How Pink Eye Is Diagnosed

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

Both viral and bacterial conjunctivitis tend to start in one eye but often spread to the other eye. Bacterial conjunctivitis, however, tends to affect the other eye more quickly than the viral form.

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 a thinner watery discharge. While this discharge can be sticky, it is unlikely to force the eye shut.

Subconjunctival Hemorrhage

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.

"Pink eye" gets its name from the color of inflamed blood vessels.

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 and can occur from trauma, valsalva maneuver (sneezing, coughing fit, vomiting) and or systemic disease conditions (blood or vascular disorders), and can be more prone in those who take certain medications like blood thinners.

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

Flipping your eyelids inside out allows the healthcare provider to examine the palpebral conjunctiva under the upper and lower lids for redness and types of inflammation (pappilae/follicles) and can help determine if the conjunctivitis is viral, bacterial, or allergic. Your healthcare provider will also want 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 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 and then use a slit lamp during this procedure to look at the eye. 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 or optometrist's office but some primary care offices, urgent care clinics, and the 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. Many healthcare providers often treat based on their clinical exam alone.

Bacterial conjunctivitis may require antibiotics, but viral infections are self-limited and heal on their own.


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 are chlamydia and gonorrhea.

Adenovirus accounts for 70% of all viral conjunctivitis cases.

Adenoviruses and herpes simplex viruses also have PCR tests available. The 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 healthcare provider 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 or optometrist. Conjunctivitis can be caused by bacteria and viruses but other causes like allergies, chemical exposures, and trauma are also common.

Red flag symptoms include fever, severe eye pain, or impaired vision.

These symptoms should prompt emergent evaluation with an ophthalmologist or an optometrist.

Frequently Asked Questions

  • How can you tell if pink eye is caused by a virus?

    A virus is the most likely cause if, in addition to eye redness, you have a cold and symptoms of an upper respiratory infection. Bacterial pink eye is more likely if the discharge from the eyes is thick and crusty. Allergies may be the cause if the discharge is watery and you have other allergy symptoms.

  • Do allergies cause pink eye?

    No, allergies cause allergic conjunctivitis, not pink eye. Seasonal allergies caused by pollen, grass, or airborne allergies and year-round allergies caused by animal dander, mold, or dust can cause inflammation of the eye tissue. Pink eye is usually caused by a virus.

7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Wood M. Conjunctivitis: Diagnosis and Management. Community Eye Health. 1999; 12(30): 19–20.

  2. Centers for Disease Control and Prevention. Conjunctivitis (Pink Eye): For Clinicians.

  3. Centers for Disease Control and Prevention. Conjunctivitis (Pink Eye): Treatment.

  4. Prokopich CL, Hrynchak P, Elliott DB, Flanagan JG. Ocular Health Assessment. In: Elliott DB, ed. Clinical Procedures in Primary Eye Care. 4th ed. Philadelphia, PA: Elsevier; 2014: Chap 7.

  5. Holtz KK, Townsend KR, Furst JW, et al. An Assessment of the AdenoPlus Point-of-Care Test for Diagnosing Adenoviral Conjunctivitis and Its Effect on Antibiotic Stewardship. Mayo Clinic Proc: IQO. 2017 Sept; 1(2):170-175. doi:10.1016/j.mayocpiqo.2017.06.001

  6. Center for Disease Control and Prevention. Conjunctivitis (pink eye) diagnosis.

  7. Hopkins Medicine. Allergic conjunctivitis.

Additional Reading
  • Jacobs DS. Conjunctivitis. In: Sullivan DJ, ed. UpToDate (Internet), Waltham, MA.

By Tanya Feke, MD
Tanya Feke, MD, is a board-certified family physician, patient advocate and best-selling author of "Medicare Essentials: A Physician Insider Explains the Fine Print."