How can I differentiate between influenza and other respiratory infections in a primary care setting?

In a primary care setting, differentiating between influenza and other respiratory infections primarily relies on clinical assessment, as rapid point-of-care microbiological tests or influenza tests are generally not recommended to determine antimicrobial prescribing ².

• Clinical Assessment: An acute respiratory infection (ARI) is defined as an acute illness (21 days or less) affecting the respiratory tract with symptoms such as cough, sore throat, fever, sputum production, breathlessness, wheeze, or chest discomfort or pain, without an alternative explanation ². Influenza typically presents with fever and associated systemic symptoms that usually resolve within about one week, though cough and fatigue may persist for up to two weeks ¹. When assessing, consider the person's overall health and social circumstances, as the threshold for treatment or referral may be lower for those with comorbidities or frailty ². Always consider the possibility of sepsis and assess accordingly ².
• Role of Testing: Routine rapid point-of-care microbiological tests or influenza (flu) tests are not offered to determine whether to prescribe antimicrobials for suspected ARI ². However, testing may be indicated for surveillance or infection control purposes ². If COVID-19 is a possible differential diagnosis when considering prescribing an antiviral for influenza, a PCR test should be arranged to rule it out ¹. While COVID-19 lateral flow devices can inform antiviral use, they are not a substitute for PCR testing ¹. If influenza is highly probable (e.g., following close contact with a confirmed case), an influenza antiviral should be started promptly without awaiting PCR results if point-of-care testing is unavailable ¹. Testing for influenza can be undertaken alongside COVID-19 testing if available, but it is not required for influenza antiviral initiation ¹. Point-of-care tests for acute community-acquired lower respiratory tract infections have varying diagnostic accuracy (Gentilotti et al., 2022). For lower respiratory tract infections where antibiotic need is unclear, a C-reactive protein (CRP) test can support decision-making: immediate antibiotics if CRP is more than 100 mg/litre, a back-up prescription if CRP is between 20 mg/litre and 100 mg/litre, and no routine antibiotics if CRP is less than 20 mg/litre ².
• Identifying Complications and Red Flags: Influenza can lead to complications, with pneumonia being the most common serious one ¹. Pneumonia may be caused by the influenza virus itself or by bacterial superinfection ¹. Symptoms that may indicate complicated influenza or pneumonia include lower respiratory tract distress (characterised by laboured breathing, shortness of breath, pleuritic chest pain, and haemoptysis) ¹. If symptoms are not improving or are worsening after one week, reassessment of the diagnosis should be considered ¹. Urgent medical attention should be sought if a person develops shortness of breath, pleuritic chest pain, or starts to cough up blood (haemoptysis) ¹. Children, particularly those under 5 years (and especially under 2 years), are at high risk of developing complications ¹. For suspected pneumonia, the CRB65 scoring system (Confusion, Respiratory rate ≥30 breaths/min, Blood pressure systolic <90 mmHg or diastolic ≤60 mmHg, Age ≥65 years) helps assess risk and inform decisions about hospital assessment ².
• Antiviral Considerations for Influenza: Antiviral drugs (oral oseltamivir or inhaled zanamivir) may be prescribed for influenza if the national surveillance scheme indicates influenza is circulating, the person is in an 'at risk' group (e.g., over 65, under 6 months, pregnant, or with chronic respiratory disease, asplenia, or diabetes), and treatment can start within 48 hours of symptom onset (36 hours for zanamivir in children) ¹. Previously healthy people not in an 'at risk' group but at serious risk of developing complications may also be considered for antivirals under similar conditions ¹.