Finding the right tests…
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Finding the right tests…
Microbiology
A conjunctival culture swabs the inside lining of your eyelid and the surface around your eye to find out what's causing an eye infection. Most eye redness and discharge in adults clears up on its own or is viral, so your doctor usually diagnoses pink eye just by looking at it. A culture gets ordered when the discharge is thick and pus-like, the infection keeps coming back, it hasn't improved on standard eye drops, or it shows up in a newborn, where a fast-moving bacterial infection can threaten the cornea. The lab grows whatever organism is on the swab, then checks which eye drops will actually work against it.
This test looks for bacteria growing from a swab of your eye discharge or eyelid lining. A newborn with conjunctivitis, an adult whose infection hasn't responded to standard antibiotic drops, or someone with unusually heavy, colored discharge are the situations where a culture adds real information beyond an exam. Some bacteria that cause eye infections, including one linked to gonorrhea, can damage the cornea quickly, so identifying the exact organism changes how urgently and with what your doctor treats it, rather than just prescribing a generic drop.
A doctor can usually tell conjunctivitis is present just by looking at the eye, but telling a viral cause from a bacterial one by sight alone is unreliable, since both can cause redness, discharge, and irritation. A culture answers the question an exam can't: which organism, if any, is actually growing there.
The swab is streaked onto special growth plates and incubated. If bacteria are present, they multiply into visible colonies over the following days; a lab technician then identifies the species and exposes it to different antibiotic eye drops to see which ones stop it from growing.
A few bacteria that cause conjunctivitis, most notably Neisseria species, can progress fast enough to threaten the cornea and vision within days, which is why hyperacute cases are treated with injectable or oral antibiotics alongside drops rather than drops alone. That urgency is also why a newborn with eye discharge gets cultured rather than simply treated on suspicion.
If nothing grows, it tells your doctor the infection is probably viral or allergic, which means antibiotic drops won't help and should usually be stopped rather than continued indefinitely.
For a first, mild case, looking is often enough, since most conjunctivitis is viral and clears on its own. A culture gets added when the case is severe, recurring, not improving, or in a newborn, because those situations need to know exactly what's growing.
A healthcare worker gently wipes a sterile swab across the inside of your lower eyelid or any visible discharge. It takes a few seconds and feels like a light touch, not a scrape.
It means no bacteria grew from your sample, which usually points to a viral or allergic cause instead. Antibiotic drops won't help a viral or allergic case, so your doctor may stop them if the culture comes back negative.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
The routine bacterial culture can pick up Neisseria species, but chlamydial conjunctivitis usually needs a separate, specific test since chlamydia doesn't grow on standard culture plates. Tell your doctor if a sexually transmitted cause is suspected so the right test gets ordered.
Ask your doctor or the centre, since instructions can depend on how the infection looks and whether contact lens wear is a suspected factor. Don't stop or restart lens wear on your own once an infection is suspected.
Not usually. Most positive cultures are still treated with antibiotic eye drops; pills or injections are reserved for the fast-moving, sight-threatening infections your doctor screens for on exam.