Sputum Culture, Bacterial

Also Known As:
Respiratory Culture
Bacterial Culture
respiratory
Bacterial Sputum Culture

Formal Name:
Culture
sputum

This article was last modified on January 14, 2020

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Gram Stain

At a Glance

Why Get Tested?

To detect and identify the cause of bacterial pneumonia or some other lower respiratory tract infections; to monitor the efficacy 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 or any other lower respiratory tract infection

Sample Required?

A sputum sample (deep respiratory secretions, not saliva), usually collected first thing in the morning; sometimes, depending on the infection, up to 3 sputum samples might be collected over consecutive days.

Test Preparation Needed?

You may be instructed to rinse your mouth out with water prior to collection and to avoid food for 1-2 hours before the sample is collected.

Looking For Test Results?

Looking For Reference Ranges?

You may be able to find your test results on your laboratory’s website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab’s website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab’s website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

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 from the upper respiratory tract. Sputum samples may be expectorated or induced (See the section below on sample collection.)

Bacterial sputum cultures detect the presence of disease-causing bacteria (pathogens) in people who are suspected of having bacterial pneumonia or other lower respiratory tract infections. Bacteria in the sample are identified and susceptibility testing is performed to guide antibiotic treatment.

Sometimes a respiratory infection is caused by a pathogen that cannot be grown and identified with a routine bacterial sputum culture. Other tests, such as an AFB smear and culture, fungal culture, or viral culture, may be ordered in addition to or instead of a routine culture.

Typically, the first step in the routine analysis of a sputum sample is a Gram stain to identify the general type of bacteria that may be present. The sample is then 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. That means that when a person has a bacterial respiratory infection, there will typically be harmless bacteria that are normally present in the mouth, throat, etc. (normal flora) as well as disease-causing (pathogenic) bacteria present.

A trained laboratorian differentiates normal flora from pathogenic bacteria and identifies the various types of bacteria present in the culture. Identification is a step-by-step process that may involve several biochemical, immunological, and/or molecular tests and observations of the organism’s growth characteristics.

Antimicrobial susceptibility testing is frequently required to guide the treatment 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 health practitioner which pathogen(s) are present and which antibiotic therapies are likely to inhibit their growth.

How is the sample collected for testing?

Sputum samples may be coughed up or induced. Samples that are coughed up are expelled into a sterile cup provided by the laboratory. 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 following instructions provided and inhaling a sterile saline or glycerin aerosol for several minutes to loosen phlegm in the lungs. Steam inhalation or a hot shower can also be useful in loosening the phlegm. Sometimes, induction of sputum might be assisted by a respiratory therapist technician.

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. If examination of a Gram stain of the sample reveals that it 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 body’s response to an infection, then it is considered to be an adequate sample for culturing.

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

You may be instructed to brush your teeth and rinse your mouth with water prior to sample collection. You may also be instructed to avoid food for at least 1-2 hours before the sample is collected, which is usually first thing in the morning.

Common Questions

How is it used?

A bacterial sputum culture is used to detect and diagnose bacterial lower respiratory tract infections such as bacterial pneumonia or bronchitis. It is typically performed with a Gram stain to identify the bacteria causing a person’s infection.

Sometimes lower respiratory tract infections are caused by pathogens that cannot be detected with routine bacterial sputum cultures. This is either because the pathogens require very specific nutrients to grow in culture or because they grow very slowly. When this is suspected to be the case, specialized tests may be done in addition to or instead of a routine culture to help identify the cause of infection. These additional tests include, for example, an AFB smear and culture to detect tuberculosis and non-tuberculous mycobacteria infections, a fungal culture, or a Legionella culture.

A bacterial sputum culture may be ordered by itself, along with a complete blood count (CBC) 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 septicemia.

Sputum is not sterile, so when a person has an infection, there will typically be both normal flora and pathogenic bacteria present. 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 ordered?

A bacterial sputum culture is ordered when a healthcare practitioner suspects that someone has a bacterial infection of the lungs or airways, such as bacterial pneumonia. This may show as changes in the lungs as seen on a chest x-ray. Signs and 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 efficacy.

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. A positive culture report typically identifies the pathogen that was detected.

The most common cause of bacterial pneumonia in adults in the U.S. is Streptococcus pneumoniae (pneumococcus). Other common bacteria include:

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

A culture that is reported as “no growth in 24 or 48 hours” is usually considered negative. 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; see AFB Testing)
Pneumocystis jiroveci—a fungus that can be seen in those who have had organ transplants or who have HIV/AIDS
Other fungi (see Fungal Tests)
Viruses
Rarely, a parasitic infection may also cause symptoms of a lower respiratory infection.

Is there anything else I should know?

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 discolored – yellowish, greenish, grayish, or (rarely) rusty or bloody – and may have an unpleasant odor.

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 by Haemophilus influenza type b.

Why would my healthcare practitioner collect more than one sputum sample?

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

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

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

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

It may be that your healthcare practitioner 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.

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.

View Sources

Sources Used in Current Review

Gamache, J. et. al. (2017 June 20, Updated). Bacterial Pneumonia. Medscape Pulmonology. Available online at https://emedicine.medscape.com/article/300157-overview#showall. Accessed on 5/05/18.

Couturier, M. and Fisher, M. (2018 April, Updated). Streptococcus pneumoniae – Pneumococcal Disease. ARUP Consult. Available online at https://arupconsult.com/content/streptococcus-pneumoniae. Accessed on 5/05/18.

Murdocy, D. et. al. (2017 June 15, Updated). The Diagnostic Utility of Induced Sputum Microscopy and Culture in Childhood Pneumonia. Clin Infect Dis. 2017 Jun 15; 64(Suppl 3): S280–S288. Available online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5447842/. Accessed on 5/05/18.

Ibrahim, M. (2014 May 7, Updated). Sputum Culture. Medscape Laboratory Medicine. Available online at https://emedicine.medscape.com/article/2119232-overview#showall. Accessed on 5/05/18.

Baer, S. (2017 June 16, Updated). Community-Acquired Pneumonia (CAP). Medscape Infectious Diseases. Available online at https://emedicine.medscape.com/article/234240-overview#a1. Accessed on 5/05/18.

(© 1995–2018). Bacterial Culture, Aerobic, Respiratory. Mayo Clinic Mayo Medical Laboratories. Available online at https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/8095. Accessed on 5/05/18.

Martin, L. et. al. (2017 November 20, Updated). Routine sputum culture. MedlinePlus Medical Encyclopedia. Available online at https://medlineplus.gov/ency/article/003723.htm. Accessed on 5/05/18.

Sources Used in Previous Reviews

Levy, D. (Updated 2009 October 15). Routine sputum culture. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003723.htm. Accessed April 2010.

Stephen, J. (Updated 2010 April 6). Bacterial Pneumonia eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/807707-overview. Accessed April 2010.

Mayo Clinic Staff (2009 May 9). Pneumonia. MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/pneumonia/DS00135. Accessed April 2010.

Cunha, B. (2010 April 14). Pneumonia, Community-Acquired. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/234240-overview. Accessed April 2010.

Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, pp1195-1196.

Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL eds, (2005). Harrison’s Principles of Internal Medicine. 16th Edition, McGraw Hill Pp 1506-1507.

Dugdale, D. (Updated 2011 December 12). Routine sputum culture. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003723.htm. Accessed November 2013.

Ibrahim, M. (Updated 2012 August 13). Sputum Culture. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/2119232-overview. Accessed November 2013.

Cunha, B. (Updated 2013 September 13). Nursing Home Acquired Pneumonia. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/234916-overview#showall. Accessed November 2013.

Cunha, B. (Updated 2013 October 14) Community-Acquired Pneumonia. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/234240-overview#aw2aab6b2. Accessed November 2013.

Bennett, N. and Domachowske, J. (Updated 2013 February 16). Pediatric Pneumonia. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/967822-overview. Accessed November 2013.

Fisher, M. and Lehman, C. (Updated 2013 January). Health Care-Associated Pneumonia – HCAP. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/HCAP.html?client_ID=LTD#tabs=0. Accessed November 2013.

Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott’s Diagnostic Microbiology 12th Edition: Mosby Elsevier, St. Louis, MO; 2007, Pg 69.

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