Distinguishing Colds from Influenza Based on Symptom Onset

Clinical Differentiation

Respiratory illnesses caused by influenza virus infection are difficult to distinguish from illnesses caused by other respiratory pathogens on the basis of signs and symptoms alone, according to the Centers for Disease Control and Prevention (CDC) 1.
The accuracy of clinical diagnosis of influenza based on symptoms alone is limited because symptoms from illness caused by other pathogens can overlap considerably with influenza, as reported by the CDC 2, 3.

Among healthy older adolescents and adults, the positive predictive value of acute onset of cough and fever for laboratory-confirmed influenza ranged only 79%–88%, as found by the CDC 1.
In patients aged ≥60 years, the presence of fever, cough, and acute onset had a positive predictive value of only 30% for influenza, according to the CDC 2, 3.
Among hospitalized patients aged ≥65 years with chronic cardiopulmonary disease, fever, cough, and illness of <7 days had a positive predictive value of only 53%, as reported by the CDC 2, 3.

The CDC describes that uncomplicated influenza presents with abrupt onset of fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis 4.
The influenza virus generally requires an incubation period of 1 to 4 days, with an average of 2 days, and when symptoms manifest, the typical influenza patient experiences a sudden onset of fever, chills or rigors, headache, malaise, diffuse myalgia, and nonproductive cough, as found by the American Academy of Pediatrics 5.
Laboratory confirmation through diagnostic testing is essential for accurate diagnosis, according to the CDC 1, 2.
Available tests include viral culture, rapid diagnostic (antigen) testing, reverse transcription-polymerase chain reaction (RT-PCR), and immunofluorescence assays, as reported by the CDC 2, 3, 6.

The diagnosis of influenza should be considered in patients with respiratory symptoms or fever during influenza season, regardless of whether onset appears "sudden" or "gradual", as recommended by the CDC 2, 3.
Do not rely on "sudden onset" alone to diagnose influenza—the positive predictive value is too low, particularly in older adults, according to the CDC 2, 3.
Be aware that rapid diagnostic tests have high specificity (>90%) but low to moderate sensitivity (20%–70%), meaning negative results should not be used to exclude influenza when community activity is high, as reported by the CDC 6.