased corneal uptake of fluorescein demonstrates subtle damage to the corneal epithelium. In humans, measurements of the penetration of fluorescein across the corneal epithelium can be used in diagnosing or monitoring dry eye disease. It is an excellent test but the need of the machine itself makes it a costly test so it not much used in clinical practice.

#### *3.3.3 To test the tear volume*

#### *3.3.3.1 Tear meniscus assessment by Meniscometry*

Tear meniscus height (TMH), curvature (TMR), and cross-sectional area (TMA) are used in clinical practice widely and have good accuracy rate and correlate well with other tests of DED [39, 40]. However, they are very much operated dependent. They have other drawbacks such as dependency on time from blink, fluorescein instillation. It can be influenced by the temperature, humidity, air velocity, illumination and location of measurement along the lid margin.

Now-a-days portable digital meniscometry with application software being installed in the iPod touch are being used. They have good reproducibility, good correlation with both the conventional video and Optical coherence tomography (OCT) meniscometry and it detects tear meniscus changed after the instillation of artificial tears.

At present, OCT meniscometry studies parameters such as upper and lower TMH, TMA, TMR and tear meniscus depth most commonly. Intra-observer and inter-observer repeatability are good with spectral-domain OCT meniscometry. All

#### *Diagnosis of Dry Eye DOI: http://dx.doi.org/10.5772/intechopen.98479*

these measurements in the OCT meniscometry are machine dependent and can be influenced by the following conditions - conjunctivochalasis, LIPCOF, disorders of lid margin congruity, and apposition between the lid and ocular surface [41–44]. But it has many advantages such as it is non-invasive and image is taken rapidly and its simple but analysis of the image may take time [45]. It is an excellent but costly tool to test tear volume.
