Vital Signs in Influenza Patients

Core Vital Sign Abnormalities

Fever >38°C is the hallmark vital sign and a diagnostic criterion for influenza-like illness requiring antiviral treatment, according to the Infectious Diseases Society of America 1, 2
Temperature >37.8°C indicates clinical instability and should prompt continued hospitalization, as recommended by the American College of Physicians 1, 2
Tachycardia >100 beats/min is common and represents one of the clinical instability criteria, as stated by the American Heart Association 1, 2, 3
Tachypnea ≥24 breaths/min indicates clinical instability and is part of the CURB-65 severity assessment, according to the European Respiratory Society 1, 2, 3
Systolic blood pressure <90 mmHg or diastolic <60 mmHg indicates severe illness and is part of severity scoring systems, as recommended by the American College of Cardiology 1, 2, 5
Oxygen saturation <92% on room air is abnormal and requires supplemental oxygen therapy, according to the American Thoracic Society 1, 2, 3

Temperature, respiratory rate, pulse, blood pressure, mental status, and oxygen saturation should be monitored at least twice daily in hospitalized patients, with more frequent monitoring for those with severe illness or requiring oxygen therapy, as recommended by the Society of Critical Care Medicine 1, 2, 3, 8

Do not dismiss patients with normal vital signs if they have bilateral chest findings, as primary viral pneumonia can progress rapidly and should prompt hospital referral regardless of severity scores, according to the British Thoracic Society 9
Mental status changes are a vital sign equivalent and score in severity assessments, indicating serious illness, as stated by the Neurocritical Care Society 1, 5, 6
Patients meeting two or more instability criteria should remain hospitalized, as recommended by the Infectious Diseases Society of America 1, 2, 3