Lactate vs Lactic Acid Lab: What to Order
Order "serum lactate" or "blood lactate" - these terms are clinically interchangeable and refer to the same test that measures lactate concentration in blood to assess tissue hypoperfusion. There is no separate "lactic acid" test in clinical practice; laboratories measure lactate (the conjugate base), not lactic acid itself, because at physiological pH, lactic acid exists almost entirely in its dissociated lactate form.
Understanding the Terminology
The terms "lactate" and "lactic acid" are often used interchangeably in clinical settings, but laboratories specifically measure lactate concentration. At physiological pH (7.35-7.45), lactic acid (pKa ~3.9) is almost completely dissociated into lactate anion and hydrogen ion. When you order a test, request:
- "Serum lactate"
- "Blood lactate"
- "Plasma lactate"
All refer to the same measurement.
Clinical Application for Tissue Hypoperfusion
The most recent 2025 ESICM guidelines on circulatory shock strongly recommend serial measurements of blood lactate to guide, monitor, and assess shock 1. Key points include:
- Lactate >2 mmol/L is typically elevated in shock states and indicates inadequate tissue perfusion 1
- Serial lactate measurements should be performed to follow shock evolution and assess adequacy of resuscitation 1
- The 2016 Surviving Sepsis Campaign guidelines note that while serum lactate is not a direct measure of tissue perfusion, increased levels are associated with worse outcomes regardless of the source 2
Why Lactate Matters for Mortality
Lactate-guided resuscitation significantly reduces mortality compared to resuscitation without lactate monitoring (RR 0.67; 95% CI 0.53-0.84) based on five randomized controlled trials involving 647 patients with septic shock 2. This mortality benefit makes lactate measurement a critical component of shock management.
Sample Collection Options
You can obtain lactate from:
- Arterial blood (traditional gold standard)
- Venous blood (clinically acceptable, correlates well with arterial)
- Capillary blood (point-of-care devices, faster turnaround)
Research shows capillary lactate has 81% sensitivity and 70% specificity compared to arterial lactate, with significantly faster turnaround time (77 minutes vs 112 minutes for arterial) 3. Point-of-care devices show strong correlation with standard laboratory methods 4.
Common Pitfalls to Avoid
- Don't assume elevated lactate always means tissue hypoxia: Hyperlactatemia can result from accelerated aerobic glycolysis during stress response, beta-adrenergic stimulation, or liver failure 2, 5
- Don't order a "lactic acid" test thinking it's different: This will result in the same lactate measurement
- Don't rely on a single lactate value: Serial measurements and lactate clearance provide better prognostic information 1, 6
- Don't over-resuscitate: Normalizing lactate may represent over-resuscitation if hyperlactatemia is unrelated to tissue dysoxia 5
Practical Ordering Approach
- Order "serum lactate" or "blood lactate" - specify arterial or venous source if relevant to your institution
- Obtain initial measurement within 60 minutes of recognizing severe sepsis/shock 7
- Repeat at 6 hours to assess lactate clearance and guide ongoing resuscitation 6
- Target lactate <2 mmol/L as part of resuscitation goals 1
The 6-hour lactate level has better prognostic value than 6-hour lactate clearance for predicting 30-day mortality (AUC 0.720 vs 0.656) 6, though both provide useful information for risk stratification.