Microbiology Test

Sputum culture

Dr. Simi Paknikar
Medically Reviewed by Dr. Simi Paknikar, MD
Last Updated on Apr 19, 2014
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Test : Sputum culture

Indications : The test is ordered to identify the causative organism, when a patient presents with chronic upper respiratory tract infection.

For the test, sputum located deep in the airways or throat must be brought up. Sputum sample is collected early in the morning before you eat anything. You are required to rinse your mouth, take three deep breaths and cough with force. If you find it difficult to cough up a sample, you may be required to inhale an aerosol mist which would help you to cough and bring out the sputum.

Do not use mouth wash as it contains bactericidal agents which will interfere with test results.

Spit the sputum that comes up into a sterile cup. Transport the cup to the lab where it is transferred on to petriplates containing special media that would allow the growth of bacteria or fungi.

It would help if you drink plenty of water or fluid the night before sample collection. Also inform the doctor regarding any antibiotic that you may have recently taken as it could affect test results.

Care must be taken to ensure that adequate amount of sputum is collected and that it is transferred to the laboratory without delay. In some people bronchoscopy or the suction method may be employed to collect sputum. Sometimes, as many as three samples will have to be collected to ensure proper diagnosis.

Physiology : Sputum culture test is carried out to diagnose upper respiratory tract and lung infections and identify the micro organisms, such as bacteria and fungi that may be causing it.

Sputum culture is carried out to avoid the usage of diagnostic techniques such as bronchoscopy, which are more invasive, laborious and expensive.

Normal Range : Normal upper respiratory flora, Tracheal aspirate and bronchscopy speciemen should not have any growth.

Interpretation : Normal

No disease-causing microorganism is present. Sometimes bacteria belonging to the normal mouth flora will grow in sputum culture. This does not indicate infection.


The culture result is positive indicating growth of microorganisms when viewed through a microscope. This indicates the presence of an infection of the respiratory tract and /or the lungs.

If the result is positive then a sensitivity test should also be conducted to determine the right antibiotic treatment.

Sample : Sputum, tracheal aspiration, bronchoscopy specimen, transtracheal aspirate.

Test Method : Aerobic culture.

Related Tests : Blood culture, Bordetella pertussis, Bronchoalveolar lavage, Gram stain, Legionella culture, Mycoplasma pneumoniae, Sputum cytology.

References :

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