33 Lactic acid

Lactate: normal level, hyperlactataemia and lactic acidosis, types A and B, and its role in sepsis and shock.

Lactate (lactic acid) is formed from pyruvate at the end of glycolysis, the breakdown of glucose. When cells have enough oxygen and functioning mitochondria, pyruvate is burned completely to produce ATP. When oxygen is lacking, or production of energy in the mitochondria is impaired, pyruvate is converted to lactate. Lactate is cleared mainly by the liver and kidneys. A raised blood lactate (hyperlactataemia) is a marker of tissue hypoperfusion and of the severity of illness in shock and sepsis. When it is high enough to lower blood pH, the condition is called lactic acidosis.

MetabolismShockAcid–base balance

Normal at rest< 2 mmol/L (< 18 mg/dL)

Septic shock criterion> 2 mmol/L with vasopressors

Severe hyperlactataemia≥ 4 mmol/L

Conversion1 mmol/L ≈ 9 mg/dL

Figure 1. Blood lactate zones at rest. During intense exercise lactate can temporarily exceed 10 mmol/L in healthy people and falls within an hour.

Causes of raised lactate

Type A, tissue hypoxia: shock of any cause (septic, cardiogenic, haemorrhagic), severe hypoxaemia, severe anaemia, carbon monoxide poisoning, bowel ischaemia, seizures, extreme exercise
Type B, without obvious hypoxia: liver failure, sepsis (also by metabolic mechanisms), metformin accumulation in kidney failure, alcohol, thiamine deficiency, some tumours, medicines (linezolid, antiretrovirals, propofol infusion), adrenaline and salbutamol, inherited mitochondrial diseases

Clinical significance

In sepsis, a lactate above 2 mmol/L despite adequate fluids, together with the need for vasopressors, defines septic shock, which carries a high mortality. Repeated measurements guide resuscitation: a falling lactate suggests improving perfusion. Lactic acidosis accompanying low blood flow or sepsis is associated with a sharp rise in mortality. Treatment is aimed at the underlying cause.

Seek emergency care

for confusion, rapid breathing, cold clammy skin, low blood pressure, little urine or fever with shivering. These may indicate sepsis or shock, in which lactate is often raised.

References

Singer M, Deutschman CS, Seymour CW, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA. 2016;315(8):801–810. doi:10.1001/jama.2016.0287
Kraut JA, Madias NE. Lactic acidosis. N Engl J Med. 2014;371(24):2309–2319. doi:10.1056/NEJMra1309483

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 9 October 2024