Blood gas lactate: why it is measured and what a high level means

Lactate is the charged form of lactic acid. Lab Tests Online UK explains that it is produced by muscle cells, red blood cells, the brain and other tissues during anaerobic, oxygen-deficient energy production, and is normally present at low concentrations. When cells lack oxygen or mitochondria are not working, the body switches to less efficient energy production whose main by-product is lactic acid, which can build up faster than the liver can break it down.

A marked rise is called hyperlactataemia and, when severe enough to disturb pH, lactic acidosis. The body can often compensate for hyperlactataemia, but lactic acidosis can cause muscular weakness, rapid breathing, nausea, vomiting, sweating and even coma.

The test is mainly requested to detect and measure the severity of oxygen shortage and lactic acidosis. It may be used with blood gases to look at acid-base balance and oxygenation, or with a metabolic screen or full blood count in a patient with acidosis. In rapidly developing conditions such as shock or heart attack, or slowly developing ones such as severe heart failure, lactate may be repeated to monitor the response to treatment.

It is used in the first assessment of suspected sepsis. If the level is above normal limits, treatment is generally started without delay, and prompt diagnosis and treatment of sepsis significantly improve the chances of recovery, according to Lab Tests Online UK.

Lactate is mainly measured on venous blood, but arterial lactate may be requested when it accompanies arterial gases. Arterial lactate is thought to be more accurate and, because no tourniquet is used, is less affected by the sampling process. Fasting may or may not be needed, and exercise before the test can raise lactate, so ask your doctor. Cerebrospinal fluid lactate is occasionally used to help separate bacterial from viral meningitis.

Type A lactic acidosis is more common and results from inadequate oxygen uptake in the lungs or decreased blood flow; shock from trauma and blood loss is the most common reason, along with heart attack, congestive heart failure and pulmonary oedema. Type B arises from conditions that increase lactate without reduced oxygen availability, such as liver and kidney disease, diabetes, leukaemia, AIDS, glycogen storage diseases, drugs and toxins, severe infections including sepsis and meningitis, and inherited metabolic or mitochondrial diseases. Strenuous exercise also raises lactate, but it returns to normal on resting.

Metformin and isoniazid are named among medicines that may cause excess lactate, and thiamine (vitamin B1) deficiency can also increase it. The presence of excess lactate is not diagnostic: it does not pinpoint the cause but helps doctors confirm or exclude possibilities.

Lowering lactate means treating the cause: restoring circulation and oxygen, giving fluids and antibiotics in sepsis, controlling diabetes, or stopping a drug that contributes. Falling concentrations over time reflect a response to treatment. Usually you cannot lower lactate yourself, but avoiding alcohol and managing diabetes helps where those are the reason. Call 999 for severe breathlessness, collapse, confusion or signs of sepsis such as mottled skin and a very high or very low temperature.