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 diagnose 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.