When to order a throat swab for a patient with a respiratory infection and risk factors for complications?

When to Order a Throat Swab for Patients with Respiratory Infections

Order a throat swab (culture or rapid antigen detection test) for patients with acute pharyngitis when clinical and epidemiological features suggest Group A Streptococcal (GAS) infection, but do not order throat swabs for patients whose symptoms clearly indicate a viral etiology. 1

Clinical Decision Framework

Do NOT Order Throat Swabs When:

• Viral features predominate: Patients presenting with cough, coryza (runny nose), conjunctivitis, hoarseness, oral ulcers, or diarrhea have viral pharyngitis and should not be tested 1
• Low clinical probability: Adults with 0-1 clinical features suggesting streptococcal infection do not require throat swabs 2

DO Order Throat Swabs When:

For Children (<15 years):

• Order throat swabs for all children presenting with acute pharyngitis, as they have higher rates of GAS infection (up to 30%) and are at risk for acute rheumatic fever 3, 1

For Adults (≥15 years):

• High probability scenario: Order throat swabs when patients have pain on swallowing PLUS at least 3 of the following 4 features:

  • Enlarged or hyperemic tonsils
  • Tonsillar exudate
  • Enlarged or tender anterior cervical lymph nodes
  • Fever ≥38°C (100.4°F) 3, 2

• Moderate probability scenario: For adults aged 15-44 years with pain on swallowing and 1-2 of the above features, order throat swabs ONLY if cough and coryza are both absent 3

Key Clinical Predictors

The most valuable clinical features for predicting GAS pharyngitis in adults include:

• Pharyngeal exudate (odds ratio 5.49, strongest predictor) 2
• Chills (odds ratio 2.45) 2
• Absence of cough 3, 2
• Fever >38°C 2

Critical Caveats

The Carrier Problem:

• Up to 20% of school-aged children are asymptomatic GAS carriers during winter and spring 1
• Carriers can develop intercurrent viral infections that mimic streptococcal pharyngitis, leading to positive throat cultures despite viral etiology 1
• Pitfall to avoid: Do not routinely retest asymptomatic patients after treatment completion, as positive results likely represent carriage rather than treatment failure 4

Special Circumstances Requiring Testing:

Even with atypical presentations, order throat swabs in these high-risk situations:

• Personal or family history of acute rheumatic fever 1
• Community outbreak of acute rheumatic fever, post-streptococcal glomerulonephritis, or invasive GAS infection 1
• Outbreak in closed or semi-closed communities 1

Practical Algorithm Summary

Step 1: Assess for viral features (cough, coryza, hoarseness, conjunctivitis)

• If present → No throat swab needed

Step 2: If viral features absent, count clinical features:

• Tonsillar exudate
• Fever ≥38°C
• Tender anterior cervical lymphadenopathy
• Absence of cough

Step 3: Apply age-specific criteria:

• Children (<15 years): Order throat swab regardless of score 3
• Adults with ≥3 features: Order throat swab (46% GAS positive) 3
• Adults 15-44 years with 1-2 features and no cough/coryza: Order throat swab 3
• Adults with 0-1 features: No throat swab needed (only 21% GAS positive) 3, 2

This selective approach achieves 95% sensitivity for GAS detection while reducing unnecessary testing in 20% of patients 3. The algorithm performs better than routine physician judgment, particularly in children where sensitivity reaches 96.9% 5.