Diagnosing Conjunctivitis

Conjunctivitis—also known as pinkeye—is a condition in which the protective membrane that lines the eyelid and covers the outer surface of the eye, called the conjunctiva, becomes inflamed. NYU Langone ophthalmologists are skilled at identifying and helping people manage this common condition.

Conjunctivitis can affect people of any age. It tends to spread easily among people who work or live together.


There are several different causes of conjunctivitis, and our ophthalmologists are experienced at identifying them. If you have any symptoms, see your ophthalmologist as soon as possible.


Viruses that cause colds are the most common cause of conjunctivitis. Like other mucus membranes, such as the nose and ears, the conjunctiva in your eye is vulnerable to infectious agents.

Viral conjunctivitis typically begins in one eye but can easily spread to the other eye. The condition can remain contagious for up to two weeks, but it tends to go away on its own within a few days or up to two weeks.

Symptoms include redness in the eye, itching, and clear, watery discharge. Many people with viral conjunctivitis find that their eyelids are stuck together or that their vision is blurry when they wake up in the morning. This is due to the discharge that accumulates on the eyelids while you are asleep.

Bacterial Infections

Bacterial conjunctivitis is common in adults and is usually caused by staphylococcus and streptococcus bacteria. Like the viral form, bacterial conjunctivitis is highly contagious.

Most strains of bacterial conjunctivitis are mild and easily managed, but some strains can lead to serious eye problems if they are not treated right away. For example, gonococcal conjunctivitis is caused by the same bacterium that leads to the sexually transmitted disease gonorrhea. These bacteria can penetrate the cornea, the clear surface that covers the front of the eye.

Symptoms include redness, discomfort, itching, and thick, pus-like, yellow discharge. As with viral conjunctivitis, you may have difficulty opening your eyes when you wake up because of the sticky buildup of discharge on the eyelids.

Bacterial conjunctivitis can cause vision loss if it is not treated immediately and aggressively with prescription antibiotics.


Allergic conjunctivitis, which is not contagious, occurs when the conjunctiva has an inflammatory response to certain allergens, such as pollen, dust, mold, or pet dander. When you’re exposed to these allergens, your body releases a chemical called histamine, which causes redness, tearing, and itching in the eye.

Many of these allergens occur only at certain times of the year or in specific environments. Other allergens, including cigarette smoke and pet dander, can irritate your eyes year-round. Symptoms of allergy-related conjunctivitis include intense itchiness and string-like, mucusy, clear, or white discharge.

Eye Irritants

If you’re exposed to smoke, chemical fumes, or other irritants, you may develop a type of conjunctivitis that is not contagious. Common eye irritants include secondhand cigarette smoke, smoke from a fireplace or a wood burning stove, and chlorine. Symptoms include redness, burning, pain, and tears.

A toxic chemical irritant, like acid or bleach, that gets in your eye can cause severe injury and requires immediate treatment to prevent complications. People with a chemical burn usually have red eyes and are in pain. They may also have excessive tearing and sensitivity to light. These people may have damage to the epithelium, the layer of cells that covers and protects the cornea.

If your eye is red due to exposure to a chemical irritant and you are in pain, go to the nearest emergency room.

Diagnostic Tests

Often, your ophthalmologist can diagnose conjunctivitis simply by examining your eye. Based on your symptoms, he or she can usually determine whether the inflammation is due to a viral or bacterial infection. He or she may perform the following tests to confirm a diagnosis:

Medical History

Your ophthalmologist can rule out many causes of conjunctivitis simply by asking about your symptoms and how they came about. He or she also asks whether you’ve been in close contact with other people who have conjunctivitis and if any irritant has come into contact with your eye.

Slit Lamp Exam

Most of the time, your doctor can diagnose conjunctivitis by using a slit lamp—an instrument that consists of a microscope and a high-energy beam of light. During a slit-lamp exam, your ophthalmologist shines a thin beam of light into your eye. This beam allows your doctor to examine the entire eye, including the conjunctiva; the sclera, or the white of the eye; the iris; and the cornea.

For a more detailed look at the eye, your doctor may put a drop of a yellow dye called fluorescein into your eye, which allows him or her to see any damage to the surface of the eye.

Visual Acuity Tests

Doctors also check to see if conjunctivitis has affected your vision by conducting a visual acuity test. This test checks to see how well you can read letters or symbols from 20 feet away, while covering one eye at a time.

Eye Culture

If you have had conjunctivitis for more than two or three weeks and it has not gone away on its own or with the help of home treatments, your doctor may want to perform an eye culture. During this test, your doctor takes a sample of the cells on the inside of your eyelids with a cotton swab and sends it to a laboratory to be examined by a pathologist.

A pathologist, who studies diseases under a microscope, can determine whether your conjunctivitis is caused by viruses or bacteria. This helps your doctor determine the most effective treatment.

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