Understanding Lactate vs. Lactic Acid in Metabolic Derangements
Lactate and lactic acid are biochemically distinct entities with important clinical implications in the assessment and management of metabolic derangements, with lactate measurement being the standard clinical test that reflects tissue hypoperfusion and metabolic stress.
Biochemical Distinction
- Lactic acid is the protonated form (H⁺ + lactate⁻) that exists at low pH
- Lactate is the predominant form found in the body at physiological pH (deprotonated anion)
- At physiological pH (7.35-7.45), nearly all lactic acid exists as lactate anion and hydrogen ions
Clinical Significance
Diagnostic Value
- Lactate measurement is the standard clinical test performed in laboratories and point-of-care devices 1
- Normal blood lactate levels are ≤2 mmol/L in most clinical settings 2
- Elevated lactate (>2 mmol/L) serves as a sensitive marker for:
- Tissue hypoperfusion
- Cellular hypoxia
- Shock states
- Metabolic derangements
Prognostic Significance
- Blood lactate serves as both a diagnostic parameter and prognostic marker of hemorrhagic shock 1
- Serial lactate measurements provide valuable prognostic information in circulatory shock 2
- Lactate clearance (decrease by 10-20%) is used as a resuscitation target in sepsis 2
- Lactate normalization strategies are associated with reduced mortality in septic patients 2
Monitoring and Measurement
Proper Collection Technique
- Lactate levels must be measured correctly using:
- Prechilled fluoride-oxalate tubes
- Transport on ice
- Processing within 4 hours
- Collection without tourniquet or fist-clenching 2
Interpretation Guidelines
- Lactate threshold in exercise testing is defined as the oxygen uptake (VO₂) at which blood lactate level increases 1
- In trauma patients, base deficit may be a better predictor than lactate when alcohol consumption is involved 1
- In obstetric patients, lactate >2 mmol/L in the absence of labor indicates end-organ injury, but may be physiologically elevated during labor 2
Clinical Applications
Trauma and Shock Management
- Lactate is recommended as a sensitive test to estimate and monitor the extent of bleeding and tissue hypoperfusion 1
- Base deficit may represent a suitable alternative when lactate measurements are unavailable 1
- Serial lactate measurements should be performed every 4-6 hours to assess lactate clearance and treatment efficacy 2
Resuscitation Targets
- Initial management includes crystalloid fluids (30 mL/kg IV) if signs of hypoperfusion are present 2
- Further resuscitation should be guided to normalize lactate levels 2
- Abramson et al. showed that all patients whose lactate levels normalized (≤2 mmol/L) within 24 hours survived, while survival decreased to 77.8% if normalization occurred within 48 hours 1
Cardiopulmonary Exercise Testing
- Lactate can be measured to determine the anaerobic threshold (AT) 1
- The AT is referenced to the VO₂ at which lactate rapidly increases and is expressed as a percentage of predicted VO₂max 1
- In CHF patients, lactate threshold <11 mL·min⁻¹·kg⁻¹ is associated with a mortality risk ratio of 2.7 at 6 months 1
Common Pitfalls and Caveats
Terminology confusion: Clinical laboratories measure and report "lactate," not "lactic acid," though these terms are often used interchangeably in clinical practice
Measurement errors: Improper collection techniques can lead to falsely elevated lactate levels
Contextual interpretation: Elevated lactate doesn't always indicate tissue hypoxia; consider:
- Type A lactic acidosis (tissue hypoperfusion)
- Type B lactic acidosis (metabolic causes including metformin, liver dysfunction) 2
Alcohol interference: In alcohol-associated trauma, base deficit may be a better predictor than lactate 1
Physiological elevation: During exercise or labor, lactate may be physiologically elevated 2
In conclusion, while the terms are often used interchangeably in clinical settings, understanding the distinction between lactate (what we actually measure) and lactic acid is important for proper interpretation of laboratory results and clinical management of metabolic derangements.