Aerobic metabolism, which supplies 90% of the body's energy needs, requires oxygen. If oxygen isn't readily available to body cells, anaerobic metabolism occurs, with lactic acid as a byproduct.1 A serum lactate level measures the amount of lactic acid in the blood and is a fairly sensitive and reliable indicator of tissue hypoperfusion and hypoxia.
Any disorder that causes an imbalance between lactate production and clearance can lead to lactic acidosis, a serious and sometimes life-threatening condition. Lactic acid production can increase with any condition that results in anaerobic metabolism, such as hemorrhagic shock or pulmonary embolism. Normally, the liver clears most lactic acid from the blood, but hepatic dysfunction decreases lactic acid clearance.
Assessing for the problem. Most patients who develop lactic acidosis are critically ill. The signs and symptoms, which may vary from patient to patient, include altered mental status; pale, cool, clammy skin; nausea and vomiting; diaphoresis; hypotension; tachypnea; and tachycardia.
Performing the test. Explain the procedure to the patient. Tell him not to clench his hand and don't apply a tourniquet, if possible-these actions can raise levels of lactic acid from the hand muscles.2 Follow facility policy for collecting venous or arterial blood, pack the sample on ice, and send it to the lab right away. (Analysis should be performed within 30 minutes of collection.) Apply manual pressure to the site until hemostasis is achieved.
What do the results mean? Lactate values differ for venous and arterial blood, and normal ranges vary among labs. Most labs define normal as 0.5 to 2.2 mmol/L for venous blood and 0.5 to 1.6 mmol/L for arterial blood.
Serum lactate levels increase in lactic acidosis, severe dehydration, heart failure, respiratory failure, hemorrhage, ketoacidosis, severe infections, alcohol abuse, salicylate toxicity, shock, and liver disease.3,4
What can affect the results? In noncritically ill adults, exercising just before blood sampling can increase lactate levels. Lactic acid levels normally rise during strenuous exercise when perfusion can't meet the increased metabolic demands of skeletal muscles.
Administering metformin with intravascular iodinated contrast media for radiologic studies or procedures can cause lactic acidosis. Metformin should be discontinued in select patients (according to facility policy and procedure) at the time of the study or procedure and for 48 hours afterward.
Not placing the specimen on ice may affect the results.
What care does the patient require? A patient with lactic acidosis is critically ill, and the use of buffering agents alone is unlikely to be effective. To resolve lactic acidosis, identify and treat the underlying disorder.