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Emergency Medicine
Emergency Medicine

Lactate in Sepsis: From Physiology to Bedside Interpretation

Why raised lactate matters in sepsis, what causes it, and how to use it without anchoring on it.

Published
18 May 2026
Read time
3 min read
Clinical ReviewFoundation / Junior DoctorSpecialty Trainee

Lactate is one of the most-cited and most-misunderstood numbers in resuscitation. A value over 4 mmol/L on a sepsis screen triggers the protocol, but the value alone tells you almost nothing about what is wrong.

Where it comes from

Lactate is a normal product of glycolysis. Pyruvate is reduced to lactate by lactate dehydrogenase, and the reaction goes both ways. In health, lactate is generated by red cells, skeletal muscle, brain, and gut and is cleared mainly by the liver and kidneys via the Cori cycle.

Elevated lactate in sepsis arises through at least four mechanisms — usually in combination:

  1. Tissue hypoperfusion — the textbook story. Inadequate oxygen delivery shifts pyruvate to lactate.
  2. Catecholamine surge — endogenous and exogenous adrenaline stimulate Na/K-ATPase activity, driving aerobic glycolysis.
  3. Hepatic clearance failure — septic livers don't metabolise lactate as efficiently.
  4. Mitochondrial dysfunction — cytokine-mediated impairment of oxidative phosphorylation.

Evidence

Surviving Sepsis 2021 retains lactate ≥4 mmol/L as a trigger for aggressive resuscitation, and recommends remeasuring within 2-4 hours to assess clearance. The evidence base is observational; the absolute threshold is pragmatic rather than physiologic.

How to interpret a raised lactate at the bedside

Ask three questions, in order:

  • Is this anaerobic (type A) or aerobic (type B)? Type A — cold, mottled, hypotensive, urine output low. Type B — warm, well-perfused, no shock features.
  • Has it cleared in the last 2-4 hours? Clearance ≥10% (or absolute drop) over 2-4 hours is the prognostic signal that matters more than the absolute number.
  • What else explains it? Metformin, beta-2 agonists, propofol, antiretrovirals, malignancy, thiamine deficiency, ethanol, regional ischaemia (mesenteric, limb).

Clinical pearl

A patient with lactate 6 mmol/L who is warm, alert, well-perfused, and on a salbutamol nebuliser may need their nebuliser stopping more than they need a fluid bolus.

What it should change

A raised lactate in sepsis should make you:

  • Look for the source — blood cultures, urine, CXR, soft-tissue exam, line review. The lactate is screaming for a source, not for more fluid.
  • Reassess perfusion — capillary refill, mottling, mentation. These are at least as good as lactate, and free.
  • Plan to remeasure at 2-4 hours. A falling lactate is reassuring; a rising lactate is a senior phone call.

Red flag

Lactate not falling despite source control and adequate MAP — escalate to critical care. Consider mesenteric ischaemia (especially in older AF patients), occult bleeding, and missed source.

Common traps

  • Treating the lactate, not the patient. Fluid bolus on a well-perfused, normotensive patient with lactate 3 mmol/L from salbutamol is a harm.
  • Tourniquet artefact. A sample drawn with prolonged stasis can spuriously elevate lactate by 1-2 mmol/L. If the value doesn't match the patient, repeat it.
  • Anchoring on the first value. Trend matters far more than the spot value.

Key points

  • Lactate elevation in sepsis is multifactorial, not just hypoperfusion.
  • Use clearance over 2-4 hours, not absolute values, as the resuscitation guide.
  • Type A vs type B distinction changes management.
  • Always look for the source — and a second cause for the lactate itself.

References

  1. [1]Evans L, Rhodes A, Alhazzani W, et al. Surviving Sepsis Campaign: international guidelines for management of sepsis and septic shock 2021. Intensive Care Med. 2021;47(11):1181-1247. Link
  2. [2]Jansen TC, van Bommel J, Schoonderbeek FJ, et al. Early lactate-guided therapy in intensive care unit patients (LACTATES). Am J Respir Crit Care Med. 2010;182(6):752-761. Link

About the author

Dr. Theo LindqvistVerifiedVerified medical practitioner

MBChB, FRCEM

Emergency Medicine· Royal London Hospital

Emergency medicine consultant with a teaching role for foundation and core trainees. Interests in resuscitation, sepsis, and time-critical decisions.

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