A Low Bicarbonate with a Raised Anion Gap
Bicarbonate is the buffer your blood uses to soak up acid, so a low value means acid is winning. The anion gap says which kind. A raised gap means an acid is present that the panel does not measure by name, and the short list of candidates includes several situations that need treating within hours rather than days.
The pattern on your report
- Bicarbonate Low · moderate Key
- Chloride Normal Key
- Sodium Normal Supporting
- Glucose High-normal Supporting
Printed as: Bicarbonate in mmol/Lor mEq/L— The same figure. Labeled bicarbonate, HCO3 or total CO2 depending on the laboratory.Chloride in mmol/Lor mEq/LGlucose in mmol/Lor mg/dL— An eighteenfold difference, so 10 mmol/L is 180 mg/dL.Sodium in mmol/Lor mEq/L
Why the numbers look like this
Blood is electrically neutral, so the positive and negative charges must balance. The panel measures the main positive ion, sodium, and the two main negative ones, chloride and bicarbonate. Sodium minus the other two leaves a gap, filled in health by proteins and other unmeasured negative charges.
Add an acid to the blood and bicarbonate is consumed neutralizing it, while the acid's own negative partner takes bicarbonate's place in the balance. Chloride does not change, so the gap widens. That is the calculation, and it identifies the presence of an unmeasured acid without identifying which one.
The contrast makes it useful. When bicarbonate falls because it has been lost directly, through the gut or the kidney, chloride rises to fill the space and the gap stays normal. Same low bicarbonate, entirely different causes, and the gap is what tells them apart.
Not being flagged is not the same as normal
The gap is calculated rather than printed, so most reports do not show it at all and it has to be worked out from numbers that are on the page. Its normal range also depends on the laboratory's methods and on your albumin: a low albumin lowers the expected gap, so a gap that looks unremarkable can be genuinely raised in someone with a low albumin.
What else on the report can hide this
Glucose and ketones come first, because diabetic ketoacidosis is the commonest of the urgent causes and both tests are quick. One version matters in particular: on SGLT2 inhibitors, ketoacidosis can occur with a glucose that is barely raised, so a normal glucose does not exclude it in anyone taking one of those drugs.
Lactate is the next question, and it separates the situations where tissue is not getting enough oxygen from everything else. Kidney function accounts for another group, since a failing kidney retains acid directly. Ask what has been taken, too, because aspirin in overdose, methanol and ethylene glycol all present this way and all have specific treatments that work only if started early.
What usually causes it
Listed from most to least common — not from most to least serious.
- Very common
Diabetic ketoacidosis
Ketones in blood or urine with a raised glucose. On SGLT2 inhibitors the glucose can be near normal, which is the presentation that gets missed.
- Very common
Lactic acidosis
Sepsis, shock, or any state where tissue is short of oxygen. Also metformin in someone whose kidney function has dropped. A lactate is the test.
- Common
Kidney failure
The kidney normally excretes acid, so a failing one retains it. Creatinine and urea are raised alongside, and the picture builds over weeks.
- Common
A sample artifact
Bicarbonate falls in a tube that sat for hours before analysis or was underfilled, and the calculated gap rises with it. A mildly low value in someone who feels well is repeated before it is investigated.
- Common
Starvation or alcoholic ketoacidosis
Ketones with a normal or low glucose, after a period of not eating or heavy drinking. Frequently mistaken for the diabetic form and treated differently.
- Uncommon
Aspirin overdose, or pyroglutamic acidosis
Aspirin overdose: ringing in the ears, fast breathing, and a mixed picture where the blood is both too acidic and being blown off too fast; a salicylate level is the test. Pyroglutamic acidosis is a rare cousin seen with regular paracetamol use in someone malnourished or unwell.
- Rare
Toxic alcohols
Methanol or ethylene glycol. A widened osmolal gap alongside the anion gap. Rare, and there are antidotes that only work early.
- Rare
Inborn errors of metabolism
Usually already known, presenting in childhood or during severe illness in an adult.
What is usually checked next
- Glucose and ketones Covers the commonest urgent cause in one step, and is done at the bedside.
- Lactate Identifies the group where tissue oxygen delivery has failed, which changes what happens immediately.
- Kidney function and albumin Kidney failure explains one group, and the albumin corrects the gap so a raised one is not missed.
- Blood gas Confirms the acidosis, measures how well breathing is compensating, and shows a mixed disturbance the panel alone cannot.
- Salicylate level and an osmolal gap Tests for the poisoning causes, both of which have antidotes that depend on being given early.
When to seek care sooner
- Emergency Deep rapid breathing, or breathlessness at rest
- Emergency Vomiting with abdominal pain and a sweet smell on the breath
- Emergency Confusion, drowsiness, or difficulty waking
- Emergency Ringing in the ears with fast breathing
- Emergency Any suspicion of having swallowed antifreeze, screen wash or industrial alcohol
- Emergency Vomiting and abdominal pain on an SGLT2 inhibitor, even with a normal glucose
Questions worth bringing to your appointment
- Has the anion gap been calculated, and was it corrected for my albumin?
- Have ketones and a lactate been checked?
- I take an SGLT2 inhibitor. Could this be ketoacidosis even with a normal glucose?
- Could any medicine or substance I have taken explain this?
- Does this need a blood gas rather than just the panel?