Sputum Culture: Preparation, Procedure, and Results

What Is a Sputum Culture?

Sputum, also called phlegm, is thick mucus produced in the lungs and airways. It is brought up when a person coughs deeply from the chest. Saliva, by comparison, is the thin fluid naturally present in the mouth.

A culture is a laboratory method in which a sample is placed under conditions that allow microorganisms to grow. If bacteria or other organisms grow from the sputum sample, laboratory professionals can identify them and determine whether they may be associated with an infection.

Routine sputum cultures mainly look for bacteria that can cause lower respiratory tract infections. Fungi and mycobacteria, including the bacteria that cause tuberculosis, usually require separate culture methods or additional tests.

Why Is a Sputum Culture Done?

A healthcare provider may order a sputum culture when symptoms or imaging findings suggest a significant infection in the lungs or lower airways, such as pneumonia.

Possible reasons include:

• A persistent cough that produces sputum
• Fever with a suspected lung infection
• Symptoms of pneumonia
• Recurrent respiratory infections
• Symptoms that do not improve with initial treatment
• Worsening symptoms in someone with a chronic lung condition
• Suspected tuberculosis or another unusual infection

Symptoms that may lead to testing include shortness of breath, chest pain, chills, fatigue, confusion, or coughing up large amounts of sputum. A provider may also order the test after a chest X-ray or another imaging test shows signs of a possible lung infection.

Because bronchitis and pneumonia can cause similar symptoms, additional testing may be needed to distinguish bronchitis from pneumonia.

A sputum culture is not required for every cough. Many respiratory illnesses are caused by viruses, which generally cannot be diagnosed with a routine bacterial sputum culture.

What Can a Sputum Culture Detect?

Bacterial Infections

A routine sputum culture may identify bacteria associated with pneumonia, bronchiectasis, and other lower respiratory tract infections. Other findings, including symptoms and blood tests for inflammation, may help a provider assess the infection.

The laboratory may report the type of bacteria found and how much growth occurred. However, the result must be interpreted carefully because bacteria from the mouth can enter the sample during collection.

Fungal Infections

A separate fungal culture may be requested when a fungal respiratory infection is suspected. This may be particularly important for people with weakened immune systems, certain chronic lung conditions, or unusual findings on imaging tests.

Fungal cultures often use different laboratory media and may take longer than routine bacterial cultures.

Mycobacterial Infections

Suspected tuberculosis requires specialized testing rather than only a routine bacterial sputum culture.

Testing may include:

• An acid-fast bacillus, or AFB, smear
• A mycobacterial culture
• A nucleic acid amplification or molecular test
• Drug-resistance testing

An AFB smear can provide relatively quick information, but it cannot always distinguish Mycobacterium tuberculosis from other mycobacteria. A negative smear also does not completely rule out tuberculosis. Culture and molecular testing may therefore be needed.

What It Cannot Reliably Detect

Routine sputum cultures do not reliably detect most viruses or every atypical respiratory organism. Depending on the suspected infection, a provider may use a viral marker test or another specialized method.

Some bacteria, including certain mycobacteria, Legionella, and Mycoplasma pneumoniae, may require specialized culture methods, antigen testing, or molecular methods such as PCR testing.

How to Prepare for a Sputum Culture

Preparation instructions vary between laboratories, so follow the directions provided by the healthcare professional or testing facility.

The sample may be collected early in the morning because mucus can accumulate in the airways overnight. Some laboratories may recommend collecting the sample before eating or drinking.

You may be instructed to:

• Rinse your mouth with plain water
• Avoid using mouthwash immediately before collection
• Collect the sample before starting antibiotics, when possible
• Tell the provider about antibiotics you are currently taking or recently completed

Do not stop an antibiotic or any other prescribed medication unless the healthcare provider specifically tells you to do so.

Correct specimen collection, labeling, storage, and prompt transportation help preserve the quality of the sample. Collection requirements can vary, so always follow the laboratory’s specific instructions.

How Is a Sputum Culture Collected?

A sputum sample should contain mucus from the lungs, not only saliva from the mouth. Follow the laboratory’s instructions, even if they differ slightly from the steps below.

1. Wash your hands.
   Clean hands reduce the chance of contaminating the container.
2. Prepare the sterile container.
   Open it carefully without touching the inside of the container or lid.
3. Take several slow, deep breaths.
   Deep breathing can help move mucus upward from the lungs.
4. Cough deeply from the chest.
   A shallow throat-clearing cough may produce mostly saliva. Try to cough strongly enough to bring up thick mucus from the lower airways.
5. Spit the sputum into the container.
   Avoid touching the inside of the container with your mouth or fingers.
6. Check the sample.
   Sputum is usually thicker than saliva and may appear clear, white, yellow, green, brown, or blood-streaked. Changes in appearance, such as brown specks in phlegm, can occur with old blood, smoke exposure, infection, or airway irritation.
7. Close the container securely.
   Replace the lid immediately and make sure it is tightly sealed.
8. Deliver the sample as directed.
   Take it to the laboratory promptly or store it according to the instructions you received.

The most important part of sputum sample collection is producing material from the lungs. A container filled mainly with watery saliva may be rejected or reported as an inadequate specimen.

What If You Cannot Produce Sputum?

Some people have difficulty coughing up enough mucus.

Drinking water before collection may help loosen secretions, but only do this if the collection instructions allow it. A healthcare professional may also ask you to breathe in a fine mist of saline through a nebulizer. This procedure, called sputum induction, helps stimulate a productive cough.

In some situations, a sample may be collected through bronchoscopy. During this procedure, a thin tube is passed into the airways, allowing the provider to collect material using suction, a small brush, or fluid washing.

What Happens to the Sample in the Laboratory?

The laboratory first checks whether the sample is suitable for testing. A specimen containing too much saliva may not accurately represent the microorganisms present in the lungs.

The testing process may include the following steps:

1. A small amount of the specimen is examined under a microscope.
2. A Gram stain may provide early information about bacteria and specimen quality.
3. The sample is placed on culture media that support microorganism growth.
4. Any significant organisms that grow are identified.
5. Susceptibility testing may be performed when appropriate.

A Gram stain may provide preliminary information sooner than a culture, but the culture is needed to grow and identify the organism more precisely.

The sputum culture may also be interpreted with other types of laboratory tests, depending on the person’s symptoms and suspected condition.

Sputum Culture and Sensitivity Testing

A sputum culture and sensitivity test combines organism identification with antibiotic susceptibility testing.

After a bacterium grows, the laboratory exposes it to selected antibiotics to determine which medications are most likely to stop or inhibit its growth. The results commonly use categories such as:

• Susceptible: The antibiotic is likely to be effective when used appropriately.
• Intermediate: The antibiotic may work under certain conditions, such as with a higher dose or when the medicine reaches high concentrations at the infection site.
• Resistant: The organism is unlikely to respond adequately to the antibiotic.

The final treatment decision does not depend on the susceptibility result alone. The provider also considers the infection site, medication allergies, kidney and liver function, other medications, illness severity, and local treatment guidelines.

How Long Does a Sputum Culture Take?

The turnaround time depends on the microorganism, culture method, and laboratory.

Preliminary findings may be available before the final report. A routine bacterial sputum culture commonly takes a few days because the bacteria need time to grow and undergo identification.

Fungal cultures may take longer. Mycobacterial cultures used for tuberculosis and related infections can require several weeks because these organisms grow more slowly.

Molecular tuberculosis tests may produce information much sooner, but culture is still important for confirming the organism and performing some forms of drug-susceptibility testing.

Understanding Sputum Culture Results

Negative or Normal Sputum Culture

A negative sputum culture generally means that no significant disease-causing bacteria grew under the routine culture conditions.

However, a negative sputum culture does not completely rule out infection. Possible reasons include:

• Antibiotics were taken before collection
• The sample contained too little sputum
• The infection was caused by a virus
• The organism required a specialized test
• The organism did not grow under routine culture conditions
• The sample was transported or stored incorrectly

The provider may order another sample or a different type of respiratory test if infection remains likely.

Positive Sputum Culture

A positive sputum culture means that one or more microorganisms grew from the specimen.

The report may identify the organism and include susceptibility results. However, growth alone does not prove that the organism is causing the person’s symptoms.

The provider interprets a positive sputum culture together with:

• Symptoms
• Chest X-ray or CT findings
• Medical history
• Immune status
• Chronic lung conditions
• Gram stain findings
• Specimen quality
• Previous culture results

For example, a productive, deep cough may occur with pneumonia, but cough sounds and characteristics cannot identify the responsible organism without appropriate testing.

Mixed Respiratory Flora

“Mixed respiratory flora” means that several microorganisms commonly found in the mouth and upper airways grew in the sample.

This result may suggest that the specimen was contaminated with saliva. It can also mean that the laboratory did not identify one clearly dominant disease-causing organism.

Depending on the person’s symptoms and specimen quality, the provider may request another sputum sample.

Contaminated or Inadequate Sample

A report may describe a sample as contaminated, inadequate, or poor quality when it contains too much saliva, too little sputum, or evidence of excessive material from the mouth.

The laboratory may decline to perform the full culture because the result would not reliably represent the lower respiratory tract. A new sample may be needed.

Colonization vs Infection

Finding bacteria in sputum does not always mean there is an active infection.

Colonization occurs when an organism is present in the airways but is not currently causing disease. This may occur in people with bronchiectasis, cystic fibrosis, chronic obstructive pulmonary disease, tracheostomies, or repeated healthcare exposure.

Signs such as new fever, increased sputum, breathing difficulty, imaging changes, and inflammation help providers distinguish colonization from infection.

What Can Affect Sputum Culture Accuracy?

Several factors can reduce the accuracy of a sputum culture:

• Collecting saliva instead of deep sputum
• Taking antibiotics before collection
• Providing too little material
• Delayed delivery to the laboratory
• Improper storage
• Contamination from the mouth
• Using the wrong container
• Organisms that do not grow in routine culture conditions

Delayed processing may allow normal mouth microorganisms to overgrow and make it more difficult to isolate the true respiratory pathogen.

What Happens After the Results?

The next step depends on the culture findings, symptoms, and other test results.

The healthcare provider may:

• Continue the current antibiotic
• Change to an antibiotic supported by susceptibility testing
• Stop an antibiotic if a bacterial infection appears unlikely
• Request another sputum sample
• Order fungal, molecular, or tuberculosis testing
• Compare the result with a chest X-ray or CT scan
• Investigate a noninfectious cause of symptoms

Do not start, stop, or change an antibiotic based only on the wording of the laboratory report. The result requires clinical interpretation.

Someone who is unsure whether their symptoms may fit pneumonia can also use the pneumonia symptom checker, although it cannot replace professional diagnosis or laboratory testing.