Bedside Snapshot
- Normal (Adult)
- Venous: pH 7.32–7.42, about 0.03 below arterial
PCO₂ and PO₂: no fixed conversion to arterial - Critical Values
- Lab-specific; no universal venous limit. Typical examples: pH <7.20 or >7.60. Severe DKA (pH <7.00) is a DKA class, not a call limit. Escalate at any venous pH with altered mentation
- High Suggests
- High venous PCO₂: hypoventilation, opioids, tiring COPD or asthma
- High pH: hyperventilation, vomiting, diuretics
- Low Suggests
- Low pH with low HCO₃⁻: DKA, lactic acidosis, toxins
- Low pH with high PCO₂: ventilatory failure
- Act Now If
- Venous pH <7.30 and/or HCO₃⁻ <18 with ketones, plus glucose ≥200 mg/dL or known diabetes (euglycemic DKA possible): DKA pathway, K⁺ and ECG; falling consciousness or rising PCO₂: support ventilation; arterial if PaO₂ or exact PaCO₂ matters
- Watch Out For
- A venous PO₂ cannot exclude hypoxemia. Use SpO₂ and an arterial sample when oxygenation matters.
Reference & Critical Ranges
Lab-specific ranges: Every lab sets its own ranges, and they depend on the analyzer and the population too. Use the range on your report. Then read the sample type printed on it: venous or arterial. The values below are typical.
Reference ranges
| Population | Conventional | SI | Notes |
|---|---|---|---|
| Adult: peripheral venous pH | 7.32–7.42 | [H⁺] about 38–48 nmol/L | About 0.03 lower than arterial pH (Byrne 2014 pooled offset, 95% CI 0.029–0.038). The range is derived from the arterial reference 7.35–7.45 minus the mean offset; confirm with your laboratory range |
| Adult: venous PCO₂ | Lab-specific; no fixed conversion to PaCO₂ | Lab-specific | Usually higher than PaCO₂ by a variable amount. Byrne 2014: the 95% prediction interval of the PCO₂ bias was −10.7 to +2.4 mmHg (sign convention not stated in the abstract); too wide to convert |
| Adult: venous PO₂ | Lab-specific; not a usable oxygenation value | Lab-specific | Arterial PO₂ averaged about 37 mmHg higher than venous (95% CI 27–47 mmHg) |
| Adult: calculated HCO₃⁻ | Calculated from venous pH and PCO₂ | Calculated | Inherits the venous pH and PCO₂ error; compare with serum total CO₂ when results disagree |
| Pregnancy | Not established for VBG | — | Use arterial sampling when PaO₂ or an exact PaCO₂ matters |
| Children | Use the laboratory's pediatric venous range | — | Venous pH is used to screen for DKA (ISPAD 2022: venous pH <7.3); neonates need neonatal arterial or cord values |
Severity & critical values
| Level | Conventional | SI | What it means / typical findings |
|---|---|---|---|
| Critical (severe DKA class) | Venous pH <7.00 | [H⁺] >100 nmol/L | 2024 consensus severe DKA band (pH <7.00, bicarbonate <10 mmol/L, or stupor or coma). A DKA class, not a general call limit. Escalate: ICU, insulin and fluids per protocol, check K⁺ |
| Marked acidemia (moderate DKA band) | Venous pH 7.00–7.19 | [H⁺] about 65–100 nmol/L | Moderate DKA band by pH (consensus 7.00–7.25). Check bicarbonate: below 10 mmol/L is severe by bicarbonate. Escalate per protocol |
| Acidemia | Venous pH 7.20–7.31 | [H⁺] 49–63 nmol/L | Find the cause: PCO₂ (respiratory) or HCO₃⁻ (metabolic). Arterial gas if it is low and the patient is unstable |
| DKA screen positive (disease-specific) | Venous pH <7.30 (and/or HCO₃⁻ <18 mmol/L) with ketones (β-hydroxybutyrate ≥3.0 mmol/L or urine 2+) and glucose ≥200 mg/dL or known diabetes | [H⁺] >50 nmol/L | Start the DKA pathway; confirm with serum electrolytes, β-hydroxybutyrate and an anion gap. This cut-off overlaps the acidemia band; it is a disease criterion, not a band boundary. Glucose may be normal in euglycemic DKA (SGLT2 inhibitors, pregnancy, starvation, alcohol); check ketones in any unexplained acidosis |
| Normal | Venous pH 7.32–7.42 | [H⁺] 38–48 nmol/L | Does not exclude a mixed disorder or hypoxemia. Venous PO₂ cannot rule out hypoxemia |
| Alkalemia | Venous pH 7.43 or higher | [H⁺] 37 nmol/L or lower | Hyperventilation or metabolic alkalosis; check K⁺ and ionized calcium |
Critical values are lab-specific: Many labs call a venous pH below about 7.20 to 7.25, or above about 7.55 to 7.60. Your own lab's policy is the one that counts. Severe DKA (pH <7.00) is a DKA severity class. It is not a general call limit. Escalate at any venous pH with altered mental status, and sooner when the pH is falling fast, the patient is hypotensive or there is a toxin exposure. Notify the prescriber or medical control. Confirm with an arterial sample when the decision depends on PaO₂ or an exact PaCO₂.
Conversion: [H⁺] in nmol/L = 10(9 − pH). Venous pH 7.32 works out to about 48 nmol/L. Every 0.1 drop in pH pushes [H⁺] up by about 25%.
Estimated arterial pH from venous pH
Arterial pH ≈ venous pH + 0.03 (population average; individual error is larger)
Normal: venous pH 7.35 corresponds to an arterial pH near 7.38. The offset is for rough estimates only. It is not a correction you can use for treatment decisions.
Causes of Abnormal Results
Why venous and arterial results differ
Venous blood has been through the tissues. The cells there add CO₂ and pull out O₂. So a venous sample reads a lower pH and a higher PCO₂ than arterial blood, plus a much lower PO₂. How big the gap is depends on the patient and on how well they are perfused. You cannot correct it with one fixed factor.
| Mechanism | Common situations | Bedside clue |
|---|---|---|
| Tissue CO₂ uptake | Every venous sample; more in low flow states | Venous pH about 0.03 lower; PCO₂ higher than arterial |
| Tissue O₂ extraction | Shock, low cardiac output, high oxygen demand | Very low venous PO₂; wide gap from arterial |
| Local acid build-up | Prolonged tourniquet, fist clenching, poor peripheral perfusion | Venous pH and lactate higher or lower than the patient's real state |
| Acid–base disturbance | Metabolic acidosis (DKA, lactic acidosis), respiratory failure | Same direction in venous and arterial samples; the venous value is the screen |
Causes of an abnormal venous pH
Read the venous pH the same way you read the arterial pH. A low venous pH points to either a high PCO₂ (respiratory acidosis) or a low HCO₃⁻ (metabolic acidosis). The full list of causes is on the pH page. In the field, the most time-critical are opioid or sedative hypoventilation, tiring COPD or asthma, DKA, shock, and toxins such as salicylate and toxic alcohols.
Do not assume the venous gas is wrong when it differs: A gap from the arterial sample is expected. Judge the venous value on its own terms. Use the arterial sample only for the decisions it is validated for (see Interpretation).
Interpretation & Bedside Approach
Step-by-step approach
- Confirm the sample type. Venous, arterial or capillary, and whether it came from a line. Write it in the chart: the same patient needs the same sample type to trend.
- How sick is the patient? Mental status, breathing pattern, perfusion, EtCO₂ and SpO₂. A venous pH within the normal range does not reassure a patient who is tiring or in shock.
- Read the pH first. Below 7.32 is acidemia and above 7.42 alkalemia on a venous sample. Allow for the 0.03 offset when the arterial reference is used.
- Read PCO₂ and HCO₃⁻ against the pH. PCO₂ moving opposite to pH means respiratory; HCO₃⁻ moving with pH means metabolic. Apply the same logic as on the pH page.
- Decide if a venous gas is enough. Use the table below. If the answer depends on PaO₂, an exact PaCO₂, or perfusion is poor, send an arterial sample.
- Escalate. Venous pH <7.30 and/or HCO₃⁻ <18 with ketones, plus glucose ≥200 mg/dL or known diabetes (normal glucose possible in euglycemic DKA: check ketones): start the DKA pathway. Severe DKA features (pH <7.00, bicarbonate <10 mmol/L, or stupor or coma): ICU-level care per protocol. Rising PCO₂ with falling consciousness: ventilatory support now.
When a venous gas is enough
| Question | Venous gas adequate? | Get an arterial sample when |
|---|---|---|
| Acidemia or alkalemia (pH, HCO₃⁻) | Yes, for most adults who are not in shock | Shock, cardiac arrest, or any decision where a 0.03 pH shift changes management |
| DKA screen | Yes, as a screen: venous pH and HCO₃⁻ with glucose and ketones; the consensus does not specify sample type, so follow local policy | Results disagree with serum electrolytes or β-hydroxybutyrate, or local protocol requires arterial sampling |
| Rule out hypercarbia | Yes as a rule-out only: high sensitivity, low specificity | A high venous PCO₂ must be confirmed before intubation or NIV decisions |
| Oxygenation (PaO₂, A–a gradient) | No | Always: use SpO₂ and an arterial sample |
| Exact PaCO₂ for ventilator changes | No | Always: match the arterial value or the EtCO₂ gap |
Look at what the 2025 Cochrane review of peripheral venous gases actually showed. A venous gas caught respiratory failure and isolated hypercarbia with high sensitivity (about 97%). Its specificity was low (about 37% and 54%). All six studies carried a high risk of bias, and the certainty is low to very low. Cut-offs were poorly reported, so no specific venous PCO₂ threshold is validated. A normal venous PCO₂ may help exclude hypercarbia when your local protocol cut-off is met. An abnormal one needs arterial confirmation.
Worked example: Picture a DKA patient with venous pH 7.18, HCO₃⁻ 8 mEq/L and glucose 520 mg/dL. The pH of 7.18 lands in the moderate band (7.00–7.25). A bicarbonate of 8 falls in the severe range (below 10). Treat it as at least moderate and possibly severe, with ICU or step-down care per protocol. The consensus weighs pH, bicarbonate, β-hydroxybutyrate and mental status together. You do not need every criterion. Confirm with serum electrolytes, β-hydroxybutyrate and anion gap.
Field & Transport Considerations
Point-of-care testing
Cartridge analyzers in ambulances and emergency departments give you pH and PCO₂, and they calculate HCO₃⁻. Some also report PO₂. On a venous sample, that PO₂ is a venous value. Stay inside the cartridge's operating temperature and hematocrit limits. Trend on one device. Write the sample type on every result.
Before and during interfacility transport
| Setting | Consideration |
|---|---|
| Sample for transfer | Label the gas as venous or arterial with time, FiO₂ and ventilator or NIV settings; a venous pH with a clear sample type is useful for the receiving team |
| Oxygen decisions | Do not use a venous PO₂ to set oxygen; use SpO₂ and an arterial sample when oxygen targets matter |
| Ventilator changes | Repeat the gas after rate or tidal-volume changes if a device is available; trend EtCO₂ between samples |
| Suspected DKA en route | Venous pH, glucose and ketones guide the pathway; confirm the receiving center can give insulin and monitor electrolytes |
| Redirect | Venous pH <7.00 or toxin-related acidemia: go to a center with ICU and dialysis capability |
Special Populations
| Population | What changes | Clinical implication |
|---|---|---|
| Neonates & infants | Transitional values after birth; cord and capillary samples differ | Use neonatal ranges and arterial or cord values; a venous gas is not the standard |
| Children & adolescents | DKA screen by venous pH (ISPAD 2022: pH <7.3; severe at pH <7.1) | Venous pH is a practical screen; bicarbonate is avoided in pediatric DKA except in exceptional cases |
| Pregnancy | Progesterone-driven ventilation; PaCO₂ near 30 mmHg; venous values not validated | Use arterial sampling when a PCO₂ or PO₂ decision matters |
| Older adults | Offset not specifically studied in older adults; interpret as for adults, and allow for comorbidity and drugs | Interpret as for adults; review drugs that change acid–base status |
| Chronic CO₂ retainers | High baseline PCO₂ with high HCO₃⁻ | Compare with the patient's baseline; a rising venous PCO₂ over baseline is the concern |
| Shock or poor perfusion | Tissue CO₂ uptake and local acid build-up are larger | Venous values drift further from arterial; send an arterial sample |
Interfering Factors & Pitfalls
Collection & handling
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Prolonged tourniquet or fist clenching | ↓ pH; ↑ PCO₂, lactate and K⁺ | Local acid build-up in the limb | Release the tourniquet within about one minute; no fist clenching |
| Delay at room temperature | ↓ pH; ↑ PCO₂ | Cells keep producing CO₂ and acid | Analyze promptly; cap and keep the syringe as the lab directs |
| Air bubbles | ↑ PO₂ (artifact); PCO₂ may fall | Gas exchange with air | Expel air at once and cap |
| Excess liquid heparin | PCO₂ and HCO₃⁻ falsely ↓ | Dilution | Use dry-heparin blood-gas syringes |
| Line sample or IV contamination | Variable | Saline or drug contamination | Discard dead space before drawing |
Physiologic & patient factors
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Shock or low flow | Gap from arterial widens | More tissue CO₂ uptake and O₂ extraction | Arterial sample when the decision depends on it |
| Hypothermia | Reported values may differ from in-body values | Gas solubility changes with temperature | Report the temperature; follow local alpha-stat practice |
| Anxiety or painful puncture | ↑ pH; ↓ PCO₂ | Hyperventilation before the draw | Note the breathing pattern; calm the patient |
Assay & device-specific
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Calculated HCO₃⁻ | Inherits pH and PCO₂ error | Derived by the analyzer, not measured | Compare with serum total CO₂ when they disagree |
| Cartridge outside its limits | Unreliable results | Temperature or hematocrit outside the device range | Follow the device manual; repeat on a validated device |
| Venous PO₂ read as arterial | Falsely low oxygenation | Venous PO₂ is far lower than PaO₂ | Never use venous PO₂ to judge oxygenation |
Common misinterpretations
- “A normal venous pH excludes acidosis”: a mixed disorder can hide behind a normal pH. Read PCO₂ and HCO₃⁻ as well.
- “Venous PO₂ shows the patient is fine”: hypoxemia can still be present when the venous PO₂ looks fine.
- “Venous and arterial results can be swapped”: the gap is not constant. Use the sample type the protocol names.
Prolonged tourniquet changes pH, K⁺ and lactate: A long tourniquet or a fist clench can make the sample look acidotic and high in potassium. Before you act on a result that does not match the patient, draw it again with a short tourniquet.
Physiology & Test Basics
What it measures
This page is about the venous blood gas (VBG). That is pH, PCO₂, PO₂ and calculated HCO₃⁻, drawn from a peripheral vein. You compare it with the arterial blood gas on the ABG panel. For acidosis and ventilation in emergency and transport care, the VBG is the everyday test. PaO₂ still needs an arterial sample.
Physiology & regulation
- Tissue gas exchange: venous blood picks up CO₂ from the tissues and has given up its O₂. Venous PCO₂ runs higher than arterial, and venous PO₂ runs lower.
- Buffering: the same CO₂ and HCO₃⁻ buffer system sets the pH in venous blood as it does in arterial.
- Lungs and kidneys: after tissue exchange, the venous sample shows the same ventilation and renal compensation as the arterial one.
How it is measured
- Specimen: it is whole blood, drawn from a peripheral vein into a pre-heparinized blood-gas syringe. Use a short tourniquet, expel air, cap and mix gently.
- Method: pH electrode and gas sensors at 37 °C, on a laboratory or point-of-care analyzer. Results come back in minutes.
- Fasting: not required.
Why it matters
A venous gas is fast and cheap, and it spares the patient an arterial puncture. It screens well for acidosis, DKA and hypercarbia. It also helps you decide whether the patient needs an arterial sample, ventilation support or transfer.
Clinical Pearls
Arterial is about 0.03 higher: Expect venous pH to read about 0.03 under arterial. A venous 7.32 lands near an arterial 7.35.
A venous gas can rule out, not rule in, hypercarbia: Normal venous PCO₂ makes hypercarbia unlikely. Abnormal? Confirm it on an arterial sample.
A venous PO₂ cannot exclude hypoxemia: Use SpO₂ and, when the decision needs it, an arterial sample. Give oxygen to the SpO₂ target. A venous PO₂ is not that target.
Prehospital: the breathing pattern hints at the result: Deep, fast breathing with no hypoxia points toward metabolic acidosis (DKA, sepsis, toxin). Slow, shallow breathing with EtCO₂ climbing points toward respiratory acidosis. Ventilate.
DKA screen with a venous sample: Under the consensus criteria, a venous pH and HCO₃⁻ plus glucose and ketones can start the DKA pathway. Local policy decides which sample type counts. Then confirm on serum electrolytes and β-hydroxybutyrate.
Normal glucose does not exclude DKA: Euglycemic DKA can come from SGLT2 inhibitors, pregnancy, starvation or alcohol. For any acidosis you cannot explain, check ketones and bicarbonate.
Nursing: keep one sample type and one analyzer: Keep the device and the sample type the same when you trend. Redo the gas after a ventilator or bicarbonate change.
Do not compare venous and arterial numbers in one trend: Blend venous and arterial numbers and a pH can seem to shift when nothing has changed. Put the sample type on the chart and on the handover.
References
- Byrne, A. L., Pace, N. L., Thomas, P. S., Symons, R. L., Chatterji, R., & Bennett, M. (2025). Peripheral venous blood gas analysis for the diagnosis of respiratory failure, hypercarbia and metabolic disturbance in adults. Cochrane Database of Systematic Reviews, 6, CD010841. https://doi.org/10.1002/14651858.CD010841.pub2
- Byrne, A. L., Bennett, M., Chatterji, R., Symons, R., Pace, N. L., & Thomas, P. S. (2014). Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology, 19(2), 168–175. https://doi.org/10.1111/resp.12225
- Menchine, M., Probst, M. A., Agy, C., Bach, D., & Arora, S. (2011). Diagnostic accuracy of venous blood gas electrolytes for identifying diabetic ketoacidosis in the emergency department. Academic Emergency Medicine, 18(10), 1105–1108. https://doi.org/10.1111/j.1553-2712.2011.01158.x
- Umpierrez, G. E., Davis, G. M., ElSayed, N. A., Fadini, G. P., Galindo, R. J., Hirsch, I. B., et al. (2024). Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care, 47(8), 1257–1275. https://doi.org/10.2337/dci24-0032
- Glaser, N., Fritsch, M., Priyambada, L., Rewers, A., Cherubini, V., Estrada, S., Wolfsdorf, J. I., & Codner, E. (2022). ISPAD Clinical Practice Consensus Guidelines 2022: Diabetic ketoacidosis and hyperglycemic hyperosmolar state. Pediatric Diabetes, 23(7), 835–856. https://doi.org/10.1111/pedi.13406
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