How do I differentiate and treat viral vs bacterial infections?

Differentiating and Treating Viral vs Bacterial Infections

Use strict clinical criteria to distinguish bacterial from viral infections: for respiratory infections, diagnose bacterial disease only when symptoms persist beyond 10 days without improvement, worsen after initial improvement, or present severely with high fever (≥39°C/102.2°F) and purulent discharge for ≥3 consecutive days. 1

Clinical Differentiation Algorithm

Respiratory Tract Infections

Acute Bacterial Sinusitis vs Viral URI:

• Persistent pattern: Nasal discharge or daytime cough lasting >10 days without improvement indicates bacterial infection 1
• Worsening pattern ("double-sickening"): New or worsening fever, daytime cough, or nasal discharge after initial improvement from a viral URI suggests bacterial superinfection 1
• Severe onset: Fever ≥39°C (102.2°F) with purulent nasal discharge for ≥3 consecutive days at illness onset indicates bacterial disease 1
• Do not obtain imaging for uncomplicated cases, as 68% of children with viral URI show sinus opacification on imaging, making it unreliable for differentiation 1, 2

Pharyngitis:

• Test for Group A Streptococcus (GAS) only if ≥2 of the following are present: fever, tonsillar exudate/swelling, swollen/tender anterior cervical nodes, absence of cough 1
• Never treat empirically without rapid antigen detection test or culture confirmation, as colonization rates reach 15-20% in asymptomatic children 1
• Viral pharyngitis typically presents with cough, conjunctivitis, and rhinorrhea—features absent in bacterial GAS pharyngitis 1

Skin and Soft Tissue Infections

Bacterial vs Fungal Rash:

• Bacterial indicators: Purulent exudate, pustules, honey-colored crusting, rapid progression with surrounding erythema and warmth 3
• Fungal indicators: Peripheral scaling, satellite lesions, absence of purulent drainage, maculopapular lesions with hyphae and budding yeast on biopsy 3
• Obtain biopsy or aspiration early for histological and microbiological evaluation in immunocompromised patients, as clinical appearance alone is unreliable 1

Treatment Approach

When to Prescribe Antibiotics

Antibiotics are indicated only for confirmed or highly probable bacterial infections:

• Acute bacterial sinusitis meeting the above criteria 1
• Culture-confirmed GAS pharyngitis 1
• Bacterial skin infections with purulent features 3

Antibiotics should NOT be used for:

• Viral URIs, common cold, acute bronchitis, or nonspecific cough illness 1, 4, 5
• Viral gastroenteritis, even if bacterial etiology is suspected, as antibiotics prolong carrier state and cause superinfection 6

First-Line Antibiotic Choices

Acute Bacterial Sinusitis:

• Children and adults: Amoxicillin alone or amoxicillin-clavulanate (high-dose: 90 mg/kg/day in children, 4g/250mg per day in adults) 1
• Penicillin allergy: Cefdinir, cefuroxime, or cefpodoxime 1
• Avoid azithromycin and TMP-SMX due to high resistance rates in S. pneumoniae and H. influenzae 1
• Duration: Treat for 7 days after symptom resolution, minimum 10 days total 1

GAS Pharyngitis:

• First-line: Amoxicillin or penicillin 1
• Once-daily amoxicillin dosing improves adherence 1

Bacterial Skin Infections:

• Localized impetigo: Topical mupirocin 2% ointment 3
• Extensive disease or systemic symptoms: Oral cephalexin or dicloxacillin 3

Watchful Waiting Strategy

Consider observation without immediate antibiotics for:

• Children >2 years with unilateral acute otitis media without severe symptoms 1
• Patients with persistent sinusitis symptoms (not worsening or severe) for 72 hours before initiating antibiotics 1

Reassess at 72 hours: If symptoms worsen or fail to improve, initiate or change antibiotic therapy 1

Critical Pitfalls to Avoid

• Do not diagnose bacterial sinusitis in the first 3-4 days of illness unless presentation is unusually severe, as viral and bacterial infections are indistinguishable early 1
• Obtain cultures before antibiotics whenever possible, particularly blood cultures in suspected sepsis and NAATs for sexually transmitted pathogens in orchitis 1, 7
• Recognize that only 0.5-2% of viral URIs progress to bacterial infection, so most respiratory symptoms resolve spontaneously without antibiotics 1
• Monitor for complications: Suspect orbital or intracranial complications in sinusitis if there is eye swelling, proptosis, impaired extraocular movement, severe headache, photophobia, or seizures—these require immediate imaging and specialist consultation 1
• Avoid antibiotic overuse in viral infections, as this increases C. difficile risk, antibiotic resistance, and adverse events including diarrhea and dermatitis without providing benefit 1, 4, 8

Adjunctive Measures

• Intranasal corticosteroids may reduce inflammation in acute bacterial sinusitis, though evidence is limited 1
• Saline nasal irrigation, decongestants, antihistamines, and mucolytics lack high-quality evidence for efficacy in acute sinusitis 1
• Supportive care for viral infections: Cleanse gently with sterile saline, use nonadherent dressings to reduce bacterial superinfection 3