What lab tests are recommended for an adult patient with no significant underlying medical conditions presenting with respiratory symptoms to check for flu and Covid-19?

Lab Testing for Flu and COVID-19 in Symptomatic Adults

For an adult with respiratory symptoms and no significant comorbidities, obtain a nasopharyngeal swab for SARS-CoV-2 RT-PCR (molecular NAAT) testing, and if influenza is also suspected based on local epidemiology, perform concurrent influenza molecular testing or a combined flu/COVID molecular test. 1

Primary Testing Recommendation

COVID-19 Testing

• Nasopharyngeal swab RT-PCR (molecular NAAT) is the gold standard with sensitivity of 60-78% and should be the first-line test 2
• Testing should be performed within the first 5 days of symptom onset when viral loads are highest and test sensitivity peaks at 89% 3
• After day 5, sensitivity drops substantially to only 54%, making false negatives much more likely 3

Influenza Testing

• Molecular NAAT testing for influenza should be performed concurrently when influenza is circulating in the community 1
• Combined respiratory viral panels that include both SARS-CoV-2 and influenza are appropriate and efficient 1

When to Use Rapid Antigen Tests

For COVID-19

• Rapid antigen tests are acceptable alternatives when molecular NAAT results cannot be obtained rapidly, but they have lower sensitivity (81% pooled sensitivity, 100% specificity) 1
• The IDSA suggests using molecular NAAT over rapid antigen testing for symptomatic individuals when available 1
• If antigen testing is negative but symptoms persist, proceed immediately to confirmatory molecular NAAT testing, as false negatives are common 1, 3

Critical Timing Considerations

• Test as soon as symptoms appear, ideally within 3 days of onset for optimal performance 3
• Do not wait to test "to make sure the virus shows up"—this is a dangerous misconception, as viral loads are highest early in illness 3

Specific Symptoms That Support Testing

Red Flag Symptoms for COVID-19

• Anosmia (loss of smell) or ageusia (loss of taste) are the strongest predictors, present in 85.6% and 88.8% of COVID-19 patients respectively 2
• These symptoms have a positive likelihood ratio of 4.55-4.99, approaching "red flag" status 4
• Fever is present in 92.8% of COVID-19 cases 2
• Cough (present in 69.8%) supports testing but is non-specific 2

Differentiating Features

• Loss of taste and smell are significantly more common in COVID-19 than influenza and should prompt COVID-19-specific testing 2, 5
• Fever, vomiting, and otorhinolaryngological symptoms are more common with influenza 5
• Neurologic symptoms and diarrhea are statistically more frequent in COVID-19 5

Testing Algorithm Based on Clinical Presentation

Symptomatic Patient Presenting Within 5 Days

1. Obtain nasopharyngeal swab for molecular NAAT (RT-PCR) for SARS-CoV-2 1, 2
2. Add influenza molecular testing if influenza is circulating locally 1
3. If molecular testing is unavailable, use rapid antigen test with plan for confirmatory NAAT if negative 1, 3

If Initial Test is Negative

• Repeat molecular NAAT testing within the 5-day window if symptoms persist or worsen, as false negatives are common particularly with timing of sample collection 2, 3
• One negative viral nucleic acid test is inadequate to rule out SARS-CoV-2 infection 1

Testing Beyond Day 5

• Do not rely on antigen testing alone after day 5 of symptoms 3
• If testing is performed late, use molecular NAAT and interpret negative results with caution 3

Common Pitfalls to Avoid

• Never skip testing in patients with only upper respiratory symptoms like sore throat alone, as current evidence does not support withholding PCR testing based on symptom type alone 4
• Do not use clinical symptoms alone to differentiate COVID-19 from influenza—molecular testing is required as clinical differentiation is unreliable 6
• Avoid single-site sampling errors by ensuring proper nasopharyngeal swab technique, as poor sampling contributes to false negatives 1
• Remember that co-infection is possible—a positive test for one pathogen does not exclude the other 2

Special Considerations

High-Risk Patients

• Testing is particularly important when results will guide treatment decisions, as antiviral therapies are time-sensitive 1
• Patients with risk factors for severe disease (age >65, cardiovascular disease, diabetes, hypertension) should be tested promptly 2

Immunocompromised Patients

• May have prolonged viral shedding beyond typical timeframes, requiring adjusted testing strategies 3

Isolation Pending Results

• Patients should isolate immediately while awaiting test results to prevent potential transmission 2