At a glance

Also known as

Respiratory Culture; Sputum MC&S

Why get tested?

To detect and identify the cause of bacterial pneumonia and other lower respiratory tract infections; to monitor the effectiveness of treatment

When to get tested?

When you have symptoms associated with a lower respiratory tract infection; when you have been treated for bacterial pneumonia

Sample required?

A fresh sputum sample (deep respiratory secretions, not saliva), usually collected first thing in the morning

Test preparation needed?

Rinse mouth out with water prior to collection

What is being tested?

Sputum cultures detect the presence of pathogenic bacteria in those who have bacterial pneumonia and other lower respiratory tract infections. Bacteria in the sample are identified and susceptibility testing is performed to guide 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. Care must be taken in the sample collection process to ensure that the sample is from the lower airways and not the upper respiratory tract. 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 pathogenic bacteria in the lungs.

The first step in the analysis of a fresh sputum sample is a Gram stain to identify the general type of bacteria that may be present and to determine sample adequacy. If a sample contains a significant number of normal cells that line the mouth (squamous epithelial cells), then the sample is not generally considered adequate for culture and a re-collection of the sample may be required. If the sample contains a majority of white blood cells that indicate a host response to an infection, then it is considered to be an adequate sample for culturing.

Once a sputum sample has been accepted, it is placed on or in appropriate nutrient media and incubated. The media encourages the growth of bacteria that are present, allowing for further testing and identification. Sputum is not sterile, so when a person has a bacterial respiratory infection, there will typically be both normal flora and any pathogens present.

The next step is to identify the different types of bacteria present and categorise them as normal flora or a potential pathogen causing an infection. Identification is a step-by-step process that may involve several biochemical tests and observations of the organism's growth characteristics.

Antimicrobial susceptibility testing is frequently required to guide the treatment of identified pathogens and to determine whether the bacteria present are likely to respond to specific antibiotics.

The sputum culture, Gram stain(s), and susceptibility testing all contribute to a report that informs the doctor which pathogen(s) are present and what antibiotic therapy is likely to inhibit their growth.

The infection may be caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture, so that other tests, such as an Acid fast bacilli (AFB) culture, fungal culture, or viral culture, may be ordered in addition to or instead of a routine culture.

How is the sample collected for testing?

Sputum samples may be expectorated or induced. Expectorated samples are coughed up and expelled into a sterile cup provided by the laboratory. The person's mouth should be rinsed with water or saline prior to sample collection. Deep coughing is generally required, and the person should be informed that it is phlegm/mucus from the lungs that is necessary, not saliva. If someone cannot produce a sputum sample, then it can often be induced by inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs.

All samples collected should be taken to the laboratory promptly for processing while they are fresh. Sputum samples must be evaluated and accepted by the laboratory before they are processed. Useful sputum culture results rely heavily on good sample collection. A sample that is not considered "adequate" must be rejected and a recollection requested. Adequate means that it is considered representative of conditions in the lower respiratory tract.

Is any test preparation needed to ensure the quality of the sample?

Rinse mouth out with water prior to collection to remove loose cells in the mouth.

The Test

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, and 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 secondary to 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 conditions and diseases that affect lung function, such as cystic fibrosis, are at increased risk.

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

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

What does the test 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 infection, or that 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?

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.


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.

Last Review Date: February 11, 2020

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