Lactic acidosis is a dangerous buildup of lactic acid in the bloodstream that makes the blood too acidic. Normal lactate levels sit below 1.0 mmol/L; lactic acidosis is typically diagnosed when levels climb above 5 to 6 mmol/L and blood pH drops below 7.35. It’s most often a sign that something serious is happening in the body, usually involving organs or tissues that aren’t getting enough oxygen.
How Lactic Acid Builds Up
Your cells normally produce energy using oxygen. When oxygen is plentiful, a byproduct called pyruvate gets funneled into your mitochondria (the energy-producing structures inside each cell) and converted into usable fuel. But when oxygen runs low, or the mitochondria can’t process pyruvate properly, your cells switch to a backup system that doesn’t require oxygen. This backup system converts pyruvate into lactate instead.
Small amounts of lactate are produced all the time, and your liver and kidneys clear it efficiently. Lactic acidosis happens when production overwhelms clearance. Lactate floods the bloodstream faster than the body can remove it, and hydrogen ions accumulate alongside it, driving down blood pH. The result is a metabolic crisis that affects the heart, brain, and other organs.
Type A vs. Type B
Lactic acidosis falls into two categories based on its underlying cause.
Type A is the more common form. It happens when tissues aren’t receiving enough oxygen, forcing cells into that anaerobic backup mode. Sepsis, severe blood loss, heart failure, cardiac arrest, and any form of shock can trigger it. Essentially, if blood flow or oxygen delivery drops significantly, type A lactic acidosis can follow.
Type B occurs without any obvious oxygen shortage. Instead, something disrupts mitochondrial function directly. Certain medications, liver disease, kidney failure, toxins like cyanide or carbon monoxide, and some cancers can all cause type B lactic acidosis. Despite different triggers, the core problem is the same in both types: mitochondria can’t keep up with pyruvate processing, and lactate accumulates.
Common Causes
Sepsis and shock are the leading causes. In patients with sepsis, some of the excess lactate comes from tissues starved of oxygen, but research has shown that not all of it is purely anaerobic. Inflammation itself can drive lactate production through stress-related metabolic changes, even in tissues that are technically receiving adequate blood flow.
Metformin, one of the most widely prescribed diabetes medications, is another well-known trigger. Metformin works partly by inhibiting a key step in the mitochondrial energy chain. In normal doses with healthy kidneys, this effect is mild and manageable. But if metformin accumulates in the blood, typically because the kidneys aren’t clearing it properly, it can shut down mitochondrial energy production more aggressively. Hydrogen ions pile up, and acidosis develops. Kidney injury, liver failure, heart failure, dehydration, and critical illness all raise the risk of this happening. This is why metformin is usually paused before surgery and during any acute illness involving low blood pressure or poor oxygen delivery.
Other causes include severe asthma attacks, carbon monoxide poisoning, seizures (which massively increase muscle oxygen demand), and certain chemotherapy drugs. Heavy alcohol use can also impair the liver’s ability to clear lactate, tipping the balance toward acidosis.
Symptoms to Recognize
The symptoms of lactic acidosis often overlap with whatever underlying condition is causing it, which can make it tricky to identify on its own. Common signs include nausea, vomiting, muscle cramping, weakness, and shortness of breath. As blood pH drops further, breathing often becomes rapid and deep as the body tries to blow off carbon dioxide to compensate for the acidity. Mental status changes, including confusion, drowsiness, or agitation, can develop as the condition worsens.
These symptoms tend to come on quickly and escalate. Someone who was alert and talking may become confused within hours. The rapid breathing pattern is particularly telling: it reflects the body’s attempt to self-correct by removing acid through the lungs, and it signals that the acidosis is significant.
How It’s Diagnosed
Diagnosis relies on blood tests. A blood gas measurement shows the pH of arterial blood, and a separate lactate level confirms how much lactic acid is circulating. Lactic acidosis produces what’s called a “high anion gap” pattern, meaning there are unmeasured negatively charged particles (in this case, lactate) throwing off the normal balance of electrolytes in the blood.
Lactate levels between 1.0 and 2.0 mmol/L are considered borderline. Above 2.0 mmol/L is classified as hyperlactatemia and warrants close attention. Once lactate reaches 4.0 mmol/L, the risk of serious complications and death rises sharply, and in most hospital systems this triggers an immediate urgent response. The formal threshold for lactic acidosis, combining elevated lactate with a low blood pH, sits at roughly 5 to 6 mmol/L.
Doctors also track how quickly lactate levels fall during treatment. The speed at which the body clears lactate from the bloodstream serves as a real-time indicator of whether treatment is working. Slow clearance suggests the mitochondria are still struggling, and may point to worsening organ function.
Why It’s So Dangerous
Lactic acidosis carries significant mortality. In a study of over 5,500 sepsis patients, those with both acidosis and elevated lactate had a 38% mortality rate, compared to 14% in patients with neither. Even moderate elevations in lactate with mild acidosis pushed mortality to 20%. The combination of falling pH and rising lactate is one of the strongest predictors of death in critically ill patients.
Acid itself is harmful to the body. As blood pH drops, the heart becomes less responsive to the hormones that maintain blood pressure. Blood vessels dilate inappropriately. Enzymes that depend on a narrow pH range start to malfunction. Below a pH of about 7.1, the heart’s ability to pump effectively deteriorates markedly, creating a vicious cycle: poor cardiac output reduces oxygen delivery, which generates more lactate, which drives pH even lower.
How It’s Treated
The single most important intervention is treating whatever caused the acidosis in the first place. If sepsis is the trigger, that means antibiotics and fluids to restore blood pressure. If shock from blood loss is the cause, transfusions and surgery to stop the bleeding take priority. For metformin-related cases, stopping the drug and supporting kidney function (sometimes with dialysis to remove the metformin from the blood) are central to recovery.
Optimizing oxygen delivery is a core part of the strategy. This can involve supplemental oxygen, mechanical ventilation to reduce how hard the body has to work to breathe, and medications to support blood pressure and cardiac output. Sedation and ventilator support also reduce tissue oxygen demand, giving the body breathing room to recover.
One treatment you might expect to help, giving sodium bicarbonate to directly neutralize the acid, is actually controversial and generally not recommended. Major critical care guidelines advise against using bicarbonate in lactic acidosis when pH remains above 7.15. The reasoning: bicarbonate raises pH temporarily but doesn’t improve blood pressure, heart function, or survival. It also generates carbon dioxide as a byproduct, which can worsen acidosis inside cells even as blood pH looks better on paper. Bicarbonate has been removed from advanced cardiac life support protocols for this reason. In extreme cases where pH drops below 7.15, some clinicians will consider it, but even then it’s given slowly and with careful monitoring for side effects like drops in calcium levels that can further weaken the heart.
Recovery depends heavily on how quickly the underlying cause is reversed. When the trigger is identified and treated early, lactate levels can normalize within hours and organs recover fully. When acidosis persists or worsens despite treatment, it reflects ongoing tissue injury, and outcomes are significantly worse.