Does a negative rapid diagnostic test for influenza (influenza) rule out the disease?

Negative Rapid Diagnostic Tests for Influenza Do Not Rule Out Infection

A negative rapid diagnostic test for influenza does not rule out influenza infection due to the test's limited sensitivity (20-70%), and treatment decisions should not be delayed or withheld based on negative results, especially in high-risk patients or during periods of high influenza activity. 1

Diagnostic Performance of Rapid Influenza Tests

Rapid influenza diagnostic tests (RIDTs) have the following characteristics:

• Specificity: Excellent at >90-95%, meaning positive results are generally reliable 1, 2
• Sensitivity: Poor to moderate (20-70%), with significant variability 1
  • As low as 19% in adults in some studies 1
  • Higher in children (70-90%) than adults (40-60%) 2
  • Better when viral load is high (early in illness) 1
  • Better in young children who typically shed higher viral concentrations 1

The sensitivity of RIDTs is substantially lower than RT-PCR (86-100% sensitivity) or viral culture (100% sensitivity) 1, 2.

Factors Affecting Test Performance

Several factors impact the reliability of rapid influenza tests:

• Time since symptom onset: Specimens collected within 48-72 hours of symptom onset yield better results 1
• Specimen type: Nasopharyngeal and nasal specimens generally have higher yields than throat swab specimens 1
• Patient age: Higher sensitivity in children compared to adults 1, 2
• Viral load: Higher sensitivity with higher viral loads 1, 3
• Community prevalence: The positive and negative predictive values vary with local influenza activity 1, 2

Clinical Implications of Negative Results

The low sensitivity of RIDTs has important clinical implications:

• Negative results should not be used to rule out influenza infection 1, 3
• Negative results should not guide treatment or infection-control decisions, especially during periods of high influenza activity 1
• Patients with high clinical suspicion of influenza but negative RIDT results may still require antiviral treatment 1
• Studies show that RIDT-negative patients may actually have more severe disease and experience delays in antiviral treatment 4

Recommended Approach to Diagnosis and Treatment

When evaluating patients with suspected influenza:

1. Consider the pretest probability: Assess clinical presentation and local influenza activity 1

2. Interpret test results appropriately:

   • Positive RIDT: Reliable for treatment decisions, especially during high influenza activity 1
   • Negative RIDT: Does not rule out influenza; consider clinical presentation and risk factors 1

3. For high-risk patients or severe illness:

   • Do not delay antiviral treatment while waiting for test results 1, 2
   • Consider empiric treatment despite negative RIDT results if clinical suspicion is high 1
   • Consider confirmatory testing with RT-PCR or viral culture if a definitive diagnosis is needed 1

4. For hospitalized patients:

   • PCR confirmation should be considered 1
   • Start presumptive treatment in high-risk patients before receiving PCR results 1

Common Pitfalls to Avoid

• Delaying treatment while waiting for test results in high-risk patients 1
• Withholding treatment based solely on a negative RIDT result 1, 4
• Failing to consider local influenza activity when interpreting test results 1
• Not recognizing that RIDT-negative patients may actually have more severe disease 4
• Relying on RIDTs alone when definitive diagnosis is needed; more sensitive tests like RT-PCR should be used 1, 2

By understanding the limitations of rapid influenza diagnostic tests and interpreting results in the context of clinical presentation and local influenza activity, clinicians can make more informed decisions about diagnosis and treatment of influenza.