Sputum culture test explained: how it works and what results mean

Sputum is the thick mucus or phlegm that is coughed up from the lower airways and lungs. It is different from saliva or spit. A sputum culture is a laboratory test in which a sample of this phlegm is placed on nutrient media and kept at body temperature so that any bacteria present can grow and be identified.

Lab Tests Online UK says the test is used to detect and diagnose bacterial infections of the lower respiratory tract, such as bacterial pneumonia, and to guide treatment. A doctor may request it when they suspect a bacterial chest infection, when an infection has not responded as expected to treatment, or occasionally after treatment to check its effectiveness.

The NHS says that for chest infections you may need a blood test or a sample of your mucus tested to see what is causing the infection, and that antibiotics are only used to treat bacterial chest infections, not viral ones such as flu or viral bronchitis.

The quality of the sample largely determines the usefulness of the result. Lab Tests Online UK describes a fresh sputum sample of deep respiratory secretions, usually collected first thing in the morning. Rinse your mouth out with water before collecting it, to remove loose cells from the mouth.

Samples are coughed up (expectorated) into a pot supplied by the laboratory. Deep coughing is generally needed, and it is phlegm from the lungs that is wanted, not saliva. If someone cannot produce a sample, it can often be induced by inhaling sterile saline for several minutes to loosen the phlegm. Samples should be taken to the laboratory promptly while fresh.

The laboratory checks that a sample is adequate before testing. A sample that is not adequate must be rejected and a further sample collected, which is one reason a second sample may be requested. A second sample may also be requested if the doctor wants additional tests.

The sputum is broken up so that bacteria are evenly distributed, placed on suitable nutrient media and incubated. Lab Tests Online UK explains that sputum is not normally free of bacteria, so a person with a bacterial infection will typically have both normal and infection-causing bacteria present. A trained scientist identifies the different types and categorises them as normal or potentially infection-causing, and follows up suspected infection-causing bacteria with confirmatory tests.

Where infection-causing bacteria are found, antimicrobial susceptibility testing is usually performed, which exposes the bacteria to different antibiotics in the laboratory to see which are likely to work and to detect resistance.

Lab Tests Online UK says that if disease-causing bacteria are detected in a person with symptoms of a lower respiratory tract infection, it is likely that the symptoms are due to a bacterial infection. The most common cause of bacterial pneumonia in adults in the UK is Streptococcus pneumoniae (pneumococcus), and other common bacteria include Staphylococcus aureus, Haemophilus influenzae, Moraxella catarrhalis, Klebsiella pneumoniae and Streptococcus pyogenes.

If no disease-causing bacteria are found, it may be that the symptoms are not due to an infection, that there were too few organisms in the sample, or that the organism responsible cannot be detected by a routine bacterial culture. A negative or mixed result therefore does not always mean there is no infection.

Symptoms that may prompt a request, according to Lab Tests Online UK, include cough, fever and chills, muscle aches, fatigue, trouble breathing, chest pain and confusion. It also says to see a GP if symptoms get worse, you feel very unwell, you cough up blood, a cough persists beyond 3 weeks, or you are pregnant, over 65, have a weakened immune system or a long-term health condition.

For long-term lung conditions the test has a particular role. The NHS says people with bronchiectasis will usually be asked to give a sample of phlegm when they are unwell or if their symptoms change, which is checked for a chest infection, and if one is found antibiotics are needed. The sample is also one of the tests used when bronchiectasis is being diagnosed.

• Rinse your mouth with water before producing the sample and cough deeply from the chest.

• Take the sample to the surgery or laboratory promptly, as it needs to be fresh.

• Ask whether other tests, such as an AFB or fungal culture, have been requested if a persistent cough is being investigated.

• The sputum may be thick, discoloured (yellowish, greenish or greyish), or rarely rusty or bloody, and can have an unpleasant odour in a bacterial infection, according to Lab Tests Online UK. Tell your clinician if you cough up blood.