How Pink Eye Is Diagnosed

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Although pink eye most commonly refers to conjunctivitis, there are other conditions that can also cause the eye to become red. A careful physical examination and use of proper lab tests can help to distinguish between conjunctivitis and more serious ocular conditions.

pink eye diagnosis
Illustration by Joshua Seong. © Verywell, 2018. 

Physical Examination

Conjunctivitis is most commonly diagnosed by simple physical examination.

Number of Eyes Affected

Bacterial conjunctivitis tends to start in one eye but often spreads to the other eye. Viral conjunctivitis, however, tends to affect only one eye.

Eye Discharge

Conjunctivitis often causes discharge from the eye. When the cause is a bacteria, that discharge is often thick and purulent, i.e., yellow or green. It tends to cause crusting that can make it difficult to open the eye in the morning.

Viral conjunctivitis, on the other hand, tends to have thinner watery discharge. While this discharge can be sticky, it is unlikely to force the eye shut.

Subconjunctival Hemorrhage

Pink eye gets its color from inflamed blood vessels. A subconjunctival hemorrhage develops when one of these blood vessels breaks. Instead of thin red lines in the white part of your eye, you will see a bright patch of red.

Although it can be conspicuous in appearance, it is not dangerous and usually recovers in a week or two. These hemorrhages are more common with viral conjunctivitis.

Swollen Lymph Nodes

A proper physical exam is not limited to the eyes. Lymph nodes around the ear and neck can sometimes get swollen and tender with viral, but not bacterial, conjunctivitis.

Special Tests

Depending on your history and symptoms, your healthcare provider may choose to perform additional testing during your physical exam.

Eversion of the Eyelid

Pink eye can occur when a foreign body gets into your eye. Until that foreign body is removed, you are likely to have an inflammatory reaction. Your healthcare provider may need to flip your upper eyelid inside out to make sure there is not something stuck between your eyelid and your eyeball that could be causing irritation.

The procedure may sound painful but it is not. In some cases, anesthetic eye drops can be used to make you more comfortable during the exam.

Fluorescein Eye Stain

A dark orange water-soluble dye called fluorescein can be placed in your eye to look for irritation and injury that can not be seen on routine exam. The dye stains the cornea and lights up over areas where superficial epithelial cells are loose or otherwise stripped away.

Areas that light up with the dye can be a sign of a corneal abrasion or may show a dendritic pattern often seen with herpes simplex eye infections. The dye can also make it easier to locate a foreign body within the eye.

Fluorescein is placed in your eye by having you blink onto a strip of dye-coated paper or by using eye drops. Your healthcare provider will then look at your eye under cobalt blue light. Altogether, the test takes only minutes to perform.

At first, the whites of your eye will take on a yellow color but natural tears wash out the fluorescein over minutes to hours. Any fluorescein that touches the skin around the eye could stain your skin for a day or two.

Slit Lamp Examination

A more formal eye exam may be performed using a slit lamp. This is essentially a microscope that shines a thin beam of light into your eye. Your healthcare provider will use different lenses to evaluate the front chambers as well as the back chambers of your eye.

This equipment is most often found in an ophthalmologist office but some primary care offices, urgent care clinics, and emergency department may also have access to a slit lamp.

Lab Tests

Lab testing can improve the accuracy of the diagnosis and may help to guide more effective treatments. After all, bacterial conjunctivitis may require antibiotics, but viral infections are self-limited and heal on their own. That said, many healthcare providers often treat based on their clinical exam alone.

Culture

The gold standard for diagnosing any infection is culture. Not only will the causative bacteria be identified, but it can then be tested against different antibiotics to show which ones are most effective.

For conjunctivitis, a sample of tears or other ocular discharge can be collected with a swab and sent to the laboratory. The problem with cultures is that it can take days to get results. That is too long to wait for treatment.

Unless you have had resistant or recurrent infections, cultures are rarely used to diagnose conjunctivitis.

PCR Testing

Polymerase chain reaction (PCR) is a more advanced technique that uses DNA from a sample to see if an infection is present. Unlike traditional culture, it cannot check for antibiotic susceptibility. 

When it comes to conjunctivitis, PCR can be used to screen for both bacteria and viruses. The most common bacteria screened for are chlamydia and gonorrhea. Adenovirus, which accounts for 70 percent of all conjunctivitis cases, and herpes simplex are viruses which also have PCR tests available. Results are often available within 24 hours.

Rapid Adenovirus Screening

While PCR can speed up the process, it still does not allow healthcare providers to make a diagnosis at the time of your visit. That could mean a delay in treatment.

A rapid point-of-care test is now available. It screens for all serotypes of adenovirus and can be run in your healthcare provider's office. In 10 minutes, you will know if you have the virus. In this case, you do not need antibiotics and can save on the cost of treatment. Unfortunately, not all offices offer the test.

If offered, the test is quite simple. Your doctor will give you a special eye drop, stretch your lower eyelid down a bit, then place the testing rod to your inner eyelid, gently rubbing it to collect a sample.

Differential Diagnosis

Most patients with pink eye have a benign or self-limited condition and do not require referral to an ophthalmologist. Conjunctivitis can be caused by bacteria and viruses but other causes like allergies, chemical exposures, and trauma are also common.

Red flag symptoms that should prompt further evaluation include fever, severe eye pain, or impaired vision. This should prompt emergent evaluation with an ophthalmologist.

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