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Sputum Culture

Sputum culture and analysis

Sputum is the liquid that comes from your respiratory tract when you cough. Sputum contains:

  • mucus
  • bacteria
  • cellular fragments
  • blood
  • pus

Analyzing your sputum can help your doctor determine if you have a respiratory infection.

What is the purpose of the test?

Your doctor may use the results from a sputum culture and analysis to help them diagnose a fungal or bacterial infection of the respiratory tract. They may order a sputum culture if you have the following symptoms:

  • coughing
  • chills
  • a fever
  • difficulty breathing
  • chest pain
  • fatigue
  • muscle aches
  • confusion

Lab technicians may find disease-causing bacteria or fungi in your sputum. This can help your doctor diagnose the cause of your symptoms, which may be:

  • bronchitis
  • lung abscess
  • pneumonia
  • tuberculosis
  • complications from chronic obstructive pulmonary disease
  • other infections

The treatment plan your doctor prescribes will vary depending on your diagnosis.

What are the benefits of the test?

Providing a sample of sputum is noninvasive, meaning it doesn’t require entering your body, and it requires little time. It can give your doctor valuable information to help them determine the cause of a respiratory infection. This can help them determine an appropriate treatment plan. Your doctor can also use the test to learn if your course of treatment for a respiratory infection is working.

For some people, a bacterial respiratory infection can cause serious consequences. If you’re elderly or you have a suppressed immune system, lung damage, or a lung condition, such as chronic obstructive pulmonary disease or cystic fibrosis, it’s important to get early diagnosis and treatment.

What are the risks of the test?

Providing a sample of sputum by coughing is safe. If you have a respiratory infection, the coughing required to produce the sample might cause some discomfort.

How should you prepare for the test?

Ask your doctor if you should temporarily stop taking any medication before giving a sputum sample. Antibiotics, anti-inflammatory drugs, and steroids can affect the results of a sputum culture.

Your doctor may advise you to drink plenty of water and other liquids the night before giving your sputum sample. This will make it easier to cough up a sample the next morning.

How is the sample collected?

You can give a sputum sample in your doctor’s office, a lab testing site, or a hospital. In some cases, your doctor may ask you to collect your specimen at home. After collecting a sputum sample at home, take it to a laboratory as quickly as possible to help it stay fresh.

You’ll get the best results if you provide a sample first thing in the morning, before you have anything to eat or drink. This will help you produce a sample of sputum from the deepest part of your chest.

Before providing the sample, your doctor will ask you to rinse out your mouth with water or saline solution. This helps clear microorganisms from your mouth. Then, your doctor will ask you to breathe deeply and cough deeply. As you cough up sputum, you’ll deposit it into a sterile collection cup.

For the best results, it’s important to provide a sample that includes sputum, not just saliva. Saliva is watery. Sputum is usually yellow and thick. If you have an infection, your sputum may also be green or spotted with blood. Saliva will contain microorganisms found in your mouth, while sputum will contain microorganisms from your lungs. They may not be the same types of microorganisms.

If you can’t produce a sample of sputum on your own, your doctor or lab technician can help you provide a sample. They may ask you to inhale sterile saline solution or a glycerin aerosol. This will help loosen the sputum deep in your lungs.

How is the sample analyzed?

A lab technician will analyze your sputum sample using a variety of methods:

Gram’s stain

They’ll use what’s called a "Gram’s stain" to learn if the sample is adequate and contains enough bacterial cells to proceed. In this test, they’ll place a portion of your sputum sample on a slide. Then, they’ll stain the slide with red and purple dyes and examine it under a microscope. This test may also help them identify the types of bacteria present. By looking at the bacteria’s reaction to the dyes, the technician can identify the bacteria as either gram-positive or gram-negative.

Sputum culture

If your sputum sample is adequate, the technician will place it on a special plate that encourages bacteria or fungi to grow. This is the sputum culture. A bacterial infection may require up to 48 hours to grow. It may take a week or more for fungi to reproduce.

Other methods

After the sputum culture is complete, a technician will use chemical tests and microscopes to examine the different types of bacteria and fungi present. They may classify the microbes as either normal or disease-causing organisms. They may also test the disease-causing organisms to determine which treatment methods, such as antibiotics, may work to treat your infection.

How are the results interpreted?

Your doctor will receive the results of the sputum culture and analysis from the lab. Your results may be available within a few days. However, some types of fungi may take a week or longer to grow for analysis.

It’s normal and healthy for sputum to have certain types of bacteria. If your results are "negative" or normal, the lab found no evidence of disease-causing bacteria or fungi in your sputum sample. If your symptoms persist, you may have an infection due to a virus or other microorganism that wasn’t identified in your sputum sample. Some types of organisms can’t be grown and identified using a sputum culture. Additional testing may be necessary.

If your results are "positive" or abnormal, the lab found evidence of disease-causing bacteria or fungi in your sample. Your doctor will use the results of the analysis to choose which treatment they recommend. Ask your doctor for more information about your specific diagnosis, treatment plan, and outlook.

Content licensed from:

Written by: Anna Giorgi
Medically reviewed on: May 09, 2016: University of Illinois-Chicago, College of Medicine

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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