Should I check for influenza (flu) and COVID-19 (Coronavirus Disease 2019) after 13 days of symptoms?

Testing for Flu/COVID After 13 Days of Symptoms

At 13 days of symptoms, testing for acute influenza or COVID-19 is generally not recommended for diagnostic purposes, as both infections are typically past their peak viral shedding period and most patients are no longer infectious. 1, 2

Rationale for Not Testing at This Timepoint

Viral Dynamics and Test Performance

• COVID-19 viral loads peak around day 4-5 of symptoms in highly immune populations, with median viral loads rising from symptom onset and reaching maximum levels by the fourth or fifth day 3
• Antigen test sensitivity for COVID-19 is optimized within the first 5 days of symptom onset, with performance declining significantly thereafter 1
• Influenza viral loads peak around day 2 of symptoms, making testing at 13 days well beyond the optimal detection window 3
• The World Health Organization notes that antigen test sensitivity drops to approximately 80% when testing occurs more than 5 days after symptom onset 2

Infectiousness Considerations

• Contagiousness for mild-moderate COVID-19 typically lasts an average of 10 days from symptom onset, meaning by day 13 most immunocompetent patients are no longer infectious 2
• For immunocompromised patients or those with severe/critical COVID-19, contagiousness can extend up to 15 days, but this represents a minority of cases 2
• PCR tests may remain positive for up to 30 days even after a person is no longer infectious, detecting non-viable viral fragments rather than active infection 1, 2

When Testing at 13 Days Might Be Considered

Specific Clinical Scenarios

• Persistent or worsening symptoms that suggest ongoing active infection rather than post-viral syndrome 1
• Immunocompromised patients who may have prolonged viral replication and extended periods of infectiousness 2
• Severe or critical illness requiring hospitalization, where determining active infection status impacts treatment decisions 2

Alternative Diagnostic Considerations at This Timepoint

At 13 days, the focus should shift from acute infection testing to evaluating for:

• Long COVID assessment - Any patient with persisting symptoms lasting more than 12 weeks after acute COVID-19 should be referred to medical care 1
• Complications of acute infection including thromboembolic events, myocarditis, or encephalitis that may have been overlooked 1
• Other serious conditions that may have been missed initially, such as malignancy or other non-viral etiologies 1
• Post-acute COVID syndrome for symptoms lasting 4-12 weeks, which should be assessed on a case-by-case basis according to severity 1

Common Pitfalls to Avoid

• Do not rely on positive PCR results at this timepoint to determine infectiousness, as they may reflect residual viral RNA rather than viable virus 1, 2
• Do not assume a negative test rules out prior infection at 13 days, as viral loads have likely declined below detection thresholds 1
• Do not delay evaluation for complications by focusing solely on confirming acute infection status 1

Clinical Action at 13 Days

The appropriate approach at 13 days is symptom-based management rather than testing for acute infection:

• Collect detailed clinical history to rule out underlying conditions and complications related to the acute episode 1
• Perform blood tests and imaging according to specific symptoms to investigate alternative diagnoses or complications 1
• Consider referral for long COVID evaluation if symptoms persist beyond 12 weeks 1
• For patients with severe illness or immunocompromise, individualized decisions about testing may be warranted in consultation with infectious disease specialists 2