Finding the right tests…
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Finding the right tests…
Microbiology
A sputum culture grows the phlegm you cough up from your lungs to find out which bacteria, if any, are causing a chest infection such as pneumonia or a bacterial flare of bronchitis. Your doctor usually orders it when a chest X-ray or exam already points to infection and antibiotics haven't worked, or before starting treatment in someone who is very unwell, since choosing the right antibiotic from the start matters more when breathing is already affected. It is a routine bacterial culture, and a separate, specialised test is used when tuberculosis specifically is suspected.
This test reports whether bacteria grew from your sputum sample and, if they did, which type and which antibiotics work against it. A cough producing thick or discolored sputum, along with fever or breathlessness, is the usual trigger for ordering it, and the antibiotic sensitivity portion is what actually changes a prescription when a first antibiotic hasn't helped. Because sputum passes through your mouth and throat on its way out, the lab also checks that the sample is really from deep in the lungs and not mostly saliva, since a saliva-heavy sample gives a misleading result.
An X-ray or exam can show that pneumonia is present, but not which organism is behind it, and different bacteria respond to different antibiotics. A sputum culture fills that specific gap when the first-choice antibiotic isn't clearing the infection or the illness is severe enough that guessing wrong carries real risk.
You'll be asked to cough deeply rather than just clear your throat, since a sample that's mostly saliva doesn't reflect what's happening in your lungs and can be rejected by the lab as not representative. A respiratory therapist may help with deep breathing or, occasionally, a saline mist to bring up a better sample.
Unlike a stain viewed immediately under a microscope, a culture needs the bacteria present to multiply into visible colonies before they can be identified, so it takes noticeably longer than a same-day test, and slower-growing organisms extend that further.
When TB is suspected specifically, a different, dedicated laboratory process is used instead of or alongside this routine culture, since TB bacteria grow far more slowly and need specialized handling that a standard bacterial sputum culture doesn't include.
Cough deeply from your chest rather than just clearing your throat, ideally first thing in the morning before eating or drinking. Rinse your mouth with water beforehand so saliva and food particles don't contaminate the sample.
Not exactly. A routine sputum culture checks for common bacteria causing pneumonia or bronchitis, while suspected TB is tested through a separate, specialised process designed for that much slower-growing organism. Tell your doctor if TB is a specific concern.
A saliva-heavy sample doesn't reflect infection deep in the lungs and may be reported as unsuitable or misleading. If this happens, you may be asked to try again, sometimes with help from a respiratory therapist.
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.
An X-ray shows a picture right away, but a culture needs any bacteria present to multiply into a large enough colony to identify and test against antibiotics. That growth step is what takes the extra time, and it varies by organism.
It means no significant bacterial growth was found, which can point toward a viral cause, a non-infectious cause, or antibiotics already suppressing growth. Your doctor interprets this alongside your symptoms and imaging.
There's no universal restriction, but rinsing your mouth with water first is usually recommended. Ask your doctor or the centre about any antibiotics you're already taking, since these can affect what grows.