Can Influenza Cause Elevated Lactic Acid?
Yes, influenza can cause elevated lactic acid levels, particularly in patients who develop severe complications such as primary viral pneumonia, acute respiratory failure, or sepsis from secondary bacterial infections, all of which lead to tissue hypoxia and anaerobic metabolism.
Mechanisms Leading to Lactic Acidosis in Influenza
While the provided guidelines do not explicitly mention lactic acid elevation, the pathophysiological mechanisms described clearly establish how influenza causes this metabolic derangement:
Severe Respiratory Complications
Primary viral pneumonia develops within the first 48 hours of fever onset and causes rapid clinical deterioration with cyanosis, tachypnea, and bilateral interstitial infiltrates, leading to profound hypoxemia that drives anaerobic metabolism and lactate production 1.
Acute respiratory failure in patients admitted to the ICU carries very high mortality risk, particularly in immunosuppressed patients, and represents a state of severe tissue hypoxia 1.
Patients develop respiratory distress with oxygen saturation drops requiring arterial blood gas monitoring, which would reveal metabolic acidosis with elevated lactate in severe cases 2.
Secondary Bacterial Pneumonia and Sepsis
Up to 75% of influenza patients who develop pneumonia test positive for bacterial coinfection, with Staphylococcus aureus carrying 47% mortality versus 16% for non-staphylococcal pneumonias 3, 1.
Secondary bacterial pneumonia presents with a biphasic illness pattern—initial improvement followed by recurrence of fever and clinical deterioration at day 9—and can progress to septic shock with resultant lactic acidosis 3.
During the 1968 pandemic, S. pneumoniae (48%), S. aureus (26%), and H. influenzae (11%) were predominant pathogens, all capable of causing sepsis with elevated lactate 3.
High-Risk Populations Most Susceptible
Patients with Diabetes
Influenza can exacerbate diabetic ketoacidosis, which itself is associated with elevated lactate due to tissue hypoperfusion and altered metabolism 1.
The odds of ICU admission for influenza are three times higher (OR 4.29) in diabetic patients compared to those without diabetes, increasing the likelihood of severe complications that elevate lactate 1.
Hyperglycemia decreases immune cell recruitment, neutrophil degranulation, and phagocytosis, collectively limiting the immune response and allowing more severe infection 1.
Patients with Cardiovascular Disease
Influenza-related respiratory infections increase the risk of acute myocardial infarction and acute heart failure, both of which can cause cardiogenic shock with elevated lactate 1, 4.
Among 89,999 adults hospitalized with laboratory-confirmed influenza, 11.7% had acute cardiovascular events, with acute heart failure (6.2%) and acute ischemic heart disease (5.7%) being most common 4.
Older age, tobacco use, underlying cardiovascular disease, diabetes, and renal disease were significantly associated with higher risk for these cardiac complications 4.
Elderly Patients
The 2024-25 influenza season demonstrated cumulative hospitalization rates highest among persons aged ≥75 years (598.8 per 100,000 population), with rates 1.8 to 2.8 times higher than median historical rates 5.
Among hospitalized patients, 16.8% were admitted to an ICU, 6.1% received invasive mechanical ventilation, and 3.0% died in hospital—all scenarios associated with elevated lactate 5.
Elderly patients may not mount adequate febrile responses and can present with atypical symptoms including lassitude and confusion, which may represent hypoperfusion states with elevated lactate 2.
Clinical Recognition and Monitoring
When to Suspect Lactic Acidosis
Any influenza patient developing increasing shortness of breath, persistent high fever, confusion, chest pain, or inability to maintain oral intake should be evaluated for complications that may include lactic acidosis 2.
Patients with CURB-65 score ≥2 (Confusion, Urea elevation, Respiratory rate ≥30/min, Blood pressure <90/60 mmHg, age ≥65 years) warrant hospitalization and metabolic assessment 2, 3.
Rapid clinical deterioration with respiratory distress, bilateral crepitations, and leucocytosis suggests primary viral pneumonia with potential for severe hypoxemia and lactate elevation 1.
Diagnostic Approach
Arterial blood gas should be performed if oxygen saturation is low or respiratory distress is present, which will reveal metabolic acidosis and elevated lactate in severe cases 2.
Full blood count, urea, creatinine, and electrolytes should be ordered to assess for complications, with elevated urea suggesting dehydration and renal hypoperfusion 2.
Chest X-ray should be obtained to evaluate for pneumonia if respiratory symptoms are prominent or the patient appears severely ill 2, 3.
Common Pitfalls to Avoid
Do not assume normal lactate in influenza patients with chronic conditions—diabetes, heart disease, lung disease, and renal disease all increase the risk of severe complications that elevate lactate 1, 4.
Do not delay antiviral therapy while awaiting laboratory confirmation—oseltamivir 75 mg twice daily for 5 days should be initiated immediately in high-risk patients, as early treatment may prevent progression to severe complications 2.
Do not overlook the absence of fever in elderly patients—they frequently present with atypical symptoms and may have severe illness with elevated lactate despite minimal fever 2.
Do not withhold antibiotics if secondary bacterial pneumonia is suspected—biphasic illness with recrudescent fever after initial improvement strongly suggests bacterial superinfection requiring empiric co-amoxiclav or doxycycline 3, 6.