How to Order a Sputum Culture
To properly order a sputum culture, collect a deep-cough purulent sputum specimen before initiating antibiotics, ensure rapid transport to the laboratory, and include relevant clinical information on the requisition form. 1, 2
Specimen Collection
Patient Preparation
- Have the patient rinse mouth with water (not antiseptic) before collection 3
- Instruct the patient to take several deep breaths
- Direct the patient to produce a deep-cough specimen (not saliva)
- Collect early morning specimens when possible for best yield
Quality Assessment
- A good quality sputum sample must contain:
- Purulent, discolored sputum is more likely to yield pathogens 1
- Foul-smelling sputum suggests possible anaerobic infection 2
Specimen Handling
Timing
- Collect specimen before initiating antibiotic therapy 1
- Prior antibiotic use significantly reduces diagnostic yield 2
- Transport rapidly to the laboratory to prevent overgrowth of contaminants 1
Requisition Information
- Include on laboratory requisition:
- Clinical diagnosis (suspected pneumonia, bronchitis, etc.)
- Current antibiotic therapy (if any)
- Relevant risk factors (COPD, immunocompromised status)
- Specific pathogens of concern if applicable (TB, Legionella)
Indications for Sputum Culture
Recommended for:
- Hospitalized patients with suspected pneumonia 1
- Patients who fail to respond to empiric antibiotic therapy 1
- Patients with severe COPD exacerbations (FEV1 <30%) 2
- Suspected tuberculosis, especially with risk factors 1
- During outbreaks of specific respiratory pathogens 1
Not Routinely Recommended for:
- Uncomplicated outpatient cases of suspected pneumonia 1
- Patients unable to produce adequate sputum samples
- Routine bronchitis without specific risk factors
Interpretation of Results
- Gram stain results should be available within hours and can guide initial therapy 1
- Culture results are most valuable when correlated with Gram stain findings 1
- A predominant morphotype on Gram stain that matches culture results increases diagnostic confidence 1, 5
- The presence of gram-positive diplococci has high specificity (97.6%) for S. pneumoniae 5
Common Pitfalls to Avoid
- Accepting poor quality specimens (high epithelial cell count) leads to misleading results 6
- Delayed transport allows overgrowth of contaminants
- Failure to collect specimens before antibiotic administration reduces yield
- Interpreting culture results without considering clinical context
- Treating based on culture results from contaminated specimens
Alternative Approaches When Sputum Cannot Be Obtained
- Blood cultures (two sets) before antibiotics for hospitalized patients 1
- Chest radiography to confirm pneumonia 1
- Consider specialized testing for suspected pathogens (Legionella urinary antigen) 1
- In severe cases, consider more invasive sampling methods (bronchoscopy) if clinically indicated 1
Remember that while sputum culture is valuable, it can only be obtained from about 59% of patients with suspected pneumonia, and only about 14% of all patients will produce a good quality specimen with a predominant morphotype 5. Therefore, clinical judgment remains essential in the diagnostic process.