Influenza Diagnosis and Treatment Guidelines (2025 Edition) Part 2

V. Laboratory and Imaging Findings

(A) General Laboratory Tests

1. 1. Complete Blood Count (CBC): White blood cell (WBC) count is typically normal or decreased. Severe cases may show a significant reduction in lymphocyte count.
2. 2. Blood Biochemistry: Elevated levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), and creatinine may occur. A minority of cases show increased creatine kinase (CK). Electrolyte imbalances, such as hypokalemia, may be present. Shock cases may exhibit elevated blood lactate.
3. 3. Arterial Blood Gas Analysis: Severe cases may demonstrate decreased partial pressure of oxygen (PaO2), oxygen saturation (SaO2), and oxygenation index (PaO2/FiO2), along with acid-base imbalance.
4. 4. Cerebrospinal Fluid (CSF): In central nervous system (CNS) involvement, cell counts and protein levels may be normal or elevated. Acute necrotizing encephalopathy (ANE) typically shows normal cell counts with elevated protein.

(B) Etiological Testing

1. 1. Antigen Detection: Nasopharyngeal or pharyngeal swab antigen testing is rapid and simple but less sensitive than nucleic acid testing. A positive result supports diagnosis, but a negative result does not exclude influenza.
2. 2. Nucleic Acid Testing: Nasopharyngeal swabs, pharyngeal swabs, tracheal aspirates, sputum, or bronchoalveolar lavage fluid can be tested via nucleic acid amplification (e.g., RT-PCR), which has high sensitivity and specificity and can distinguish viral types/subtypes.
3. 3. Virus Culture: Influenza virus can be isolated from respiratory specimens.

(C) Serological Testing

A retrospective diagnosis can be made if convalescent-phase IgG antibodies seroconvert or show a fourfold or greater increase compared to the acute phase.

(D) Imaging Findings

Primary viral pneumonia: Imaging reveals lung patchy shadows, ground-glass opacities (GGO). Rapid progression may lead to bilateral diffuse infiltrates or consolidations. Rare cases show pleural effusion.

Acute necrotizing encephalopathy (ANE): CT or MRI may show multifocal brain lesions, including thalamus, periventricular white matter, internal capsule, putamen, dorsal upper brainstem (around the fourth ventricle and ventral midbrain aqueduct), and cerebellar medulla. Bilateral symmetrical thalamic lesions are characteristic.

VI. Diagnosis

Diagnosis is based on epidemiological history, clinical manifestations, and etiological testing. During influenza season, even with atypical symptoms, influenza should be considered for high-risk or hospitalized patients, and etiological testing is required. During non-epidemic periods, influenza testing should be performed for hospitalized patients with suspected viral pneumonia, in addition to testing for common respiratory pathogens.

(A) Clinically Diagnosed Cases

Patients with epidemiological history (close contact with suspected/confirmed influenza cases within 7 days before onset without effective protection, belonging to a cluster of influenza-like cases, or having clear evidence of transmission) and typical influenza symptoms, after excluding other causes of influenza-like illness.

(B) Confirmed Cases

Patients with influenza symptoms and at least one positive etiological test:

1. 1. Positive influenza antigen test.
2. 2. Positive influenza nucleic acid test.
3. 3. Positive influenza virus culture.
4. 4. IgG antibody seroconversion or a fourfold or greater increase in convalescent-phase titers.

VII. Clinical Classification

(A) Mild Type

Manifested as upper respiratory tract infection.

(B) Moderate Type

Fever >3 days and/or cough, shortness of breath, but respiratory rate (RR) <30 breaths/min and oxygen saturation (SpO2) >93% at rest on room air. Imaging shows pneumonia.

(C) Severe Type

1. Adults meeting any of the following:

• ✅Tachypnea (RR ≥30 breaths/min).
• ✅SpO2 ≤93% at rest on room air.
• ✅PaO2/FiO2 ≤300 mmHg (corrected for altitude >1000m using: PaO2/FiO2 × [760/atmospheric pressure (mmHg)]; 1 mmHg = 0.133 kPa).
• ✅Rapid clinical worsening with >50% progression of lung lesions on imaging within 24–48 hours.

2. Children meeting any of the following:

• ✅Extremely high fever or persistent fever >3 days.
• ✅Tachypnea (≥60 breaths/min for <2 months; ≥50 breaths/min for 2–12 months; ≥40 breaths/min for 1–5 years; ≥30 breaths/min for >5 years), excluding fever/crying effects.
• ✅SpO2 ≤93% at rest on room air.
• ✅Nasal flaring, chest retractions, wheezing, or dyspnea.
• ✅Altered consciousness or seizures.
• ✅Refusal to eat, feeding difficulties, or signs of dehydration.

(D) Critical Type

Patients meeting any of the following:

1. 1. Respiratory failure requiring mechanical ventilation.
2. 2. Shock.
3. 3. Acute necrotizing encephalopathy (ANE).
4. 4. Multi-organ failure requiring ICU care.

VIII. High-Risk Populations for Severe/Critical Illness

The following groups are at higher risk for severe/critical influenza and require early antiviral treatment and monitoring:

1. 1. Children <5 years (highest risk <2 years).
2. 2. Adults ≥65 years.
3. 3. Individuals with chronic conditions: respiratory, cardiovascular (excluding hypertension), renal, hepatic, hematologic, neurological/neuromuscular, metabolic/endocrine disorders, malignancy, or immunosuppression.
4. 4. Obese individuals.
5. 5. Pregnant and postpartum women.

To be continued...

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