Medical Disclaimer
  • For Educational Purposes Only: This content is intended for educational reference and should not be used for clinical decision-making.
  • Not a Substitute for Professional Judgment: Always consult your local protocols, institutional guidelines, and supervising physicians.
  • Accuracy Not Guaranteed: While all content has been prepared to the best of my knowledge and ability, errors or omissions may exist.
  • Verify Before Acting: Users are responsible for verifying information through authoritative sources before any clinical application.
AI Assistance Notice
The clinical content and references are curated and reviewed by myself; however, AI was used to assist in organizing, paraphrasing, and formatting the information presented.
Quick Reference
  • Normal Range: 0.5-2.0 mEq/L (or <2.0 mmol/L)
  • Mildly Elevated: 2-4 mEq/L
  • Moderately Elevated: 4-8 mEq/L
  • Severely Elevated: >8 mEq/L
  • Sepsis Threshold: >2 mEq/L = elevated risk
  • Severe Sepsis/Shock: >4 mEq/L = high mortality
  • Primary Use: Assessing tissue perfusion, diagnosing shock states, prognosticating sepsis
  • Sample Type: Arterial blood (ABG), venous blood, or point-of-care
  • Key Point: Lactate clearance >10% at 6 hours predicts better outcomes than absolute value alone

Test Description

Lactate is a byproduct of anaerobic metabolism that accumulates when tissues are deprived of adequate oxygen or when cells cannot utilize oxygen effectively. Under normal aerobic conditions, pyruvate (from glycolysis) enters the mitochondria for oxidative phosphorylation. When oxygen is insufficient or mitochondrial function is impaired, pyruvate is converted to lactate instead.

Physiological Role

Lactate serves as:

  • Marker of tissue hypoperfusion: Elevated lactate indicates inadequate oxygen delivery to tissues (Type A lactic acidosis)
  • Metabolic stress indicator: Elevated in cellular dysfunction even without hypoperfusion (Type B lactic acidosis)
  • Prognostic tool: Higher lactate levels and poor clearance correlate with increased mortality in critically ill patients
  • Resuscitation endpoint: Lactate clearance is used to guide fluid resuscitation and vasopressor therapy in sepsis and shock

Clinical Importance

Lactate measurement is critical in emergency and critical care medicine. It is incorporated into sepsis bundles (Surviving Sepsis Campaign), shock protocols, and trauma resuscitation guidelines. Serial lactate measurements and lactate clearance are more informative than a single value, providing dynamic assessment of resuscitation adequacy.

Clinical Significance

Lactic Acidosis Classification

Lactic acidosis is classified into two types based on the presence or absence of tissue hypoperfusion:

Type A Lactic Acidosis: Tissue Hypoperfusion

  • Shock (any type): Cardiogenic, hypovolemic, distributive (septic), obstructive
  • Sepsis and septic shock: Microcirculatory dysfunction and cytopathic hypoxia
  • Severe hypoxemia: Respiratory failure, severe anemia (Hgb <7 g/dL)
  • Carbon monoxide poisoning: Impaired oxygen delivery and utilization
  • Cyanide poisoning: Mitochondrial dysfunction, inability to utilize oxygen
  • Regional ischemia: Mesenteric ischemia, limb ischemia, myocardial infarction

Type B Lactic Acidosis: No Tissue Hypoperfusion

  • Medications:
    • Metformin (especially with AKI or overdose)
    • Linezolid (mitochondrial toxicity)
    • Nucleoside reverse transcriptase inhibitors (NRTIs)
    • Propofol infusion syndrome
  • Liver failure: Impaired lactate clearance (liver metabolizes lactate)
  • Malignancy: High tumor burden (Warburg effect—tumors prefer glycolysis)
  • Seizures: Intense muscle activity and metabolic demand
  • Thiamine deficiency: Impairs pyruvate dehydrogenase, shunting pyruvate to lactate
  • Inborn errors of metabolism: Mitochondrial disorders

Lactate in Sepsis and Septic Shock

The Surviving Sepsis Campaign incorporates lactate into sepsis definitions and resuscitation bundles:

  • Lactate >2 mEq/L: Indicates elevated risk; triggers sepsis bundle (obtain cultures, give antibiotics, fluid resuscitation)
  • Lactate >4 mEq/L: Defines septic shock (along with hypotension requiring vasopressors); associated with high mortality (~40%)
  • Lactate clearance: Clearance of >10% at 6 hours predicts better outcomes and is used as a resuscitation endpoint
Sepsis-3 Definition: Septic shock is defined as sepsis with persistent hypotension requiring vasopressors to maintain MAP ≥65 mmHg AND lactate >2 mEq/L despite adequate fluid resuscitation.

Lactate Clearance

Lactate clearance is a more important prognostic indicator than a single lactate value. It reflects the adequacy of resuscitation and restoration of tissue perfusion.

  • Calculation: Lactate Clearance = [(Initial Lactate - Follow-up Lactate) / Initial Lactate] × 100%
  • Goal: >10% clearance at 6 hours, or normalization of lactate
  • Prognostic value: Patients with lactate clearance >10% have significantly lower mortality than those with persistent or worsening lactate
  • Clinical use: Guides ongoing resuscitation—if lactate is not clearing, reassess fluid status, vasopressor needs, and source control
Interpretation Guidelines

Approach to Elevated Lactate

When encountering an elevated lactate, use a systematic approach:

  1. Assess for shock: Check blood pressure, heart rate, urine output, and end-organ perfusion (mental status, skin perfusion)
  2. Determine lactate type:
    • Type A (hypoperfusion): Treat underlying shock—fluids, vasopressors, source control
    • Type B (no hypoperfusion): Identify and address underlying cause (stop offending drug, treat liver failure, etc.)
  3. Initiate resuscitation: In sepsis/shock, start early goal-directed therapy (fluids, antibiotics, vasopressors)
  4. Monitor lactate clearance: Recheck lactate at 2-6 hours to assess resuscitation adequacy
  5. Reassess if lactate not clearing: Consider occult bleeding, inadequate source control, persistent infection, or Type B causes
High Mortality Indicators
  • Lactate >4 mEq/L in sepsis (septic shock—mortality ~40%)
  • Lactate >8 mEq/L in any shock state (mortality >60%)
  • Persistently elevated lactate despite resuscitation (failure to clear)
  • Rising lactate on serial measurements (worsening perfusion)

Lactate as "Poor Man's Mixed Venous Oxygen Saturation"

Lactate is often used as a surrogate for mixed venous oxygen saturation (SvO2) in resource-limited settings. While not as precise, lactate is more widely available and easier to obtain than central venous catheterization for ScvO2 monitoring. Lactate clearance parallels improvements in oxygen delivery and consumption.

Interfering Factors

Factors That Increase Lactate

  • Medications: Metformin (especially with renal impairment), linezolid, propofol infusion, NRTIs, salicylates (high dose), epinephrine (β2-mediated glycolysis), albuterol
  • Seizure activity: Intense muscle contractions and metabolic demand transiently elevate lactate
  • Vigorous exercise: Physiologic lactic acidosis from muscle exertion
  • Liver failure: Impaired lactate clearance (liver is primary site of lactate metabolism)
  • Thiamine deficiency: Impairs pyruvate metabolism, shunts to lactate
  • Collection issues: Prolonged tourniquet use, fist clenching during blood draw (causes local ischemia and falsely elevated lactate)

Pseudoelevated Lactate

  • Improper sample handling: Delayed processing or inadequate sample preservation can falsely elevate lactate (cells continue metabolizing glucose)
  • Tourniquet application: Prolonged tourniquet or fist clenching causes local anaerobic metabolism

Important Note on Metformin

Metformin-associated lactic acidosis (MALA) is a rare but serious complication. Risk is highest with acute kidney injury, overdose, or severe dehydration. In patients taking metformin who present with lactic acidosis, consider MALA if other causes are excluded and renal function is impaired.

Clinical Pearls
Clinical Pearl
"Lactate >4 in sepsis = high mortality": Lactate above 4 mEq/L in the setting of sepsis defines septic shock and carries a mortality risk of approximately 40%. This mandates aggressive resuscitation with fluids, vasopressors, and early antibiotics.
Clinical Pearl
Lactate clearance matters more than absolute value: A patient with lactate of 6 mEq/L that clears to 3 mEq/L (50% clearance) has a better prognosis than a patient with lactate of 4 mEq/L that remains at 4 mEq/L (0% clearance). Always recheck lactate at 2-6 hours.
Clinical Pearl
Check serial lactates: A single lactate value provides limited information. Serial measurements reveal trends—improving lactate indicates effective resuscitation, while persistent or worsening lactate signals ongoing hypoperfusion or inadequate treatment.
Metformin + AKI = lactic acidosis: Metformin is renally cleared. In the setting of acute kidney injury, metformin accumulates and can cause severe lactic acidosis. Always check renal function in metformin-treated patients with elevated lactate and consider holding metformin during acute illness.
"Poor man's mixed venous oxygen saturation": Lactate is often used as a surrogate for ScvO2/SvO2 in sepsis resuscitation. It's more readily available and less invasive than central venous catheterization. Lactate clearance parallels improvements in oxygen delivery.
Clinical Pearl
Thiamine deficiency can cause lactic acidosis: Thiamine (vitamin B1) is a cofactor for pyruvate dehydrogenase. In deficiency, pyruvate cannot enter the Krebs cycle and is shunted to lactate. Consider thiamine supplementation in chronic alcoholics, malnourished patients, or those with persistent lactic acidosis without clear cause.
Propofol infusion syndrome: Prolonged propofol infusion (>48 hours at high doses) can cause severe lactic acidosis, rhabdomyolysis, and cardiovascular collapse. Monitor lactate in patients on prolonged propofol, especially at doses >5 mg/kg/hr.
Clinical Pearl
Normal lactate does NOT exclude shock: Early or compensated shock may have normal lactate. Always interpret lactate in the context of vital signs, physical exam, and end-organ perfusion. Some patients in shock maintain normal lactate until late decompensation.
Clinical Pearl
Epinephrine can elevate lactate: β2-adrenergic stimulation increases glycolysis and lactate production independent of hypoperfusion. Patients on epinephrine infusions may have mildly elevated lactate (Type B) even with adequate perfusion.
Clinical Pearl
Surviving Sepsis Campaign guidelines: Lactate is central to sepsis bundles. Measure lactate within 1 hour of suspected sepsis. If lactate >2 mEq/L, initiate sepsis resuscitation (cultures, antibiotics within 1 hour, 30 mL/kg crystalloid bolus). Remeasure lactate within 2-6 hours to assess response.
Lactate in trauma: Lactate is a sensitive marker of occult hypoperfusion in trauma patients. Elevated lactate may indicate ongoing hemorrhage or inadequate resuscitation even when blood pressure is maintained. Serial lactates guide resuscitation endpoints.
References
  1. Kratz, A., Ferraro, M., Sluss, P. M., & Lewandrowski, K. B. (2004). Laboratory reference values. New England Journal of Medicine, 351, 1548-1564.
  2. Lee, M. (Ed.). (2009). Basic skills in interpreting laboratory data. Ashp.
  3. Farinde, A. (2021). Lab values, normal adult: Laboratory reference ranges in healthy adults. Medscape. https://emedicine.medscape.com/article/2172316-overview?form=fpf
  4. Nickson, C. (n.d.). Critical Care Compendium. Life in the Fast Lane • LITFL. https://litfl.com/ccc-critical-care-compendium/
  5. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
Back to ABG Panel All Lab Values