What is being tested?

A sputum culture detects the presence of pathogenic bacteria in those who have bacterial pneumonia or a lower respiratory tract infections. Pathogenetic bacteria in the sputum sample are identified and susceptibility testing is performed to help guide appropriate antimicrobial treatment.

Sputum is the thick mucus or phlegm that is expelled from the lower respiratory tract (bronchi and lungs) through coughing; it is not saliva or spit from the mouth. Care must be taken in the sample collection process to ensure it is from the lower airways and not the mouth. If a sample is mostly saliva, the microorganisms grown in culture will not be representative of those causing the infection. Furthermore, the presence of saliva and bacteria from the mouth in a sputum sample make it more difficult to identify potential pathogenic bacteria infecting the lungs.

The first step in the analysis of a fresh sputum sample is a Gram stain to identify the type of bacteria present and to determine sample adequacy. If a sample contains a significant number of epithelian (skin) cells that line the mouth, lots of normal oral bacteria and no white blood cells, then the sample is not generally considered adequate for culture and a re-collection of the sample may be required if clinically relevant. If the sample contains a high amount of white blood cells that indicate an infection, then it is considered to be an adequate sample for culturing. Sputum Gram stain can also help identify aspiration pneumonia along with clinical findings.

Once a sputum sample has been accepted, it is placed on or in appropriate nutrient media and incubated. The media and environment conditions encourage bacteria growth, this then allows for further testing and indention. Because sputum is not sterile, cultures will grow both normal oral flora as well as and any bacterial pathogens that are present.

Identification is a step-by-step process that may involve several biochemical tests and observations of the organism's growth characteristics. Culture results are reported in correlation to the Gram stain, aiding the laboratory to identify whether there is an infection present.

Antimicrobial susceptibility testing is frequently performed to guide the treatment of any identified pathogens and to test whether the pathogen will respond to specific antibiotics. The sputum culture, Gram stain(s), and susceptibility testing all contribute to a report which informs the doctor which pathogen(s) are present and what antibiotic therapy is likely to eradicate infection.

The infection may be caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture, so other tests, such as an Acid fast bacilli (AFB) culture, legionella culture, fungal culture, or viral PCR, may be ordered in addition to or instead of a routine culture. If an unusual pathogen is suspected the laboratory should be informed to ensure the correct cultures and/or extra tests are completed.

How is it used?

A sputum culture is ordered to detect and diagnose bacterial lower respiratory tract infections such as bacterial pneumonia. A bacterial infection can reach the lungs in several ways. Bacteria may spread downward from the upper respiratory tract, bacteria in oral or gastric secretions may be aspirated into the lungs, airborne droplets such as from a person sneezing or coughing can be inhaled into the lungs or bacteria from a localised infection may spread to the blood (septicaemia) and then be carried to the lungs. Bacterial pneumonia may be a person's primary infection, or it may develop as a secondary to a viral infection such as influenza, a cold or viral pneumonia.

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

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

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

When is it requested?

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

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

A sputum culture may be ordered after treatment of an infection, to verify the effectiveness of treatment.

What does the result mean?

If pathogenic 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 Australia is Streptococcus pneumoniae (pneumococcus). Other common bacteria include:

• Staphylococcus aureus (staph)
• Haemophilus influenzae
• Klebsiella pneumoniae
• Pseudomonas aeruginosa
• Acinetobacter baumanii (tropical Australia)
• Burkholderia pseudomallei (tropical Australia)
• Moraxella catarrhalis
• Streptococcus pneumoniae
• Streptococcus pyogenes

If pathogenic bacteria are not detected with a culture, then it may be that the person's symptoms are due to a viral or a another type of infection, or because the pathogen was not present in sufficient quantity in the sample collected. 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.

Is there anything else I should know?

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

Common questions

• Can bacterial pneumonia be prevented?

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. Prevention of influenza with the annual influenza vaccine can prevent viral pneumonia and subsequent infection with bacteria.

• Why would my doctor collect more than one sputum sample?

This may be done if the first sample is not considered adequate or if the doctor wants to order additional tests on sputum samples.

• Why would a separate test be ordered for fungus or an AFB culture?

These tests detect organisms that do not grow on the media typically used for a routine sputum culture. Fungus, Legionella or mycobacterium species may take several weeks to grow in the laboratory, and they require the use of specialised media to grow and special stains to be detected under the microscope.

• I have been sick for more than a week. Why would my doctor order a sputum culture now?

It may be that your doctor suspects that you have developed a bacterial infection secondary to an initial viral infection or that your infection has not responded as expected to treatment. Recent research shows that the average cough lasts 17 days, which is far longer than most people expect to be unwell. A continuing cough is not likely to mean that bacterial pneumonia has developed, as viruses are far more likely to be the cause. However symptoms such as high fever, chest pain and difficulty breathing warrant further medical care and investigation.

• Once I have been treated for a bacterial lower respiratory tract infection, can the infection return?

For most people, once the infection has been successfully treated, it will not return. If the treatment was not successful, then the infection may persist or re-emerge. If someone has an underlying condition that increases their risk of lung infection, such as a lung disease, then they may experience recurrent (new) infections. In some cases, these recurrent infections can become increasingly challenging to treat.

More information

RCPA Manual: MCS sputum