A Raised Lactate
Lactate rises when tissues are producing energy without enough oxygen, which is why it is used to gauge the severity of an acute illness. But plenty of things raise it without any shortfall in oxygen delivery, and the commonest of those is how the blood was taken. In someone who looks well, a tourniquet held too long or a delay before processing explains most raised values; in someone who looks unwell, the number should be believed and acted on.
The pattern on your report
- Lactate High Key
- Bicarbonate Normal Key
- CRP Normal Key
- eGFR Normal Key
Printed as: Bicarbonate in mmol/Lor mEq/L— Falls as lactate accumulates and consumes it, so a normal value alongside argues for a sample effect.CRP in mg/Lor mg/dLeGFR in mL/min/1.73m2— Determines the risk from metformin, which accumulates as filtration falls.Lactate in mmol/Lor mg/dL— Cells keep producing it after the blood is drawn, so a sample that sat is not comparable to one run immediately.
Why the numbers look like this
Cells generate energy most efficiently using oxygen. When oxygen delivery fails to meet demand, they switch to a less efficient pathway that produces lactate as a by-product, and the level in the blood rises. That is the mechanism behind its use in shock and severe infection, where it reports tissue that is not getting what it needs.
Several things produce lactate without any failure of oxygen delivery. Vigorous muscle activity does, which includes a seizure, and it includes clenching the fist during a blood draw. Albuterol and adrenaline drive it up directly. Metformin reduces the liver's ability to clear it, which matters when kidney function falls and the drug accumulates. Liver disease reduces clearance for a different reason. Thiamine deficiency blocks the pathway that would otherwise consume it.
The sample itself is the most common confounder in a well person. Red and white cells keep producing lactate after the blood is drawn, so a delay before analysis raises the measured level. A tourniquet left on produces local oxygen debt in the arm. Both are avoidable, and both are frequent.
The distinction that matters is not which mechanism but who the person is. In someone acutely unwell, a raised lactate predicts a poor outcome and prompts urgent treatment whatever the explanation turns out to be.
Not being flagged is not the same as normal
Lactate has to be handled carefully to mean anything: taken without prolonged tourniquet, ideally without fist clenching, and processed quickly or collected into a tube that stops cells metabolizing. A value from a sample that sat is not comparable to one run immediately. Venous and arterial samples also differ slightly, with venous running a little higher, and the trend over hours carries more information in an unwell person than any single figure.
What else on the report can hide this
How the person looks decides how much weight to give this. Observations, conscious level and how they feel outrank the number in both directions.
The sample technique is the first question in someone well: how long the tourniquet was on, whether the fist was clenched, and how long before the sample was processed.
A repeat taken properly resolves most raised values in a well person, and a lactate that has fallen on a proper repeat needs nothing further.
Where it is genuinely raised, the panel around it locates the cause. Kidney function matters for anyone on metformin, the liver panel matters for clearance, and a full blood count with CRP addresses infection.
In an unwell person the trend over hours is what matters most. A lactate that falls with treatment is reassuring in a way a single value never is, and one that keeps rising is a reason to escalate regardless of what else looks acceptable.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
How the sample was taken
A tourniquet held too long, a clenched fist, or a delay before processing. The commonest explanation by far in someone who looks well.
- Very common
Recent vigorous exercise
Muscle produces it during intense activity and clears it over the following hour or two. From the history.
- Common
A seizure
Sustained muscle activity produces a marked rise that falls quickly afterwards. The timing is characteristic.
- Common
Sepsis or shock
Tissue not receiving enough oxygen. The person looks unwell, the observations are abnormal, and the number should be believed and acted on.
- Common
Albuterol or adrenaline
Both drive lactate production directly. Common after nebulized treatment for asthma, and expected rather than alarming in that setting.
- Uncommon
Metformin with reduced kidney function
The drug accumulates and reduces clearance. Kidney function is what determines the risk, and it needs checking whenever this is considered.
- Uncommon
Liver disease
The liver clears most lactate, so impaired function raises the baseline. Usually already known.
- Rare
Thiamine deficiency
Blocks the pathway that consumes lactate. Considered with alcohol use, poor nutrition, or after bariatric surgery, and it is easily treated.
- Rare
Bowel ischemia
Severe abdominal pain out of proportion to the examination, with a rising lactate. Time-critical, and the pain rather than the number is the alarm.
What is usually checked next
- The observations and how the person looks Decides how much weight the number carries, and it outranks the number in both directions.
- How the sample was taken and how long it waited Tourniquet time, fist clenching and processing delay explain most raised values in someone well.
- A properly taken repeat A value that has fallen on a clean repeat in a well person needs nothing further.
- Kidney function, particularly on metformin The drug accumulates as filtration falls, and that is what determines the risk.
- The trend over hours in an unwell person A falling lactate is reassuring in a way a single value never is, and a rising one is a reason to escalate.
When to seek care sooner
- Emergency Fever with a fast heart rate, low blood pressure, or drowsiness
- Emergency Severe abdominal pain out of proportion to the examination
- Emergency Breathing faster than usual, or breathlessness at rest
- Emergency Confusion or a marked change in behavior
- Emergency Vomiting and abdominal pain while taking metformin
- Emergency A lactate that is rising on repeat
Questions worth bringing to your appointment
- How long was the tourniquet on, and did I clench my fist?
- How quickly was the sample processed?
- Was my bicarbonate normal?
- Am I on metformin, and is my kidney function checked?
- Should this be repeated with a careful draw?