The diagnosis of lactic acidosis is made when the serum lactate level is > 4 mmol/L. It is the most common cause of metabolic acidosis and can be associated with an anion gap metabolic acidosis or mixed acid-base disorder. Lactic acidosis typically occurs when lactic acid production exceeds clearance, often in the setting of impaired tissue oxygenation and defective mitochondrial oxygenation. Clinical presentation varies based on underlying cause and is similar in both children and adults. It can be classified as follows:

• Type A lactic acidosis – Associated with tissue hypoperfusion caused by circulatory failure from shock, which can be due to many things including sepsis, heart failure, hypovolemia, neurologic compromise, or respiratory failure. Patients typically present with hypotension, cool and clammy extremities, oliguria, and altered mental status. Seizures or excessive exercise may result in elevated lactate levels by this mechanism.
• Type B lactic acidosis – Due to drug- or toxin-induced impairment of cellular metabolism, ischemia, nutritional deficient state, or rarely malignancy. Common causes include diabetes related to metformin toxicity or diabetic ketoacidosis (DKA), solid and hematologic malignancies, chronic severe alcohol use disorder, thiamine deficiency, use of beta-adrenergic agonists (intravenous epinephrine, inhaled albuterol, inhaled salmeterol), drug-induced mitochondrial dysfunction due to high doses of propofol, human immunodeficiency virus (HIV), nucleoside reverse transcriptase inhibitors, and linezolid.
• D-lactic acidosis – Occurs in patients with short gut syndrome or malabsorption due to bacterial fermentation of large amounts of glucose and starch to organic acids, including D-lactic acid. Because of slow metabolism of D-lactic acid, systemic absorption leads to elevated plasma D-lactic acid levels and metabolic acidosis. Other causes include diabetic ketoacidosis and high-dose infusion or ingestion of propylene glycol, which can be found in various intravenous medications. Patients present with episodic metabolic acidosis as well as altered mental status, ataxia, loss of memory, or slurred speech.