Bicarbonate and total CO2 (TCO2) in cats and dogs

What it shows

Total CO2 is bicarbonate plus the carbon dioxide dissolved in the sample. According to eClinPath, most dissolved CO2 escapes into the air after collection, so on a chemistry sample TCO2 is usually equal to bicarbonate or 1–2 mmol/L above it. That is why labs use one in place of the other, and why the Merck Veterinary Manual writes “total CO2 (bicarbonate)” as a single test.

What these values do not show is the breathing side: eClinPath stresses that chemistry bicarbonate does not replace a blood gas test, which also gives pH and the CO2 pressure. Merck adds that total CO2 is less useful for respiratory disturbances.

Higher than reference — what it may be a sign of

• Loss of stomach acid: vomiting of stomach contents, for example with a pyloric obstruction or gastric dilatation-volvulus — chloride falls and bicarbonate rises.
• Diuretics: loop and thiazide types.
• Bicarbonate-containing fluids.
• Compensation for a long-standing respiratory acidosis.
• Rarely, excess aldosterone; false increases are rare, though collection into an EDTA tube can raise the value.

Lower than reference

• Acids building up (high anion gap acidosis): lactic acid from poor circulation or lack of oxygen; kidney failure, mainly in later stages of chronic disease; diabetic ketoacidosis; ethylene glycol (antifreeze) and aspirin.
• Bicarbonate lost (normal anion gap acidosis): diarrhea and some kinds of vomiting, kidney tubule defects, Addison’s disease.
• Compensation for fast breathing (respiratory alkalosis).
• Artefact: an aged sample — cells keep producing lactate and CO2 escapes into the air; too much heparin in the tube; prolonged venous stasis during collection.

Look at together with

• Chloride and sodium — chloride falling more than sodium goes with alkalosis; chloride rising more than sodium with one kind of acidosis.
• Anion gap — (sodium + potassium) − (chloride + bicarbonate); high with low bicarbonate means extra acids such as lactate or ketones.
• Creatinine and urea — kidney failure is one cause of acidosis.
• Glucose and urine ketones — to recognise diabetic ketoacidosis.
• Potassium — moves between cells and blood as acid-base balance changes.

Units and conversion

Bicarbonate is reported in mmol/L or mEq/L — the same number. TCO2 is given in mmol/L. TCO2 and bicarbonate are not converted into each other: they are different measurements. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form. TCO2 and bicarbonate each have their own range, so read each against its own interval. Because storage and contact with air can falsely lower these values, a sample analysed fresh is more reliable than one that has travelled for a long time; a surprisingly low value in a well animal is worth rechecking. A low value is a reason to look for the cause, not a diagnosis by itself.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Bicarbonate
2. eClinPath (Cornell University) — Acid-base: laboratory detection (TCO2, anion gap)
3. Whitbread TJ. Clinical Biochemistry. Merck Veterinary Manual
4. Van Vertloo L. Renal Dysfunction in Dogs and Cats. Merck Veterinary Manual
5. International Renal Interest Society — IRIS recommendations for CKD in cats (PDF, 2023)
6. VCA Animal Hospitals — Serum Electrolytes