• Best confirmatory test: RT-PCR / real-time RT-PCR or another molecular nucleic-acid amplification test on a nasopharyngeal swab, nasal swab, or combined nasal-throat specimen. The assay can identify influenza A and, when available, subtype it as H1N1pdm09.
• When to collect: Collect the specimen and test as early as possible, ideally within about 4 days of symptom onset. Molecular tests can still detect RNA later in illness.
• Rapid flu antigen tests: Useful as a quick screening test, but a negative result does not rule out influenza, particularly when clinical suspicion is high. Confirm a negative rapid test with RT-PCR if the result will change management.
• Severe or hospitalized lower-respiratory illness: If an upper-respiratory PCR is negative but suspicion remains high, clinicians may test a lower-respiratory specimen such as an endotracheal aspirate or bronchoalveolar lavage sample.
• Viral culture and serology: Not usually used for immediate clinical confirmation. Culture is mainly for surveillance, and a single antibody test is not useful for acute diagnosis.

Adults

• Trouble breathing, shortness of breath, bluish lips/face, or persistent chest pain/pressure
• New confusion, fainting, inability to stay awake, seizures, or severe weakness/unsteadiness
• Signs of dehydration: very little urine, severe dizziness, dry mouth, inability to keep fluids down
• High fever that persists, returns after improving, or rapidly worsening cough/fever
• Coughing blood
• Symptoms that improve then suddenly worsen, which can suggest pneumonia or another complication

Infants and children

• Fast or difficult breathing, ribs pulling in with breaths, blue/gray lips or face
• Not drinking or feeding, no tears when crying, or markedly fewer wet diapers/urination
• Unusual sleepiness, unresponsiveness, confusion, or seizures
• Fever in an infant under 3 months old
• Severe irritability, refusal to be held, or a rash with fever
• Symptoms that improve and then worsen again

Adults

• Trouble breathing, shortness of breath, or bluish lips/face
• Persistent chest pain, chest pressure, or severe abdominal pain
• New confusion, fainting, severe dizziness, inability to stay awake, or seizures
• Signs of severe dehydration, especially not passing urine
• Severe weakness, unsteadiness, or severe muscle pain
• Repeated or persistent vomiting
• Fever or cough that initially improves but then returns or gets worse
• A major worsening of a chronic illness, such as asthma, COPD, heart disease, or diabetes

Children

• Fast or difficult breathing, ribs pulling in with breaths, or bluish lips/face
• Chest pain
• Dehydration: no urine for 8 hours, dry mouth, or no tears when crying
• Not alert, not interacting when awake, extreme irritability, or seizures
• Severe muscle pain or refusal to walk
• Inability to eat or drink
• Fever above 104°F (40°C) not responding to fever medicine
• Any fever in an infant younger than 12 weeks
• Fever/cough that improves then returns or worsens
• Worsening of a long-term health problem

Adults

• Difficulty breathing or shortness of breath
• Persistent chest or abdominal pain/pressure
• Persistent dizziness, confusion, inability to wake or stay awake, or seizures
• Not urinating
• Severe muscle pain, severe weakness, or marked unsteadiness
• Fever or cough that improves but then returns or worsens
• Worsening of a chronic medical condition

Children

• Fast or difficult breathing, ribs pulling in with breaths, or bluish lips/face
• Chest pain
• Dehydration: no urine for 8 hours, dry mouth, or no tears when crying
• Not alert or interacting when awake, or seizures
• Severe muscle pain or refusal to walk
• Fever above 104°F (40°C) not controlled by fever medicine
• Any fever in an infant younger than 12 weeks
• Fever/cough that improves and then returns or worsens
• Worsening of a chronic medical condition