Diagnostic Approach to Suspected Respiratory Infection

Clinical Assessment

The American Thoracic Society recommends assessing for suggestive symptoms and signs of pneumonia, including cough, dyspnea, pleural pain, fever ≥38°C, sweating/shivers, aches and pains, tachypnea, and new and localizing chest examination signs 1
Evaluate vital signs, particularly respiratory rate, oxygen saturation, temperature, and blood pressure to determine severity of illness and need for hospitalization, as recommended by the American College of Chest Physicians 2, 3
Note that the absence of runny nose and presence of breathlessness, crackles, diminished breath sounds, tachycardia, and fever are particularly suggestive of pneumonia, according to the American College of Physicians 1
Consider severity assessment tools such as CURB-65 score to guide management decisions, as suggested by the British Thoracic Society 3, 4

Obtain a chest radiograph in all patients with suspected respiratory infection seen in the hospital setting to improve diagnostic accuracy and define severity, as recommended by the American College of Radiology 2, 1
Consider posterior-anterior and lateral views when possible, as portable chest radiographs have limited accuracy, according to the American Thoracic Society 2
Look for multilobar involvement, pleural effusions, or cavitation which may indicate greater severity or complications, as suggested by the Society of Critical Care Medicine 2

Obtain the following tests for all patients with suspected respiratory infection requiring hospital evaluation: full blood count, urea and electrolytes, liver function tests, and C-reactive protein measurement, as recommended by the Infectious Diseases Society of America 2, 3, 5
Consider additional tests based on clinical presentation, such as blood cultures, sputum cultures, and respiratory viral testing, including COVID-19 evaluation, as suggested by the Centers for Disease Control and Prevention 2, 3, 5, 6

Obtain samples of lower respiratory tract secretions before antibiotic changes, as recommended by the American Thoracic Society 2
Collect blood cultures in all patients with suspected pneumonia requiring hospitalization, according to the Infectious Diseases Society of America 7
Consider diagnostic thoracentesis if significant pleural effusion is present, as suggested by the Society of Critical Care Medicine 2, 7

Review current medications, particularly immunosuppressive agents that may increase risk of opportunistic infections, as recommended by the National Comprehensive Cancer Network 6, 8
Consider whether the patient is on immune checkpoint inhibitors, which can cause immune-related pulmonary reactions that may mimic infection, according to the American Society of Clinical Oncology 6

For patients with suspected influenza, consider initiating antiviral treatment within 48 hours of symptom onset, as recommended by the Centers for Disease Control and Prevention 1
In immunocompromised patients, consider broader differential diagnoses including opportunistic infections, as suggested by the Infectious Diseases Society of America 8, 5
For patients on corticosteroids or other immunosuppressants, consider Pneumocystis jirovecii pneumonia, particularly with high respiratory rates or oxygen desaturation, according to the National Institutes of Health 8

Do not rely solely on clinical presentation for diagnosis of pneumonia, as this approach is overly sensitive, as warned by the American College of Chest Physicians 2
Do not delay obtaining respiratory specimens before initiating or changing antibiotics, as recommended by the Infectious Diseases Society of America 2
Remember that tracheal colonization is common in intubated patients but does not necessarily indicate infection requiring treatment, according to the Society of Critical Care Medicine 2

The British Infection Society and British Thoracic Society recommend a full blood count for all patients admitted to hospital with influenza-like illness 9, 10
Patients requiring hospitalization due to severity of illness should have a CBC 9, 10, 11
Patients with severe illness, as indicated by a high CURB-65 score (3 or more), respiratory distress, hypoxemia, or hemodynamic instability, should have a CBC 10, 11, 12

Leukocytosis with left shift may indicate primary viral pneumonia, mixed viral-bacterial pneumonia, or secondary bacterial pneumonia 9, 10
Lymphopenia has been noted in severe viral infections, particularly avian H5N1 influenza 9, 10

When CBC is indicated, consider also obtaining urea and electrolytes to assess for hypo/hypernatremia and renal impairment 9, 10
Liver function tests should be monitored for hepatic involvement 9, 10
Creatine kinase should be tested if myositis is suspected 9, 10
C-reactive protein (CRP) should be tested when secondary bacterial infection is suspected 9, 10

During influenza pandemics, testing strategies may change based on phase and caseload 9, 11
In early pandemic phases, more comprehensive testing is recommended 9, 11
In later phases with higher caseloads, testing may be limited to severe cases 9, 11
Blood cultures, sputum gram stain and culture, and pneumococcal and Legionella urinary antigen tests should be considered when bacterial superinfection is suspected 12