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Laboratory Tests and Ancillaries
Various diagnostic tests are useful in confirming the diagnosis
but would often correlate poorly with symptoms. These tests can evaluate tear
film instability, ocular surface damage, and aqueous tear flow.
Fluorescein
Dye Disappearance Test or Tear Function Index
The
fluorescein dye disappearance test or tear function index is used to assess the
clearance or turnover of tears on the ocular surface. This test evaluates
aqueous tear production, tear volume, and tear drainage. Findings in this test
are more correlated with the severity of ocular irritation symptoms and corneal
fluorescein staining than the Schirmer test. Delayed clearance is associated
with high tear cytokine concentration which may contribute to chronic
inflammation.
Lacrimal
Gland Function Test
The
lacrimal gland function test measures lactoferrin, the most abundant tear
protein being secreted by the lacrimal gland. There is low lactoferrin
concentration in patients with Sjögren syndrome and other causes of lacrimal gland dysfunction.
Matrix Metalloproteinase-9 (MMP-9) Test
The MMP-9 test may be used to assess
the presence of inflammation and changes in disease states which may aid in the
management.
Ocular Surface Dye Staining
Ocular surface dye staining evaluates the
ocular surface by using dyes such as fluorescein dye, rose bengal, or lissamine
green dyes. The degree
oof conjunctival staining is indicative of the severity of dry eye syndrome.
In the case of fluorescein dye, it
stains areas of the corneal and conjunctival epithelia with disrupted
intercellular junctions that are adequate to allow the dye to penetrate into
the tissue. The presence of blotchy or exposure-zone punctate staining is
consistent with DES. While mild staining may be seen in normal eyes especially in
the morning. Rose bengal stains the ocular surface cells that are deficient in
mucous coating and debris in the tear film. This stain may detect involvement
of the conjunctiva by staining cells in the interpalpebral zone. Lastly,
lissamine green stains similar to that of rose bengal but is well tolerated as
fluorescein dye. It is not used in evaluating corneal epithelial disease.
In cases of viral keratoconjunctivitis
and medicamentosa, ocular surface dye staining shows diffuse staining of the
cornea and conjunctiva. Staphylococcal blepharitis, meibomian gland
dysfunction, or lagophthalmos would show staining on the inferior cornea and
bulbar conjunctiva. In the case of superior limbic keratoconjunctivitis, there
is staining on the superior bulbar conjunctiva. Staining on the interpalpebral cornea and
bulbar conjunctiva is usually observed in aqueous tear deficiency.
Schirmer Test
Schirmer test is used to assess aqueous
tear production or flow. It is performed by placing a narrow filter paper strip
in the inferior cul-de-sac and measuring the length of the strip wets during
the period of 5 minutes. This test may be done with or without anesthesia to
measure the reflex tearing or with anesthesia to measure basal tearing by
minimizing ocular surface reflex activity. Schirmer test with anesthesia is
considered abnormal if the result is ≤10 millimeters after 2 minutes. It must be remembered
that an isolated abnormal result may be nonspecific but a consistently low
result is highly suggestive of aqueous tear deficiency. However, given that
results of the Schirmer test are usually variable, it should not be used as the
sole criterion to diagnose DES.
Tear Film Break-up Time (TFBUT) test
The TFBUT test measures the lapsed time
between the last blink and the appearance of the first break in the
fluorescein-stained tear film. It is used to evaluate tear film instability. The
presence of a localized anterior basement membrane abnormality may be
considered if the tear break-up time in the same area is recurrent. An abnormal
result is considered when TFBUT is <10 seconds which may be seen in both
aqueous tear deficiency and meibomian gland disease.
Tear Osmolarity Test
The tear osmolarity test has been shown
to be more sensitive and specific in diagnosing and grading the severity of DES
as compared to corneal and conjunctival staining, TFBUT, Schirmer test, and
meibomian gland grading.
Other Tests
Videokeratography may be used to
objectively diagnose and evaluate the severity of DES, screen patients prior to
LASIK, and identify their post-LASIK chronic DES risk. Another test that can be
done is impression cytology. Impression cytology is a less invasive alternative
to ocular biopsy that is used to determine abnormalities like goblet cell loss
and squamous metaplasia.
Specific Tests in Patients with
Underlying Conditions
Antibodies such as anti-Rho (SSA), anti-La (SSB), anti-nuclear
antibody (ANA), and rheumatoid factor are detected for Sjögren syndrome. Additional
biomarkers such as salivary protein 1 (SP1), carbonic anhydrase 6 (CA6), and
parotid secretory protein (PSP) may be used to determine early Sjögren syndrome
or other forms of autoimmune DES.
Antithyroid peroxidase antibody, antithyroglobulin antibody, and
orbital imaging such as computed tomography (CT) and magnetic resonance imaging
(MRI) scan can be done to assess for thyroid disease.
MRI is especially helpful in assessing extraocular muscle
thickness. Serum lysozyme, ACE test, chest CT scan (to determine the extent of
disease) and conjunctival biopsy can be done in patients with sarcoidosis.
Lastly, conjunctival biopsy with light microscopy,
immunofluorescent or immunohistochemical studies are done for those with ocular
mucous membrane pemphigoid.