Diagnostic Tests for Pneumonia in Patients with Cough and Purulent Sputum
A chest radiograph is the most essential diagnostic test to rule out pneumonia in patients presenting with cough and purulent sputum, along with blood cultures in hospitalized patients and sputum culture in those with severe illness or risk factors. 1
Initial Assessment
Clinical Evaluation
Assess for the following vital sign abnormalities that increase pneumonia risk:
- Heart rate >100 beats/min
- Respiratory rate >24 breaths/min
- Oral temperature >38°C
- Presence of focal consolidation, egophony, or fremitus on chest examination 1
The absence of ALL these findings significantly reduces the likelihood of pneumonia and may eliminate the need for a chest radiograph 1
Important warning signs requiring immediate investigation:
- Hemoptysis
- Prominent systemic illness
- Suspicion of inhaled foreign body
- Suspicion of lung cancer 1
Diagnostic Algorithm
Step 1: Chest Imaging
- Posteroanterior and lateral chest radiograph (preferred over portable films) 1
- Helps define severity (multilobar vs. localized)
- Identifies complications (effusions, cavitation)
- Rules out other conditions
Step 2: Laboratory Testing Based on Severity
For Outpatients (Non-severe illness):
- No routine laboratory testing if clinical findings do not suggest pneumonia 1
- Consider C-reactive protein (CRP) measurement:
- CRP <10 μg/ml essentially rules out pneumonia
- CRP 11-50 μg/ml without dyspnea and daily fever also makes pneumonia unlikely 2
For Hospitalized Patients:
- Complete blood count
- Basic metabolic panel
- Blood cultures (two sets) before antibiotics 1, 3
- Arterial blood gas if respiratory distress or concern for hypoxemia 1
- Sputum analysis:
For Severe Pneumonia (CURB-65 Score 3-5 or bilateral CXR changes):
- All of the above plus:
- Pneumococcal urine antigen
- Legionella urine antigen (especially if clinically or epidemiologically suspected) 1
- Paired serological examination for influenza/other agents 1
Sputum Collection and Analysis
When to Collect Sputum:
- Hospitalized patients with suspected pneumonia 3
- Patients failing empiric antibiotic therapy 3
- Suspected tuberculosis or other specific pathogens 3
- Severe COPD exacerbations 3
Proper Collection:
- Collect deep-cough purulent sputum BEFORE initiating antibiotics 3
- Transport to laboratory within 2 hours 3
- Quality criteria: ≥25 polymorphonuclear leukocytes and <10 squamous epithelial cells per low-power field 3
Special Considerations:
- For suspected tuberculosis: collect three consecutive morning specimens 3
- For suspected Legionella: note this on requisition and consider PCR testing 3
- For immunocompromised patients: request fungal cultures and extended incubation 3
Additional Testing in Special Circumstances
For Pleural Effusions:
- Diagnostic thoracentesis when significant pleural effusion is present 1
For Non-Resolving Pneumonia:
- Consider bronchoscopy with bronchoalveolar lavage (BAL) or protected specimen brush (PSB) 1
- Consider CT thoracic scan at 6 weeks if respiratory symptoms persist, especially in smokers and those over 50 years 1
Common Pitfalls to Avoid
Assuming purulent sputum alone indicates bacterial infection (can occur with viral infections due to inflammatory cells) 3
Delaying antibiotic administration to collect specimens in severely ill patients (collect blood cultures first, then start antibiotics) 1
Relying on sputum cultures alone without correlating with Gram stain findings 3
Failing to recognize that prior antibiotic use significantly reduces diagnostic yield 3
Not recognizing that Streptococcus pneumoniae is the most common detected bacterium in community-acquired pneumonia 4
By following this systematic approach to testing, clinicians can effectively rule out pneumonia and other severe infections in patients presenting with cough and purulent sputum, while avoiding unnecessary antibiotic use in those with viral or non-infectious causes.