A doctor can often determine whether a virus, bacterium, or allergen is causing the conjunctivitis (pink eye) based on patient history, symptoms, and an examination of the eye. Conjunctivitis always involves eye redness or swelling, but it also has other symptoms that can vary depending on the cause. These symptoms can help a healthcare professional diagnose the cause of conjunctivitis. However, it can sometimes be difficult to make a firm diagnosis because some symptoms are the same no matter the cause.

It can also sometimes be difficult to determine the cause without doing laboratory testing. Although not routinely done, your healthcare provider may collect a sample of eye discharge from the infected eye and send it to the laboratory to help them determine which form of infection you have and how best to treat it.


Viral Conjunctivitis

The cause is likely a virus if

  • conjunctivitis accompanies a common cold or respiratory tract infection, and
  • discharge from the eye is watery rather than thick


Bacterial Conjunctivitis

The cause may be bacterial if

  • conjunctivitis occurs at the same time as an ear infection
  • occurs shortly after birth
  • discharge from the eye is thick rather than watery


Allergic Conjunctivitis

The cause is likely allergic if

  • conjunctivitis occurs seasonally when pollen counts are high
  • the patient’s eyes itch intensely
  • it occurs with other signs of allergic disease, such as hay fever, asthma, or eczema