HCO3 (Bicarbonate) Blood Test: What High and Low Results Mean
Other names: HCO3, HCO3-, HC03, HC03-, Bicarbonate, Bicarbonate Serum, Serum Bicarbonate, Bicarb, HCO3 Blood Test, HCO3 Meaning, HCO3 High Meaning, HCO3 Low Meaning, HC03 Meaning, What is HCO3, HCO3 Venous, HCO3 Venous High, HCO3 Venous Low, HC03 Venous High, HCO3 Venous High Meaning, Bicarbonate Blood Test, Bicarbonate High, Bicarbonate Low, High HCO3, Low HCO3, Serum HCO3, HCO3 Full Form, HCO3 Medical Abbreviation, Bicarbonate Abbreviation, HCO3 Serum, HCO3 Lab, HCO3 Lab Test, Bicarb Blood Test, Bicarb Lab, Bicarb in Blood Test, POC HCO3, POCT HCO3, HCO3 VEN, HCO3 Ven High, Venous HCO3, Venous HCO3 High, HCO3 ACT, HCO3 (TCO2), Carbon Dioxide Blood Test, CO2 Blood Test, Total CO2, CO2 CMP, Is HCO3 Bicarbonate, Is HCO3 Bicarb, Is HCO3 the Same as CO2, Bicarbonate Normal Range, Serum Bicarbonate Normal Range, HCO3 Normal Range, Que es HCO3 (Spanish), Que Significa HCO3 (Spanish), HCO3 ماهو (Arabic), HCO3 Meaning in Hindi, HCO3 Adalah (Indonesian), HCO3 คือ (Thai), HCO3 Что Это (Russian), HCO3 O Que É (Portuguese), Wat is HCO3 (Dutch), HCO3 Significado (Spanish), Bicarbonato HCO3 (Spanish/Portuguese), HCO3 Alto (Spanish)
Optimal Result:
22 - 29 mmol/L.
QUICK ANSWER
HCO3 is bicarbonate — the blood's main acid-base buffer. It keeps your blood pH in the normal range (7.35–7.45). On your lab report it may appear as HCO3, bicarb, or CO2 depending on the panel.
| Result | What it means |
|---|---|
| Low HCO3 (below ~22 mmol/L) | Blood is more acidic than normal — most commonly metabolic acidosis, diabetic ketoacidosis, kidney disease, or diarrhea |
| Normal HCO3 (~22–29 mmol/L) | Acid-base balance maintained |
| High HCO3 (above ~29 mmol/L) | Blood is more alkaline than normal — most commonly metabolic alkalosis from vomiting, diuretics, or lung disease compensation |
Common questions at a glance:
| Question | Short answer |
|---|---|
| What does HCO3 stand for? | Hydrogen carbonate — the chemical formula for bicarbonate ion |
| Is HCO3 the same as bicarbonate? | Yes — HCO3, HCO3-, bicarb, and bicarbonate all refer to the same ion |
| Is the CO2 on my CMP the same as HCO3? | Essentially yes — the CO2 value on a CMP/BMP measures total bicarbonate in the serum, not respiratory CO2 |
| What is a normal HCO3 level? | Approximately 22–29 mmol/L for serum/venous; 22–26 mmol/L for arterial blood gas |
| What does high HCO3 mean? | Metabolic alkalosis — excess base in the blood |
| What does low HCO3 mean? | Metabolic acidosis — excess acid in the blood |
WHAT IS HCO3? — THE COMPLETE TERMINOLOGY DECODER
HCO3 (also written HCO3−, HCO3-, HC03, bicarb, or bicarbonate) is the bicarbonate ion — the body's most important buffer for maintaining blood pH. It is a negatively charged ion (anion) produced when carbon dioxide from cellular metabolism dissolves in the blood.
All of these terms mean the same thing on a blood test report:
| Term on your report | Same as HCO3? |
|---|---|
| HCO3 | Yes — standard chemical abbreviation |
| HCO3- | Yes — the minus sign indicates its negative charge |
| HC03 | Yes — common typo/variant (zero instead of letter O); the same test |
| Bicarbonate | Yes — the full name |
| Bicarb | Yes — common clinical abbreviation |
| Serum Bicarbonate | Yes — indicates blood serum was the specimen |
| Serum HCO3 | Yes |
| HCO3 VEN / HCO3 Venous | Yes — indicates a venous blood sample |
| HCO3 ACT | Yes — "actual" bicarbonate, measured directly |
| HCO3 (TCO2) | Yes — total CO2, calculated from blood gas values |
| POC HCO3 / POCT HCO3 | Yes — Point-of-Care Testing; measured at bedside |
| CO2 (on CMP/BMP) | Yes — the CO2 on a metabolic panel measures serum bicarbonate |
| Carbon Dioxide (on CMP/BMP) | Yes — same as above |
| Total CO2 | Yes — another way of expressing serum bicarbonate |
IS HCO3 THE SAME AS CO2 ON MY BLOOD TEST?
This is one of the most common sources of confusion on lab reports. The short answer: yes, on a metabolic panel they are measuring the same thing.
When your CMP or BMP reports "CO2" or "Carbon Dioxide," it is not measuring the gas you breathe out. It is measuring the total bicarbonate content of your blood serum — essentially the same thing as HCO3.
| Test/Panel | What "CO2" or "HCO3" measures | Sample type |
|---|---|---|
| CMP / BMP — CO2 value | Serum bicarbonate (total CO2 in serum) | Venous blood |
| Blood gas — HCO3 | Bicarbonate calculated from pH and PCO2 | Arterial or venous blood |
| Blood gas — PCO2 | Carbon dioxide gas dissolved in blood — the respiratory CO2 | Arterial or venous blood |
Why the confusion? CO2 from cellular metabolism dissolves in blood and combines with water to form carbonic acid (H2CO3), which rapidly dissociates into bicarbonate (HCO3-) and a hydrogen ion. The serum CO2 test measures this total bicarbonate pool. The PCO2 on a blood gas measures the dissolved gas itself. They are related but different — bicarbonate is the stored form, PCO2 is the respiratory gas form.
If your CMP shows "CO2: 26 mmol/L" — that means the same as "HCO3: 26 mmol/L." If your blood gas shows "PCO2: 40 mmHg" — that is the respiratory CO2, a different value.
ARTERIAL VS VENOUS HCO3 — WHY THE REFERENCE RANGE DIFFERS
Many patients receive a "venous HCO3" result from a routine blood draw or a "HCO3 venous high" flag and are confused about how this differs from bicarbonate on a blood gas.
| Specimen type | How it's collected | Normal HCO3 range | Clinical use |
|---|---|---|---|
| Serum / Venous (CMP, BMP) | Standard blood draw from a vein | ~22–29 mmol/L | Routine metabolic monitoring; most common |
| Venous blood gas (VBG) | Venous blood sent for blood gas analysis | ~24–28 mmol/L | More detailed acid-base analysis without arterial puncture |
| Arterial blood gas (ABG) | Arterial blood (wrist or groin) | ~22–26 mmol/L | Gold standard for acid-base and respiratory assessment |
| POC / POCT (bedside analyzer) | Fingerstick, venous, or arterial via point-of-care analyzer | ~22–29 mmol/L (varies by device) | Rapid bedside assessment in hospitals/clinics |
Why is venous HCO3 slightly higher than arterial? Venous blood is returning from tissues toward the lungs, carrying more CO2 than arterial blood. This extra CO2 converts to bicarbonate in the blood, making venous HCO3 slightly higher (typically 2–4 mmol/L higher than arterial). A venous HCO3 of 28–30 mmol/L may be near the upper end of the expected venous range and should be interpreted using the laboratory's stated reference interval and clinical context.
YOUR HCO3 VALUE — WHAT DOES YOUR NUMBER MEAN?
| HCO3 value (mmol/L) | Interpretation |
|---|---|
| Below 15 | Severely low — significant metabolic acidosis; urgent clinical evaluation |
| 15–17 | Markedly low — metabolic acidosis; clinical evaluation warranted |
| 18–21 | Low — below normal range; evaluate for acidosis cause |
| 22 | Lower end of normal |
| 22–29 | Normal range (serum/venous adults) |
| 29 | Upper limit of normal for most labs |
| 30–32 | Mildly elevated — borderline; may reflect compensation or mild metabolic alkalosis |
| 33–36 | Moderately elevated — evaluate for metabolic alkalosis cause |
| Above 36 | Markedly elevated — significant metabolic alkalosis; clinical evaluation needed |
What do specific values mean?
- HCO3 18–21 — below normal; may indicate mild metabolic acidosis, chronic kidney disease, or compensation for respiratory alkalosis; evaluate in context of symptoms and other lab values
- HCO3 22–29 — normal range for most adults; no action needed in isolation
- HCO3 29–32 — borderline to mildly elevated; common with mild vomiting, diuretic use, or as compensation for chronic lung disease; check in context
- HCO3 31–35 — elevated; evaluate for metabolic alkalosis causes including prolonged vomiting, diuretics, Cushing syndrome, or COPD compensation
- HCO3 above 35 — significantly elevated; warrants clinical evaluation
MOST COMMON HCO3 RESULTS
| HCO3 result (mmol/L) | Typical interpretation |
|---|---|
| 18 | Mildly low — metabolic acidosis; evaluate kidney function, diarrhea history, or diabetes |
| 20 | Below normal — mild acidosis or compensation for respiratory alkalosis; check context |
| 22 | Lower end of normal range |
| 23–26 | Typical healthy mid-normal range |
| 27–29 | Upper-normal range |
| 29 | At or near upper limit of normal for most serum/venous labs |
| 30–32 | Mildly elevated — borderline; common in COPD, mild vomiting, or diuretic use |
| 33–35 | Elevated — evaluate for metabolic alkalosis |
| Above 36 | Significantly elevated — warrants clinical evaluation |
Always check your lab's reference range. Some labs report normal serum bicarbonate as 22–28, others as 23–29 or 22–31 for older adults. The flag on your report is the most reliable guide for your specific lab.
CAN HCO3 BE HIGH TEMPORARILY?
Yes — a single mildly elevated HCO3 reading does not always indicate an ongoing metabolic alkalosis. Several common situations can transiently raise bicarbonate:
| Cause of temporary elevation | Notes |
|---|---|
| Vomiting before the blood draw | Even one episode of significant vomiting can lower blood acid and raise HCO3 transiently |
| Dehydration | When blood volume contracts, bicarbonate concentrates (contraction alkalosis) — common cause of mildly elevated HCO3 in dehydrated patients |
| Antacids or bicarbonate-containing medications | Large doses of calcium carbonate, sodium bicarbonate, or bicarbonate-based antacids can transiently raise serum HCO3 |
| Recent administration of IV bicarbonate | IV sodium bicarbonate (given for acidosis treatment or cardiac arrest) raises HCO3 directly |
| Post-hyperventilation | Breathing fast lowers CO2; kidneys compensate by temporarily raising bicarbonate; when breathing normalizes, HCO3 may be mildly elevated briefly |
| Lab variation | Serum bicarbonate has normal test-to-test variability of approximately ±2 mmol/L; a single reading of 29–30 in someone with a baseline of 26–27 may reflect normal variation |
The practical implication: A mildly elevated single HCO3 of 29–32 mmol/L in an otherwise well patient without lung disease, vomiting, or diuretic use is often worth repeating before acting on. Persistent elevation across two or more tests is more clinically significant than a single borderline reading.
WHAT DOES HIGH HCO3 MEAN?
High bicarbonate (above approximately 29 mmol/L on serum/venous) indicates metabolic alkalosis — a state where the blood has more base than normal, driving pH above 7.45. This can occur either as a primary condition (the kidneys are retaining too much bicarbonate) or as a compensatory response (the body raises bicarbonate to offset respiratory acidosis from lung disease).
Common causes of high HCO3:
| Cause | Mechanism | Notes |
|---|---|---|
| Prolonged vomiting | Loss of stomach acid (HCl) depletes acid, leaving relative bicarbonate excess | Most common cause of metabolic alkalosis |
| Diuretic use (thiazides, loop diuretics) | Diuretics cause potassium and hydrogen ion loss; kidneys retain bicarbonate to compensate | Very common — check if patient is on diuretics |
| COPD / Chronic lung disease | Lungs retain CO2 (respiratory acidosis); kidneys compensate by retaining bicarbonate (compensatory metabolic alkalosis) | HCO3 often persistently elevated 30–36 in severe COPD |
| Cushing syndrome / Corticosteroid excess | Excess cortisol promotes kidney acid excretion and bicarbonate retention | |
| Conn syndrome / Hyperaldosteronism | Excess aldosterone causes potassium loss and bicarbonate retention | |
| Severe dehydration | Contraction of blood volume concentrates bicarbonate (contraction alkalosis) | |
| Milk-alkali syndrome | Excess calcium and absorbable alkali consumption | |
| Nasogastric suction | Removal of stomach contents removes acid | Clinical/hospital setting |
The most common outpatient cause is prolonged vomiting or diuretic use — both deplete acid and cause a relative rise in bicarbonate. The most common chronic cause is COPD, where the lungs chronically retain CO2 and the kidneys adapt by raising bicarbonate.
WHAT DOES LOW HCO3 MEAN?
Low bicarbonate (below approximately 22 mmol/L) indicates metabolic acidosis — the blood has more acid than normal, driving pH below 7.35. The body is either producing too much acid, losing too much bicarbonate, or failing to excrete acid.
Common causes of low HCO3:
| Cause | Notes |
|---|---|
| Diabetic ketoacidosis (DKA) | Ketones are acids; bicarbonate is consumed buffering them; often HCO3 drops below 15 |
| Chronic kidney disease | Kidneys fail to excrete acid and regenerate bicarbonate; common cause of mild-moderate metabolic acidosis |
| Diarrhea | Loss of bicarbonate-rich intestinal fluid |
| Lactic acidosis | Excess lactic acid from shock, severe exercise, or liver failure consumes bicarbonate |
| Respiratory alkalosis (compensation) | Hyperventilation blows off CO2; kidneys lower bicarbonate to compensate |
| Addison disease | Adrenal insufficiency impairs kidney acid excretion |
| Aspirin/salicylate overdose | Salicylic acid directly consumes bicarbonate |
| Methanol or ethylene glycol poisoning | These produce organic acids that rapidly consume bicarbonate |
| Starvation ketosis | Milder than DKA but produces ketone acids |
| High-anion-gap metabolic acidosis | Acid accumulation (lactic acid, ketoacids, toxins) — check anion gap alongside HCO3 |
What is a dangerously low HCO3? A bicarbonate below 15 mmol/L indicates severe metabolic acidosis and is clinically urgent. A bicarbonate below 10 mmol/L is a medical emergency. At low bicarbonate levels, blood pH falls to dangerous levels — below pH 7.1, cardiac and neurological function are seriously compromised.
HCO3 AND CHLORIDE TOGETHER
Bicarbonate and chloride are the two main anions in the blood and have a reciprocal relationship — when one rises, the other often falls to maintain electrical neutrality. Interpreting them together adds significant clinical information.
| HCO3 | Chloride | Most likely interpretation |
|---|---|---|
| Low | High | Hyperchloremic metabolic acidosis — diarrhea, renal tubular acidosis, saline overload |
| Low | Normal | High anion gap metabolic acidosis — DKA, lactic acidosis, toxin (acid is displacing HCO3 without raising chloride) |
| Low | Low | Mixed acid-base disorder or significant fluid loss — evaluate carefully |
| High | Low | Classic metabolic alkalosis — vomiting, diuretics, nasogastric suction; chloride falls as bicarbonate rises |
| High | Normal | Mild metabolic alkalosis or compensatory response — less severe |
| Normal | High | Possible dehydration or excess saline administration without acid-base disorder |
This HCO3/chloride relationship is why chloride is always measured alongside bicarbonate on electrolyte panels.
HCO3 AND POTASSIUM TOGETHER
Bicarbonate and potassium are closely linked through the kidney's acid-base handling. Low potassium often accompanies high bicarbonate and vice versa.
| HCO3 | Potassium | Most likely interpretation |
|---|---|---|
| High | Low | Classic vomiting or diuretic-induced alkalosis — both hydrogen ions and potassium are lost; most common outpatient pattern |
| High | High | Unusual — consider hyperaldosteronism with potassium supplementation, or check for lab error |
| High | Normal | Mild metabolic alkalosis without significant potassium depletion |
| Low | High | Advanced chronic kidney disease — kidneys retain potassium while failing to excrete acid |
| Low | Low | GI losses (diarrhea, fistula), laxative abuse — both bicarbonate and potassium lost through GI tract |
| Low | Normal | Early metabolic acidosis or compensation for hyperventilation |
Why the link? When the body loses hydrogen ions (acid), the kidneys retain potassium instead; when acid accumulates, the kidneys excrete potassium to buffer. This makes potassium a useful companion marker whenever HCO3 is abnormal.
HCO3 AND THE ANION GAP — WHY THEY'RE INTERPRETED TOGETHER
When HCO3 is low, the anion gap helps determine whether the cause is acid accumulation or bicarbonate loss:
| Low HCO3 pattern | Anion gap | What it indicates |
|---|---|---|
| Low HCO3 + High anion gap | High | Acid accumulation — DKA, lactic acidosis, toxin ingestion, renal failure |
| Low HCO3 + Normal anion gap | Normal | Bicarbonate loss or failure to produce it — diarrhea, renal tubular acidosis, early kidney disease |
The anion gap = Sodium − (Chloride + HCO3). A normal anion gap is approximately 8–12 mEq/L. When HCO3 falls and the anion gap rises, it means an unmeasured acid is being buffered by — and consuming — bicarbonate.
HCO3 IN ACID-BASE BALANCE — THE FRAMEWORK
Understanding your bicarbonate value is easier with a simple acid-base framework:
| Blood pH | HCO3 | PCO2 (CO2 gas) | Condition | Primary problem |
|---|---|---|---|---|
| Low (<7.35) | Low | Low or normal | Metabolic acidosis | Too much acid or too little base |
| Low (<7.35) | High | High | Respiratory acidosis | Lungs retaining CO2 |
| High (>7.45) | High | High or normal | Metabolic alkalosis | Too little acid or too much base |
| High (>7.45) | Low | Low | Respiratory alkalosis | Lungs exhaling too much CO2 |
For most patients seeing HCO3 on a routine CMP: the most clinically relevant scenarios are metabolic acidosis (low HCO3, most often from kidney disease or diabetes) and metabolic alkalosis (high HCO3, most often from vomiting or diuretics).
HCO3 AND CHRONIC KIDNEY DISEASE
Chronic kidney disease (CKD) is one of the most common causes of persistently low bicarbonate and deserves dedicated attention because so many HealthMatters users have CKD or are being monitored for kidney function.
Why CKD causes low HCO3: Healthy kidneys excrete acid and regenerate bicarbonate continuously. As kidney function declines, this capacity is progressively lost — acid accumulates and bicarbonate falls. This is called renal metabolic acidosis or CKD-associated metabolic acidosis.
Typical HCO3 levels by CKD stage:
| CKD stage | Estimated GFR | Typical HCO3 pattern |
|---|---|---|
| Stage 1–2 (early) | >60 mL/min | Usually normal; HCO3 may be low-normal (22–24) |
| Stage 3 (moderate) | 30–59 mL/min | Mildly low HCO3 common (20–23); metabolic acidosis beginning |
| Stage 4 (severe) | 15–29 mL/min | Low HCO3 typical (18–22); acidosis increasingly significant |
| Stage 5 / ESRD | <15 mL/min | Often HCO3 below 18; significant acidosis; may require treatment |
Why treating low HCO3 in CKD matters: Chronic metabolic acidosis in CKD accelerates kidney function decline, increases muscle protein breakdown (causing muscle wasting), impairs bone mineral density, and worsens cardiovascular risk. KDIGO guidelines recommend maintaining serum bicarbonate above 22 mmol/L in CKD patients. Treatment with oral sodium bicarbonate supplements is commonly prescribed when HCO3 falls below 22 in CKD.
The target: Most nephrologists aim to keep serum bicarbonate at 22–26 mmol/L in CKD patients. A HCO3 of 18–21 in a CKD patient is an actionable finding, not one to wait on.
WHEN IS LOW HCO3 AN EMERGENCY?
Most abnormal bicarbonate results on a routine CMP do not require emergency action. But some patterns do — and recognizing them matters.
| Situation | Concern level | Action |
|---|---|---|
| HCO3 18–21 + mild symptoms (fatigue, nausea) | Low-Moderate | Same-day or next-day medical evaluation |
| HCO3 15–18 with symptoms | Moderate-High | Same-day medical evaluation; do not wait |
| HCO3 below 15 | High | Urgent medical evaluation — significant metabolic acidosis |
| HCO3 below 10 | Medical emergency | Go to emergency department; pH may be critically low |
| Low HCO3 + known diabetes + high glucose + vomiting | High | Possible diabetic ketoacidosis (DKA) — emergency evaluation |
| Low HCO3 + confusion, severe fatigue, or rapid breathing | High | Emergency evaluation — severe acidosis affecting neurological function |
| Low HCO3 + known toxin ingestion (methanol, ethylene glycol, salicylates) | Medical emergency | Emergency department immediately |
| HCO3 mildly low (20–22) + no symptoms + chronic kidney disease | Low | Routine follow-up with nephrologist; expected in CKD |
The most important pattern: a bicarbonate below 15 mmol/L in someone with diabetes, vomiting, and high blood glucose is diabetic ketoacidosis until proven otherwise. DKA is a medical emergency regardless of the bicarbonate value alone.
High HCO3 emergencies are rare in outpatient settings. A bicarbonate above 40 mmol/L may cause neuromuscular irritability and requires evaluation. In the outpatient context, a mildly elevated HCO3 of 29–34 mmol/L rarely requires emergency action.
MOST COMMON CLINICAL SCENARIOS
| Pattern | Most likely interpretation | Recommended next step |
|---|---|---|
| HCO3 18–21 + chronic kidney disease | Metabolic acidosis of CKD — very common finding | Monitor; nephrologist may recommend sodium bicarbonate supplementation |
| HCO3 below 15 + known diabetes + glucose very high | Diabetic ketoacidosis (DKA) | Emergency evaluation |
| HCO3 30–34 + COPD or emphysema | Compensatory metabolic alkalosis from chronic CO2 retention | Expected finding in severe COPD; monitor |
| HCO3 30–35 + prolonged vomiting or diarrhea | Metabolic alkalosis from acid loss | Clinical evaluation; rehydration; electrolyte replacement |
| HCO3 30–35 + diuretic medications | Diuretic-induced metabolic alkalosis | Review diuretic dose; check potassium |
| HCO3 mildly low (20–22) + no symptoms | Possibly low-normal; lab variation; or mild acidosis | Repeat with full electrolyte panel; check for kidney function |
| HCO3 high + low potassium | Hypokalemia-driven alkalosis | Address potassium deficiency |
| HCO3 low + high anion gap | Acid accumulation — DKA, lactic acidosis, or toxin | Full acid-base evaluation; emergency if severe |
| HCO3 low + normal anion gap + diarrhea | Bicarbonate loss through GI tract | Treat underlying cause; rehydrate |
| HCO3 venous flagged high (29–32) | May be within normal range for venous specimen | Confirm with serum reference range; consider venous vs arterial distinction |
TREND INTERPRETATION
For HealthMatters users tracking bicarbonate over time:
| Trend pattern | Clinical meaning |
|---|---|
| 18 → 21 → 24 across sequential tests | Improving metabolic acidosis — response to treatment or resolving underlying cause |
| 25 → 22 → 19 over months | Progressive acid accumulation — evaluate for worsening CKD, new metabolic problem, or medication effect |
| 31 → 28 → 25 after stopping diuretics | Resolution of diuretic-induced alkalosis — bicarbonate normalizing as cause is removed |
| Stable 31–33 over years with COPD | Expected compensatory metabolic alkalosis — kidneys maintaining compensation for chronic CO2 retention; stable is reassuring |
| 22 → 20 → 18 in CKD patient | Progressing renal metabolic acidosis — discuss sodium bicarbonate supplementation with nephrologist |
| 24 → 28 → 32 without obvious cause | Evaluate for new vomiting pattern, diuretic addition, or Cushing syndrome if no explanation |
| Fluctuating 20–25 across tests | May reflect variable hydration, diet, or lab variation; check other electrolytes for context |
FAQ about Bicarbonate (HCO3), Serum
-
What does HCO3 stand for?
HCO3 is the chemical formula for the bicarbonate ion — one hydrogen atom (H), one carbon atom (C), and three oxygen atoms (O3), with a negative charge (indicated by the minus sign in HCO3-). It is the body's most abundant blood buffer and is responsible for neutralizing excess acid. In clinical shorthand, HCO3 is pronounced "H-C-O-3" or simply called "bicarb." It is not a different measurement from bicarbonate — it is the same ion expressed in chemical notation. -
Is HCO3 the same as CO2 on my blood test?
On a metabolic panel (CMP or BMP), yes — the value labeled "CO2" or "Carbon Dioxide" is measuring your serum bicarbonate (HCO3), not the respiratory gas you exhale. When CO2 from cellular metabolism dissolves in blood, most of it converts to bicarbonate. The serum CO2 test captures this total bicarbonate. So if your CMP shows "CO2: 25 mmol/L," it means the same thing as "HCO3: 25 mmol/L." The value labeled "PCO2" on a blood gas is different — that measures dissolved CO2 gas and reflects your breathing, not your bicarbonate level. -
What does high HCO3 mean?
High HCO3 (above approximately 29 mmol/L on a serum or venous sample) means the blood has more base than normal — a condition called metabolic alkalosis. The most common causes in everyday clinical practice are prolonged vomiting (which removes stomach acid), diuretic medications (which cause potassium and hydrogen ion loss), and chronic lung disease such as COPD (where the lungs retain CO2 and the kidneys compensate by raising bicarbonate). High HCO3 in the context of known COPD is an expected and usually stable finding. High HCO3 from vomiting or diuretic overuse warrants evaluation of potassium and overall electrolyte balance. -
What does low HCO3 mean?
Low HCO3 (below approximately 22 mmol/L) means the blood has more acid than normal — metabolic acidosis. The bicarbonate has been consumed buffering excess acid or lost before it could be regenerated. Common causes include diabetic ketoacidosis (ketone acids consuming bicarbonate rapidly), chronic kidney disease (kidneys fail to excrete acid), chronic diarrhea (bicarbonate-rich intestinal fluid is lost), lactic acidosis (from shock or severe illness), and salicylate (aspirin) overdose. A mildly low HCO3 of 20–21 mmol/L in an otherwise well person may reflect lab variation or early kidney disease. A HCO3 below 15 mmol/L is clinically significant and requires evaluation. -
What does HCO3 venous high mean?
HCO3 venous (or "HCO3 VEN," "HCO3 ven") refers to bicarbonate measured from a venous blood sample — the standard method for a routine blood draw. Because venous blood is returning from tissues carrying more CO2 than arterial blood, venous bicarbonate is naturally 2–4 mmol/L higher than arterial bicarbonate. A venous HCO3 of 28–31 mmol/L may be flagged as high on some lab reports but could be within the expected venous range. If your report shows "HCO3 venous high" with a value of 29–32, this is often borderline and requires interpretation alongside your clinical context, symptoms, and whether you are on diuretics or have lung disease. -
What does HC03 mean on a blood test?
(with zero, not the letter O) "HC03" with a zero is a common typo for "HCO3" — both refer to the same thing: bicarbonate. On lab reports, lab systems, and patient portals, the letter O in HCO3 is sometimes rendered as or confused with the number 0, producing "HC03." The measurement and reference range are identical regardless of whether your report shows "HCO3" or "HC03." -
What is a normal HCO3 level?
The normal range for serum or venous bicarbonate in adults is approximately 22–29 mmol/L, though some labs use 22–28 or 23–29. For arterial blood gas (ABG), the normal range is slightly lower at approximately 22–26 mmol/L, because arterial blood has less CO2 to convert to bicarbonate than venous blood. For patients over 60, some labs extend the upper limit to 31 mmol/L. Always check your lab's stated reference range on your report, as these vary slightly between institutions. -
Is HCO3 an acid or a base?
HCO3 (bicarbonate) is a base — it accepts hydrogen ions (acid) rather than donating them. This is why bicarbonate acts as a buffer: when excess acid (H+) builds up in the blood, bicarbonate neutralizes it by accepting the hydrogen ion and forming carbonic acid (H2CO3), which then breaks down to CO2 and water that can be exhaled. However, bicarbonate can also act as a weak acid in certain chemical environments — in blood physiology, its primary role is as a base and buffer. -
¿Qué significa HCO3 en un análisis de sangre? (What does HCO3 mean in a blood test?)
HCO3 es el símbolo químico del bicarbonato — el principal amortiguador ácido-base de la sangre. Mantiene el pH de la sangre dentro del rango normal (7.35–7.45). En un análisis de sangre, aparece como "HCO3," "bicarbonato," o "CO2" dependiendo del tipo de prueba. El rango normal en adultos es aproximadamente 22–29 mmol/L. Un HCO3 bajo (por debajo de 22) indica acidosis metabólica — la sangre es más ácida de lo normal, lo que puede deberse a enfermedad renal, diarrea prolongada o diabetes. Un HCO3 alto (por encima de 29) indica alcalosis metabólica — la sangre es más alcalina de lo normal, frecuentemente causada por vómitos prolongados, diuréticos o enfermedad pulmonar crónica.
Lab Results Explained and Tracked
What does it mean if your Bicarbonate (HCO3), Serum result is too high?
Elevated bicarbonate (HCO3 above approximately 29 mmol/L on a serum or venous sample) indicates metabolic alkalosis — a state where the blood contains more base than normal and pH is driven above 7.45. The most common causes in routine clinical practice are prolonged vomiting, which removes stomach acid and leaves a relative bicarbonate excess; diuretic medications such as furosemide or hydrochlorothiazide, which cause loss of potassium and hydrogen ions with compensatory bicarbonate retention; and chronic obstructive pulmonary disease (COPD) or other chronic lung conditions, where the lungs chronically retain CO2 (respiratory acidosis) and the kidneys compensate by raising bicarbonate to normalize blood pH — producing a persistently elevated HCO3 of 30–36 mmol/L that is expected and stable in well-managed COPD. Other causes include Cushing syndrome, hyperaldosteronism (Conn syndrome), severe dehydration (contraction alkalosis), and nasogastric suction in hospital settings. A mildly elevated venous HCO3 of 29–32 mmol/L requires interpretation alongside clinical context — venous bicarbonate is naturally 2–4 mmol/L higher than arterial, so a value in this range may fall within the expected venous reference range for some labs.
Related Health Conditions
All Your Lab Results.
One Simple Dashboard.
Import, Track, and Share Your Lab Results Easily
Import, Track, and Share Your Lab Results
Import lab results from multiple providers, track changes over time, customize your reference ranges, and get clear explanations for each result. Everything is stored securely, exportable in one organized file, and shareable with your doctor—or anyone you choose.
Cancel or upgrade anytime
What does it mean if your Bicarbonate (HCO3), Serum result is too low?
Low bicarbonate (HCO3 below approximately 22 mmol/L) indicates metabolic acidosis — the blood contains more acid than normal, either because acid is being overproduced, bicarbonate is being lost, or the kidneys are failing to excrete acid and regenerate bicarbonate. The most common causes include diabetic ketoacidosis (DKA), where ketone acids rapidly consume bicarbonate and HCO3 may fall below 15 mmol/L; chronic kidney disease, where progressive loss of kidney function impairs acid excretion — a persistent mild metabolic acidosis with HCO3 of 18–21 mmol/L is common in moderate to advanced CKD; chronic diarrhea, which depletes bicarbonate-rich intestinal secretions; lactic acidosis from shock, severe illness, or metformin use in kidney disease; and salicylate or toxic alcohol ingestion. A mildly low HCO3 of 20–22 mmol/L in an asymptomatic person may reflect early kidney disease, metabolic compensation for hyperventilation, or laboratory variation. A HCO3 below 15 mmol/L indicates significant metabolic acidosis and warrants clinical evaluation; below 10 mmol/L is a medical emergency.
Related Biomarkers
Article Review & Sources
All our content is backed by peer-reviewed studies, academic research, and trusted medical sources. We're committed to accuracy and transparency — see our editorial policy for details.
Laboratories
Bring All Your Lab Results Together — In One Place
We accept reports from any lab, so you can easily collect and organize all your health information in one secure spot.
Pricing Table
Gather Your Lab History — and Finally Make Sense of It
Finally, Your Lab Results Organized and Clear
Personal plans
$79/ year
Advanced Plan
Access your lab reports, explanations, and tracking tools.
- Import lab results from any provider
- Track all results with visual tools
- Customize your reference ranges
- Export your full lab history anytime
- Share results securely with anyone
- Receive 5 reports entered for you
- Cancel or upgrade anytime
$250/ once
Unlimited Account
Pay once, access everything—no monthly fees, no limits.
- Import lab results from any provider
- Track all results with visual tools
- Customize your reference ranges
- Export your full lab history anytime
- Share results securely with anyone
- Receive 10 reports entered for you
- No subscriptions. No extra fees.
$45/ month
Pro Monthly
Designed for professionals managing their clients' lab reports
- Import lab results from any provider
- Track lab results for multiple clients
- Customize reference ranges per client
- Export lab histories and reports
- Begin with first report entered by us
- Cancel or upgrade anytime
About membership
What's included in a Healthmatters membership
Import Lab Results from Any Source
See Your Health Timeline
Understand What Your Results Mean
Visualize Your Results
Data Entry Service for Your Reports
Securely Share With Anyone You Trust
Let Your Lab Results Tell the Full Story
Once your results are in one place, see the bigger picture — track trends over time, compare data side by side, export your full history, and share securely with anyone you trust.
Bring all your results together to compare, track progress, export your history, and share securely.
What Healthmatters Members Are Saying
I have been using Healthmatters.io since 2021. I travel all over the world and use different doctors and health facilities. This site has allowed me to consolidate all my various test results over 14 years in one place. And every doctor that I show this to has been impressed. Because with any health professional I talk to, I can pull up historical results in seconds. It is invaluable. Even going back to the same doctor, they usually do not have the historical results from their facility in a graph format. That has been very helpful.
Anthony
Unlimited Plan Member since 2021
What fantastic service and great, easy-to-follow layouts! I love your website; it makes it so helpful to see patterns in my health data. It's truly a pleasure to use. I only wish the NHS was as organized and quick as Healthmatters.io. You've set a new standard for health tracking!
Karin
Advanced Plan Member since 2020
As a PRO member and medical practitioner, Healthmatters.io has been an invaluable tool for tracking my clients' data. The layout is intuitive, making it easy to monitor trends and spot patterns over time. The ability to customize reports and charts helps me present information clearly to my clients, improving communication and outcomes. It's streamlined my workflow, saving me time and providing insights at a glance. Highly recommended for any practitioner looking for a comprehensive and user-friendly solution to track patient labs!
Paul
Healthmatters Pro Member since 2024
We implement proven measures to keep your data safe.
At HealthMatters, we're committed to maintaining the security and confidentiality of your personal information. We've put industry-leading security standards in place to help protect against the loss, misuse, or alteration of the information under our control. We use procedural, physical, and electronic security methods designed to prevent unauthorized people from getting access to this information. Our internal code of conduct adds additional privacy protection. All data is backed up multiple times a day and encrypted using SSL certificates. See our Privacy Policy for more details.
