What are the criteria for diagnosing influenza-like illness?

Criteria for Influenza-Like Illness (ILI)

Influenza-like illness (ILI) is clinically defined as the presence of fever and new (or, in those with chronic lung disease, worsening) cough of acute onset in the context of influenza circulating in the community. 1, 2

Core Diagnostic Criteria

• The most predictive clinical features for ILI are the three-fold combination of fever, cough, and acute onset 1, 2
• Fever is typically defined as a temperature ≥38°C (100.4°F) 2, 3
• The accuracy of this clinical definition is higher in persons aged 60 years and above compared to patient groups without age restrictions 1, 2
• The probability of influenza infection increases with increasing level of fever 1, 2

Common Associated Symptoms

• Cough (present in approximately 85-93% of cases) 1, 2, 3
• Headache (approximately 65% of cases) 1, 2
• Myalgia/muscle pain (approximately 53% of cases) 1, 2
• Malaise (approximately 80% of cases) 1, 2
• Anorexia (approximately 60% of cases) 1, 2
• Sore throat (approximately 50% of cases) 1, 2
• Chills (approximately 70% of cases) 1, 2
• Coryzal/nasal symptoms (approximately 60-91% of cases) 1, 2, 3

Diagnostic Value and Limitations

• The predictive value of ILI increases when influenza virus is known to be circulating in the community 1, 2
• In cohort studies, correlation of ILI with laboratory-confirmed influenza infection ranges from 25% to 45%, while in clinical trials, rates of 70% have been consistently reported 1, 2
• When influenza is circulating within the community, patients with both cough and fever within 48 hours of symptom onset have a positive predictive value of 79% for influenza infection 3
• Only about 51% of hospitalized patients with confirmed influenza meet the CDC criteria for ILI, suggesting these criteria may be more appropriate for outpatient settings 4

Important Caveats and Pitfalls

• ILI is not specific for influenza infection; other respiratory pathogens can present with similar symptoms 1, 2
• Other pathogens that may present with an ILI include:
  • Respiratory syncytial virus (RSV) 1
  • Adenovirus 1, 5
  • Rhinovirus 1, 5
  • Parainfluenza virus 1, 5
  • Bacterial pathogens such as Chlamydia pneumoniae, Legionella sp, Mycoplasma pneumoniae, and Streptococcus pneumoniae 1
• Testing based on fever alone (without requiring cough or sore throat) can increase case detection by up to 34% during influenza season 6
• The positive predictive value of ILI varies during the course of the influenza season 6, 7
• Including seasonality and influenza vaccination status improves the predictive value of ILI in both clinical and community settings 7

Complications

• Pneumonia (either primary viral or secondary bacterial) is the most common complication of influenza in adults 1, 2
• Neurological complications are rare in adults 1
• Patients with underlying pulmonary disease are more likely to require ventilatory support if they develop influenza 4