Is it appropriate to test for SARS‑CoV‑2 (COVID‑19) and influenza on the first day of acute respiratory symptoms such as fever, cough, sore throat, and myalgia?

Yes, Testing on Day 1 of Symptoms is Appropriate and Recommended

You should test for both COVID-19 and influenza on the first day of acute respiratory symptoms—this is the optimal testing window when viral loads are highest and test sensitivity is maximized. 1, 2

Why Test Immediately

Optimal Test Performance

• Antigen test sensitivity peaks at 89% (95% CI: 83-93%) when performed within the first 5 days of symptom onset, with day 1 being ideal 1, 2
• Testing within 3 days of symptom onset yields the best performance, so earlier is definitively better 2
• After 5 days, sensitivity drops substantially to only 54%, making false-negative results much more likely 2

Clinical Decision-Making Benefits

• Multiple COVID-19 therapies have FDA Emergency Use Authorization for use within the first 5 days of symptoms, making rapid diagnosis critical for treatment eligibility 1
• Early accurate diagnosis determines the correct treatment course, as influenza treatments are not effective for COVID-19 3
• Both viruses present with similar symptoms (fever, cough, headache, muscle aches, fatigue), making clinical diagnosis without testing unreliable 3

Testing Strategy on Day 1

Preferred Test Type

• For symptomatic individuals, the Infectious Diseases Society of America recommends standard NAAT (rapid RT-PCR or laboratory-based NAAT) over rapid antigen tests 1
• If NAAT is unavailable or results are delayed more than 1 day, use a rapid antigen test rather than waiting 1
• Combo tests for both influenza and COVID-19 are appropriate and efficient 4

If First Test is Negative

• If symptoms persist or worsen and the first antigen test is negative, repeat testing should occur within the 5-day window 2
• Consider molecular NAAT testing for confirmation rather than repeat antigen testing 2

Critical Considerations

Co-infection Risk

• Co-infection with both SARS-CoV-2 and influenza occurs and can worsen patient outcomes, particularly in those requiring intensive care 5, 6
• Prevalence of co-infection ranges from 1.35% to higher rates depending on local epidemiology 6
• Testing for both pathogens simultaneously is recommended during respiratory illness outbreaks 5

Common Pitfall to Avoid

• Do not wait to test "to make sure the virus shows up"—this is a dangerous misconception 2
• Viral loads are highest early in illness, not later 2
• Delaying testing reduces sensitivity and may miss the treatment window for antiviral therapies 1

Special Populations

• Severely immunocompromised patients may have prolonged viral shedding beyond typical timeframes, but this does not change the recommendation to test early 2
• High-risk patients benefit even more from early diagnosis to enable prompt treatment initiation 1