Conjunctivitis laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Sara Mehrsefat, M.D. [2]
Overview
Laboratory tests are not often required as conjunctivitis is often treated empirically. Obtaining conjunctival cultures is generally reserved for cases of suspected infectious neonatal conjunctivitis, recurrent conjunctivitis, conjunctivitis recalcitrant to therapy, conjunctivitis presenting with severe purulent discharge, and cases suspicious for gonococcal or chlamydial infection.
Laboratory Findings
Laboratory tests usually are not required in patients with mild conjunctivitis of suspected viral, bacterial or allergic origin. However, specimens for bacterial cultures should be obtained in patients who have severe inflammation (hyperacute purulent conjunctivitis) or chronic or recurrent conjunctivitis. Cultures also should be obtained in patients who do not respond to treatment.
Viral Conjunctivitis
In-office rapid antigen testing is available for adenoviruses and has 89% sensitivity and up to 94% specificity. This test can identify the viral causes of conjunctivitis and prevent unnecessary antibiotic use.
Bacterial Conjunctivitis
Acute bacterial conjunctivitis is usually self-limited and laboratory tests usually are not required. Swabs for bacterial culture is generally reserved for cases of suspected infectious neonatal conjunctivitis (ophthalmia neonatorum), recurrent conjunctivitis, conjunctivitis recalcitrant to therapy, conjunctivitis presenting with severe purulent discharge, and cases suspicious for gonococcal or chlamydial infection.
Chlamydial Infection
Several laboratory procedures can be used to identify chlamydial infections. These include cell culture, direct fluorescent monoclonal antibody staining of smears, enzyme immuno-assays for Chlamydia organisms, DNA hybridization assays and a polymerase chain reaction test to identify chlamydial antigens. Many ophthalmologists obtain conjunctival cytology scrapings for Gram staining and/or Giemsa staining to help characterize the conjunctival inflammatory response. The findings can be helpful, particularly for diagnosing allergic, chlamydial and certain atypical forms of conjunctivitis in which the clinical diagnosis is not immediately apparent. Conjunctival scrapes for cytology are rarely done because of the cost and the general lack of laboratory staff experienced in handling ocular specimens.
Gonococcal Infection
The diagnostic work-up for a gonococcal ocular infection includes immediate Gram staining of specimens for gram-negative intra-cellular diplococci, as well as special cultures for neisseria species on chocolate agar or Thayer-Martin.
Neonatal Conjunctivitis
Laboratory studies for neonatal conjunctivitis should include the following: Conjunctival scraping for gram stain or giemsa stain Conjunctival scraping for polymerase chain reaction assay (PCR) to detect chlamydia and gonorrhea Culture on chocolate agar and/or Thayer-Martin for N gonorrhoeae Culture on blood agar for other bacteria Culture of corneal epithelial cells for HSV if cornea is involved; PCR should also be considered in cases of possible HSV conjunctivitis
Allergic Conjunctivitis
Most cases are mild, and the patient just needs reassurance. Superficial conjunctival scrapings may help to establish the diagnosis by revealing eosinophils, but only in the most severe cases, since eosinophils are typically present in the deeper layers of the substantia propria of the conjunctiva. Therefore, the absence of eosinophils on conjunctival scraping does not rule out the diagnosis of allergic conjunctivitis. Measurement of tear levels of various inflammatory mediators, such as IgE, histamine, and tryptase, as indicators of allergic activity. Additionally, skin testing by an allergist may provide definitive diagnosis. Skin testing is now highly practical.
Conjunctival incisional biopsy is occasionally done when granulomatous diseases (e.g., sarcoidosis) or dysplasia are suspected.