Serum lactate is a substance measured in the blood, produced as a byproduct of the body’s metabolic processes. It plays a role in energy production, especially when oxygen supply to cells is limited, acting as an alternative energy source for organs like the heart and brain during high energy demand.
Understanding Lactate Levels
Normal serum lactate levels range between 0.5 and 2.2 millimoles per liter (mmol/L). An elevated level indicates the body is producing more lactate than it can clear, due to increased metabolic demand or decreased oxygen delivery to tissues. Mildly to moderately elevated levels, between 2 and 4 mmol/L, are termed hyperlactatemia. If lactate exceeds 4 mmol/L with a decrease in blood pH (making the blood too acidic), this more serious condition is called lactic acidosis.
While often used interchangeably, lactate and lactic acid are chemically distinct. Lactic acid readily donates a hydrogen ion in the body’s neutral pH, forming lactate, which is primarily what is detected in the blood. Transient increases in lactate are common during intense physical activity, as muscles switch to anaerobic energy production when oxygen cannot be supplied fast enough. This temporary rise during exercise is cleared quickly by the body once activity stops.
Common Causes of Elevated Serum Lactate
Elevated serum lactate can arise from various medical and physiological conditions, broadly categorized by inadequate oxygen delivery, increased lactate production, or impaired lactate clearance. Conditions causing inadequate oxygen delivery, known as Type A lactic acidosis, include different forms of shock: septic, cardiogenic, hypovolemic, and obstructive. In these states, reduced blood flow and oxygen supply force cells to rely on anaerobic metabolism, increasing lactate production. Severe lung diseases, respiratory failure, or pulmonary edema also cause elevated lactate due to excessive breathing work and anaerobic muscle activity. Trauma, especially with significant blood loss, leads to hypoperfusion and subsequent elevated lactate levels.
Conditions causing increased lactate production, often classified as Type B lactic acidosis, occur even when oxygen delivery is adequate. Seizures, for instance, can lead to a substantial, short-term increase in lactate due to intense muscle activity. Certain toxins and medications, such as metformin, propofol, linezolid, and nucleoside reverse transcriptase inhibitors, can interfere with cellular metabolism, increasing lactate production. Malignancies, particularly hematological cancers like leukemia and lymphoma, also contribute to elevated lactate due to increased metabolic activity of tumor cells.
Conditions that impair the body’s ability to clear lactate, predominantly Type B lactic acidosis, also result in elevated serum levels. The liver is the primary organ responsible for metabolizing lactate, converting it back into glucose or breaking it down into carbon dioxide and water. Therefore, liver dysfunction or failure hinders lactate clearance, leading to its accumulation. Kidney disease can also contribute to impaired lactate clearance, as the kidneys play a secondary role in its removal. Thiamine (vitamin B1) deficiency can also impair lactate metabolism, causing increased production and reduced clearance.
Clinical Importance and Patient Care
Serum lactate measurement is an important tool in clinical settings, particularly for diagnosing and monitoring the severity of various medical conditions. It serves as an early indicator of tissue hypoperfusion or inadequate oxygenation, especially in emergency and critical care situations. For example, elevated lactate levels are a component of diagnostic criteria for conditions like septic shock, where prompt identification and treatment can improve patient outcomes.
The measurement also helps monitor a patient’s response to treatment. Serial lactate measurements, often taken every 2 to 4 hours, show whether interventions aimed at restoring oxygen delivery and perfusion are effective. A decrease in lactate levels indicates improvement, while persistently elevated or increasing levels suggest ongoing metabolic distress or inadequate response to therapy. While a high lactate level itself does not provide a specific diagnosis, it prompts healthcare providers to investigate the underlying cause. Management of elevated lactate focuses on addressing the root problem, which might involve fluid resuscitation, oxygen supplementation, antibiotics for infections, or discontinuation of causative medications.