Useful clinical patterns

• Common cold or viral pharyngitis: cough, rhinorrhea, hoarseness, conjunctivitis, oral ulcers, and diffuse aches favor viral infection.
• Strep throat: sudden sore throat, fever, tender anterior neck nodes, tonsillar exudate, and absence of cough can raise suspicion, but confirmation needs a rapid antigen test or throat culture. The CDC strep guidance notes that clear viral symptoms can identify viral pharyngitis clinically, whereas patients without viral features need testing.
• Bacterial sinusitis: suspect when symptoms persist over 10 days without improvement, are severe with fever at least 39 C plus purulent discharge/facial pain for several days, or worsen again after starting to improve. This is the CDC “persistent, severe, or worsening” pattern for adult sinusitis.
• Pneumonia: viral and bacterial pneumonia cannot be reliably separated by symptoms alone. Focal consolidation, significant shortness of breath, low oxygen saturation, and focal chest signs need clinical evaluation and sometimes chest imaging and microbiologic testing.
• Urinary or skin infections: focal urinary symptoms with urinalysis/culture, or localized warmth, tenderness, swelling, pus, or an abscess, are more useful for identifying bacterial disease than general symptoms alone.

Tests that can help

• Rapid antigen/PCR tests: e.g., influenza, COVID-19, RSV, group A streptococcus.
• Culture: throat, urine, wound, blood, or sputum only when clinically appropriate.
• CBC, CRP, procalcitonin: may support a probability assessment in selected settings, but none independently proves “viral” versus “bacterial.” They should not be used as a standalone reason to prescribe or withhold antibiotics.

Do not use these as rules

• “High fever = bacterial” is false.
• “Green mucus = bacterial” is false.
• “Antibiotics made me better” does not establish bacterial infection because many illnesses improve naturally.
• Viral infections can be severe, and bacterial infections can be mild.

High-yield clues

• Viral leaning: cough + rhinorrhea + hoarseness + conjunctivitis.
• Bacterial leaning: focal symptoms, persistent illness, severe presentation, or “double worsening” after an initial improvement.
• Sinusitis: bacterial disease is more likely when symptoms persist more than 10 days without improvement, are severe with fever at least 39 C plus purulent discharge/facial pain, or worsen after getting better. See CDC sinusitis guidance.
• Sore throat: clinical features alone cannot reliably distinguish strep from viral pharyngitis. A rapid strep test or culture is used when indicated. See CDC strep guidance.

Practical guideline-based approach

1. Assess severity first
   Check vital signs, oxygen saturation where relevant, hydration, mental status, focal examination findings, and risk factors such as age extremes, immunosuppression, pregnancy, major comorbidity, or recent hospitalization.
2. Use the illness trajectory, not a single symptom
   Persistent symptoms, severe early disease, or deterioration after initial improvement are more informative than mucus color, fever alone, or a CBC alone.
3. Test when the result changes management
   Examples include rapid strep testing for an appropriate sore-throat presentation, viral PCR/antigen testing when it affects isolation or antiviral decisions, urinalysis/culture for urinary symptoms, and cultures or imaging when pneumonia, sepsis, or invasive infection is suspected.
4. Avoid using antibiotics as a diagnostic trial
   Antibiotic response does not establish bacterial infection. For uncomplicated viral URI and acute bronchitis, antibiotics are not recommended.

Latest pneumonia-specific guidelines, as of September 1, 2026

Key points for viral versus bacterial pneumonia

• In an immunocompetent outpatient without comorbidities who has clinical and imaging evidence of CAP and tests positive for a respiratory virus, ATS suggests not giving empiric antibiotics.
• For outpatients with comorbidities and for all hospitalized patients with CAP, empiric antibiotics are generally still recommended despite a positive viral test, because bacterial-viral co-infection remains possible.
• Lung ultrasound may be used to diagnose CAP when performed by experienced clinicians.
• For clinically stable adult outpatients and non-severe inpatients, ATS suggests an antibiotic course of less than 5 days, with a minimum of 3 days in appropriate patients.
• Systemic corticosteroids are not recommended for non-severe inpatient CAP, but may be considered for severe CAP.

Important diagnostic principle

1. Clinical severity and risk assessment
2. Imaging confirming pneumonia
3. Targeted respiratory viral testing
4. Microbiology where it will alter management, such as blood cultures, sputum culture, urinary antigens, or molecular assays
5. Reassessment and antibiotic de-escalation when results and clinical course support it