What this test measures
Bicarbonate (HCO3-) is the principal buffer that keeps blood pH in the narrow 7.35–7.45 range. Most labs measure "total CO2" on a venous sample, which is essentially serum bicarbonate plus a small amount of dissolved CO2 and carbonic acid. It is part of every standard electrolyte panel.
The kidneys reabsorb bicarbonate in the proximal tubule and generate new bicarbonate to replace what is consumed buffering acid. The lungs regulate CO2 by changing breathing rate. Together they keep acid-base homeostasis tight.
Why it matters
In India, low bicarbonate is most commonly seen in chronic kidney disease (failure to excrete acid), diabetic ketoacidosis (overwhelming acid load), severe diarrhoea (bicarbonate loss), lactic acidosis (sepsis, shock), and renal tubular acidosis. KDIGO recommends maintaining serum bicarbonate above 22 mEq/L in CKD — chronic acidosis accelerates bone loss, muscle wasting and CKD progression. Sodium bicarbonate supplementation is recommended when values drop below 22.
High bicarbonate (metabolic alkalosis) is less common but is seen with vomiting, diuretic use, primary hyperaldosteronism, and severe potassium depletion. Bicarbonate is also affected by chronic lung disease — chronic CO2 retention in COPD raises bicarbonate as a kidney compensation.
How to prepare
No fasting required. Routine venipuncture. The sample should be processed reasonably quickly — bicarbonate can fall slightly with prolonged storage as CO2 escapes from open tubes.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Bicarbonate (mEq/L)[1][2] | 22 – 29 | Low bicarbonate (metabolic acidosis). Causes — diabetic ketoacidosis, lactic acidosis (sepsis, shock, metformin overdose), severe diarrhoea (loss of bicarbonate), chronic kidney disease, renal tubular acidosis, salicylate (aspirin) overdose, methanol / ethylene glycol poisoning. The anion gap helps separate the cause (high gap = acid added; normal gap = bicarbonate lost). | High bicarbonate (metabolic alkalosis). Causes — vomiting (loss of stomach acid), nasogastric suction, diuretic use (loop and thiazide), primary hyperaldosteronism, severe hypokalemia, antacid overuse, chronic respiratory acidosis (lung disease — compensatory rise). Values >35 mEq/L need urgent evaluation. |
Interpreting a low bicarbonate — anion gap matters
| Pattern | Anion gap | Common causes |
|---|---|---|
| Low HCO3 + high anion gap | > 12 | Diabetic ketoacidosis, lactic acidosis (sepsis, shock), renal failure, salicylate / methanol / ethylene glycol poisoning |
| Low HCO3 + normal anion gap | ≤ 12 | Diarrhoea, renal tubular acidosis (type 1, 2, 4), ureteric diversion, acetazolamide |
| Low HCO3 in CKD | Often raised | Failure to excrete acid; supplement if HCO3 < 22 |
| Low HCO3 + low pCO2 (compensatory) | Variable | Body trying to correct acidosis by breathing more |
Frequently asked questions
What does the "CO2" on my electrolyte panel mean?
"Total CO2" on a venous panel is almost entirely serum bicarbonate — the body's main buffer. It is not the same as arterial blood gas pCO2 (which measures dissolved gas).
Do I need to fast?
No fasting required for a bicarbonate test.
My bicarbonate is 19. Is that serious?
Mild metabolic acidosis. Causes range from chronic kidney disease (commonest) to early diabetic ketoacidosis or diarrhoea. Your doctor will look at the anion gap and clinical picture and may add an arterial blood gas if severe.
Why does CKD lower bicarbonate?
The failing kidney cannot excrete the daily acid load (from protein metabolism) or generate enough new bicarbonate. The result is chronic metabolic acidosis, which accelerates bone loss, muscle wasting and CKD progression. Guidelines (KDIGO) recommend bicarbonate supplementation when serum bicarbonate falls below 22.
My bicarbonate is 32 — what causes that?
Metabolic alkalosis. Common causes are vomiting (loss of stomach acid), diuretic use, primary hyperaldosteronism, severe potassium depletion, or chronic respiratory acidosis (lung disease where the kidney has compensated). Your doctor will look at potassium, chloride and urine chloride to localise the cause.
Is sodium bicarbonate safe to take?
For documented CKD-related acidosis, sodium bicarbonate (or sodium citrate) tablets are routinely prescribed. Self-medicating with baking soda for "gas" is not advised — it can cause sodium overload, alkalosis and serious electrolyte problems.
What is the difference between bicarbonate and an arterial blood gas?
Bicarbonate is calculated on a venous electrolyte panel. An arterial blood gas (ABG) measures actual blood pH, pCO2 and bicarbonate together — needed to fully characterise acid-base disorders, especially in critically ill patients.
Related Kidney / Electrolytes tests
Tests commonly ordered alongside BICARBONATE, or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — CO2 Blood Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Bicarbonate · accessed 2026-05-30T00:00:00.000Z
- KDIGO 2024 CKD Guideline · accessed 2026-05-30T00:00:00.000Z
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