What is a Sputum Culture test? 

A sputum culture test is a laboratory test that helps detect infection-causing germs in a sputum sample. Sputum is mucus from the lower respiratory tract that is released through coughing, and it is different from saliva. It settles in lower airways of lungs in individuals with an infection or long-term illness and is released and expectorated while coughing deeply.

  1. Why is a Sputum Culture test performed?
  2. How do you prepare for a Sputum Culture test?
  3. How is a Sputum Culture test performed?
  4. What do Sputum Culture test results indicate?

A sputum culture test is performed to detect the presence of bacteria and other microorganisms in lungs or airways. This test is recommended in individuals who are suspected of having lung infections, particularly those experiencing excessive coughing. Individuals with cough, fever, chills, tiredness, confusion, chest pain, difficulty in breathing and muscle aches are also advised to undergo this test.

A sputum culture test is recommended in individuals suspected with the following conditions:

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It is generally advised to stop eating for about 1-2 hours before the test. The duration of fasting before the test may be longer if it is so recommended by the doctor. Increasing water and fluid intake the night before this test makes it easier to cough sputum. The doctor must be informed about any medications, such as herbs, supplements, vitamins and antibiotics, that the individual is taking. 

Antibiotic intake needs to be stopped before the test as they may give a false negative result by suppressing bacterial growth in sputum.

A sputum sample is collected for this test. The individual is first instructed to rinse their mouth with water before coughing the sputum. He/she must then cough deeply to ensure that the phlegm is released from lungs. This phlegm is collected in a special container and is tested in the laboratory by placing it in a special culture dish. 

It is then observed to check for the growth of any bacteria or disease-causing microorganisms. Certain individuals are unable to cough sufficient sputum for the test. In such cases, a narrow, flexible tube with a light source on it called bronchoscope is used to collect sputum.

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Age, medical history, sex, testing methods and other factors may cause variation in test results.

Normal results: Absence of disease-causing microorganisms in the sputum sample indicates the absence of lung infection.

Abnormal results: A positive result is considered an abnormal result, and it suggests the presence of bacteria or disease-causing microorganisms in the sputum sample.

Some of the most common bacterial pathogen causing lung infections are  Staphylococcus aureus, Haemophilus influenzae and Klebsiella pneumoniae.

However, some bacteria are generally present in lungs and do not cause disease. Therefore, the doctor can help in the correct interpretation of reports.

Occasionally, a sputum test may fail to detect lower respiratory tract infections caused by certain microorganisms. In such cases, additional tests, such as acid-fast bacilli (AFB) smear and culture and fungal culture, are recommended. A complete blood count (CBC) and blood culture test may also be recommended along with a sputum culture test.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational perspective and is in no way a substitute for medical advice from a qualified doctor.  

References

  1. David R. Murdoch et al. The Diagnostic Utility of Induced Sputum Microscopy and Culture in Childhood Pneumonia. Clin Infect Dis. 2017 Jun 15; 64(Suppl 3): S280–S288. Published online 2017 May 29. PMID: 28575362
  2. Ellison RT, Donowitz GR. Acute pneumonia. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, Updated Edition. 8th ed. Philadelphia, PA: Elsevier Saunders; 2015: chap 69.
  3. Chernecky CC, Berger BJ. Culture, routine. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:409-411.
  4. Brainard J. Respiratory cytology. In: Zander DS, Farver CF, eds. Pulmonary Pathology. 2nd ed. Philadelphia, PA: Elsevier; 2018 Chapter 36.
  5. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; Sputum Culture
  6. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Routine sputum culture
  7. Center for Disease Control and Prevention [internet], Atlanta (GA): US Department of Health and Human Services, Diagnosis of Tuberculosis Disease
  8. HealthlinkBC [internet] British Columbia; Sputum Culture Test Overview
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