Sputum Culture

A sputum culture is requested to detect and diagnose bacterial infections in the lower respiratory tract such as bacterial pneumonia. A bacterial infection can reach the lungs in several ways. Bacteria may spread from the mouth and throat to the upper respiratory tract, bacteria in oral or gastric secretions may be breathed into the lungs as droplets in the air. These droplets are produced when a person sneezes or coughs and can pass into the lungs. Bacteria can also spread to the blood (septicaemia) from a local infection and then be carried to the lungs. Bacterial pneumonia may be a person's main infection, or it may develop after a viral infection such as influenza, a cold or viral pneumonia.

Anyone can get a bacterial respiratory infection, but the elderly, those with suppressed immune systems, those with damaged lung tissue, those who are exposed to lung irritants, such as through smoking, and those with diseases that affect the lungs, such as cystic fibrosis, are at increased risk.

A sputum culture may be used by itself, with a FBC (Full Blood Count) to evaluate the type and number of white blood cells as an indication of infection, and/or with a blood culture to test for septicaemia.

If disease-causing bacteria are found during a sputum culture, then antimicrobial susceptibility testing is usually performed so that the appropriate antibiotics can be prescribed.

A sputum culture is requested when a doctor suspects that a person has a bacterial infection of the lungs or airways, such as bacterial pneumonia, which may show as changes in the lungs as seen on a chest x-ray. Symptoms may include:

• Cough
• Fever, chills
• Muscle aches
• Fatigue
• Trouble breathing
• Chest pain
• Confusion

Sometimes a sputum culture may be requested after treatment of an infection, to check its effectiveness.

If disease-causing bacteria are detected in a person with signs and symptoms of a lower respiratory tract infection, then it is likely that the person's symptoms are due to a bacterial infection. The most common cause of bacterial pneumonia in adults in the UK is Streptococcus pneumoniae (pneumococcus). Other common bacteria include:

• Staphylococcus aureus (staph)
• Haemophilus influenzae
• Moraxella catarrhalis
• Klebsiella pneumoniae
• Streptococcus pyogenes

If disease-causing bacteria are not detected with a culture, then it may be that the person's symptoms are not due to an infection, or that the pathogen was not present in sufficient quantity in the sample collected for it to be detected. It may also be due to the fact that the microorganism responsible is not detectable with a routine bacterial culture. Examples of organisms NOT detected with a routine bacterial culture of the sputum include:

• Mycoplasma pneumoniae and Chlamydia pneumonia - common in young adults
• Legionella species
• Mycobacterium tuberculosis (tuberculosis)
• Pneumocystis jiroveci - a fungus that can be seen in those who have had organ transplants or who have HIV/AIDS
• Other fungi
• Viruses
• Rarely, a parasitic infection may also cause symptoms of a lower respiratory infection.

Those people whose lungs have become damaged, through disease, exposure to toxins or chronic exposure to irritants, or from previous infections, have an increased risk of recurrent infections.

With bacterial respiratory infections, the sputum may have a thick consistency (viscous), appear discoloured – yellowish, greenish, greyish, or rarely rusty or bloody – and may have an unpleasant odour.

There is a pneumococcal vaccine that helps protect against invasive Streptococcus pneumoniae (pneumococcus) infections, the most common cause of bacterial pneumonia in adults. There is also a different pneumococcal vaccine for young children that protects them against serious infections with these bacteria, and there is a vaccine for infants to protect against serious infections with Haemophilus influenza type b.