Lactic acid, an intermediary in carbohydrate metabolism, is predominantly derived from skeletal muscle and erythrocytes. The blood lactate concentration depends on the rate of production in these tissues and the rate of metabolism in the liver. Lactic acidosis occurs in 2 clinical settings: Type A (hypoxic), associated with decreased tissue oxygenation, such as shock, hypovolemia, and pulmonary insufficiency and type B (metabolic), associated with disease (diabetes mellitus, neoplasia, liver disease…), intoxication by drugs/toxins (ethanol, methanol, salicylates…), or inborn errors of metabolism. Lactate is produced when the body switches from aerobic (oxygen-dependent) to anaerobic metabolism. This shift often occurs during intense physical activity, when the body’s demand for energy exceeds the oxygen supply. However, elevated lactate levels (hyperlactatemia) can also signal issues like oxygen deprivation, infections, or metabolic disturbances. In veterinary medicine, lactate testing is particularly useful in cases of colic in horses, shock in dogs and cats, and assessing stress levels in animals undergoing surgery. Common situations where a lactate test might be ordered include Shock or Sepsis, Respiratory or Cardiac Failure, Severe Exercise, Diabetes or Liver Disease. Elevated lactate levels can be a marker for many serious conditions like Lactic acidosis, a state where high levels of lactic acid overwhelm the body’s ability to maintain a normal pH, can result from Severe infection (Sepsis): Systemic infections often result in an inflammatory response that impairs oxygen delivery, triggering anaerobic metabolism, Hypoxia (Low oxygen levels): Diseases of the heart or lungs can restrict oxygen flow to tissues, leading to lactate buildup. Tissue Ischemia: Conditions that block blood flow, such as arterial blockages or severe trauma, can starve tissues of oxygen, increasing lactate production. Certain Medications or Toxins: Some drugs (like metformin) and toxins (like cyanide) can affect mitochondrial function and increase lactate levels. In veterinary medicine, lactate testing is invaluable in emergencies, for example, elevated lactate levels can indicate poor intestinal perfusion and are used as a prognostic marker. Similarly, in small animals, lactate testing can assess the severity of shock, respiratory distress, or other critical conditions. The lactate test is a simple yet powerful tool for diagnosing and monitoring conditions that compromise oxygen delivery or increase anaerobic metabolism. In veterinary practice, understanding lactate levels helps guide treatment decisions, improving patient outcomes.