Can influenza affect laboratory results, such as complete blood counts (CBC), electrolyte panels, and liver function tests (LFTs)?

Laboratory Effects of Influenza Infection

Yes, influenza can significantly affect multiple laboratory parameters, including complete blood counts, electrolyte panels, and liver function tests, and these changes should be anticipated and monitored in hospitalized patients. 1, 2

Complete Blood Count (CBC) Changes

Full blood count abnormalities are common and clinically significant in influenza:

• Leukocytosis with left shift may occur in patients with primary viral pneumonia, mixed viral-bacterial pneumonia, or secondary bacterial pneumonia 1, 2

• Lymphopenia is a characteristic finding, documented in 96.4% of H1N1 cases and particularly noted in severe avian H5N1 influenza infections 1, 2, 3

• Eosinopenia occurs in approximately 50% of influenza cases 3

• Leukocyte counts tend to decrease as the illness progresses beyond 4 days, with significant reduction in total WBC and neutrophil counts compared to the first three days of illness 3, 4

• Influenza B infections specifically are associated with lower leukocyte counts (mean 6,383/mm³) compared to influenza A (mean 7,639/mm³) 4

Electrolyte Panel Abnormalities

Electrolyte disturbances are frequent and require monitoring:

• Hyponatremia or hypernatremia may develop during influenza infection 1

• Decreased serum CO2 combining power occurs in approximately 60.7% of H1N1 cases, with significant improvement after 4 days of illness 3

• Elevated blood urea nitrogen (>7 mmol/L) is part of the CURB-65 severity assessment and indicates renal impairment 1

Liver Function Test Changes

Liver enzymes are typically normal but can be significantly affected in severe cases:

• Liver function tests are usually normal in uncomplicated influenza 1

• Severe hepatocyte injury can occur, with massive increases in AST and ALT (>20 times upper normal limit) in cases complicated by hypoxic hepatitis, particularly in patients with pre-existing conditions like anemia or chronic renal failure 5

• Acute liver failure has been documented as a complication of influenza A infection, characterized by extensive centrilobular hepatocyte necrosis, elevated PT-INR, and hypercytokinemia 5

Additional Laboratory Findings

Other laboratory parameters affected by influenza:

• Creatine kinase elevation occurs with severe myalgia or myositis, and is significantly higher in influenza B infections compared to influenza A 1, 4

• Hypoferremia (low serum iron) is present in 92.9% of H1N1 cases, with gradual improvement after 4 days 3

• C-reactive protein elevation occurs in 84.6% of cases, though it is most useful for detecting superimposed bacterial infection rather than viral infection alone 1, 3

• Complement activation with elevated CH50 levels (71.4% of cases) persists throughout the disease course and may significantly increase 7 days after illness onset 3

Recommended Laboratory Monitoring

The British Infection Society and British Thoracic Society recommend the following tests for all hospitalized influenza patients:

1. Full blood count 1, 2
2. Urea, creatinine, and electrolytes 1, 2
3. Liver function tests 1, 2
4. Creatine kinase (if myositis is suspected) 1
5. C-reactive protein (if secondary bacterial infection is suspected) 1

Clinical Pitfalls to Avoid

Key considerations when interpreting laboratory results:

• Do not assume normal liver function tests rule out severe influenza - hepatic complications can develop rapidly in patients with pre-existing conditions or septic shock 5

• Lymphopenia is expected and should not be misinterpreted as immunosuppression requiring antibiotics unless bacterial superinfection is clinically suspected 3

• Serial monitoring is important as laboratory parameters change significantly throughout the disease course, with most improvements occurring after 4-7 days 3

• Influenza B may present with more pronounced laboratory abnormalities including lower leukocyte counts and higher creatine kinase compared to influenza A 4