VBG Sampling 🩸
Venous blood gas (VBG) draw for UK and Irish practice. When a VBG can replace repeated arterial sampling, venepuncture technique, and GGC pH and pCO2 offsets.
🧾 Overview
- A venous blood gas can be used when monitoring of pCO2 and H⁺ is desirable without multiple arterial sampling.
- There is no way to correlate pO2 between venous blood gases and arterial blood gases.
🚨 When the first gas should be arterial
- If the patient is critically ill, in shock, or hypotensive (systolic BP <90 mmHg), the initial blood gas should be arterial.
🧠 Indications for blood gas analysis
- Critically ill patients.
- Unexpected or inappropriate hypoxaemia (SpO2 <94% on air or oxygen), or any patient requiring oxygen to reach that range. Allow for transient dips to 90% or less during sleep.
- Deteriorating oxygen saturation or increasing breathlessness in previously stable hypoxaemia (for example severe COPD).
- A previously stable patient who needs a significantly increased FiO2 to hold a constant saturation.
- Risk factors for hypercapnic respiratory failure with acute breathlessness, falling saturations, drowsiness, or other CO2 retention symptoms (for example COPD or neuromuscular disease).
- Acutely breathless, or poor peripheral circulation, when a reliable oximetry signal cannot be obtained.
- Severe metabolic disturbance, for example DKA or renal failure, hypothermia (temperature <32°C), severe sepsis, shock, or altered conscious level.
- An unexpected NEWS change, or an unexpected fall in oxygen saturation of 3% or more, even if still in the target range.
- Smoke inhalation, carbon monoxide poisoning, or cyanide poisoning.
📋 Venous pCO2 as a screen
- In patients who are not at risk of metabolic acidosis, a satisfactory SpO2 plus a venous pCO2 below 6 kPa can exclude significant arterial hypoxia or hypercapnia and may avoid an ABG. BTS did not make this a formal recommendation.
⚠️ Cautions
- Do not take blood from an existing peripheral venous access site. Haemolysis, contamination, and intravenous fluid or medication can all alter the results.
- Do not collect from hardened veins, an arteriovenous shunt, haematoma, inflammation or swelling, a vascular graft, a paretic arm, or an arm with lymphatic drain disorders.
- Do not ask the patient to clench or pump the fist. Fist clenching and pumping can cause pseudohyperkalaemia.
- Keep total tourniquet time under 1 minute.
🧰 Equipment
- Pre-heparinised syringe
- Needle of appropriate gauge
- Well-fitting non-sterile gloves
- Tourniquet
- 70% alcohol swab
- Gauze or cotton wool, and a bandage
- Laboratory labels
- Sharps container
- Container with crushed ice if analysis is not done at the point of care
🧑⚕️ Preparation
- Introduce yourself, ask the patient to state their full name, match identity to the request, explain the test, and obtain verbal consent.
- Hand hygiene, then gloves.
- Clean the site with a 70% alcohol swab for 30 seconds and allow to dry completely (30 seconds). Do not touch the cleaned site.
💉 Procedure
- Anchor the vein by placing a thumb below the venepuncture site. Insert the needle bevel up at a 30 degree angle or less.
- Peripheral venous gases are obtained by peripheral venepuncture. Fresh venepuncture is preferred.
- It is acceptable, but not ideal, to draw blood when first introducing an indwelling venous device, before connecting the cannula to intravenous fluids.
- A sample aspirated from a central line is a central venous gas, not a peripheral VBG. Specimens from central lines carry a risk of contamination or erroneous results.
- Once sufficient blood has been collected, release the tourniquet before withdrawing the needle.
- Withdraw the needle, apply gentle pressure with gauze, and ask the patient not to bend the arm. Dispose of the device immediately into a sharps container.
✅ Aftercare and processing
- Expel air bubbles, cap the syringe, and roll the specimen between the hands to mix. Cap to prevent contact with air and leaking during transport.
- Label the syringe.
- Transport immediately. Use crushed ice if analysis is not done at the point of care.
📊 Interpreting a VBG
- There is no way to correlate pO2 between venous and arterial blood gases.
pH
- Peripheral VBG vs arterial
- 0.02–0.04 lower
- Central VBG vs arterial
- 0.03–0.05 lower
H⁺
- Peripheral VBG vs arterial
- 2–4 nmol/L higher
- Central VBG vs arterial
- 3–5 nmol/L higher
pCO2
- Peripheral VBG vs arterial
- 0.4–1.1 kPa higher
- Central VBG vs arterial
- 0.5–0.6 kPa higher
| Parameter | Peripheral VBG vs arterial | Central VBG vs arterial |
|---|---|---|
| pH | 0.02–0.04 lower | 0.03–0.05 lower |
| H⁺ | 2–4 nmol/L higher | 3–5 nmol/L higher |
| pCO2 | 0.4–1.1 kPa higher | 0.5–0.6 kPa higher |
🛠️ Sampling errors
- Air in the sample.
- An improper quantity of heparin in the syringe, or improper mixing after blood is drawn.
- A delay in specimen transportation.
Based on
GuidelineGGC Medicines — Guidelines for Blood Gas Analysis (updated Sep 2025)GuidelineWHO guidelines on drawing blood: best practices in phlebotomy (2010)GuidelineBTS Guideline for Oxygen Use in Healthcare and Emergency Settings (2017)GuidelineJoint EFLM-COLABIOCLI Recommendation for venous blood sampling (Clin Chem Lab Med 2018)
Note Template
Ready-to-use clinical note structure
🕒 08 / 10 / 2026 — 16:04 Venous blood gas (VBG) Patient: [age] [sex] Indication: [pCO2 / H⁺ monitoring / DKA or renal failure / hypercapnia screen / other] Why VBG not ABG: [not critically ill, shocked, or hypotensive / other] O2: [air / device] FiO2 __ SpO2 __% 🧾 Before: • Verbal consent / ID: [Y] • Site restrictions checked (PIVC, AV shunt, lymphoedema, infection): [Y] • Tourniquet <1 min: [Y] • Fist not clenched or pumped: [Y] 💉 Procedure: • Site: [peripheral R/L / central line: specify] • Fresh venepuncture (preferred) / new cannula before IV fluids: [ ] • Pre-heparinised syringe; air expelled; mixed by rolling: [Y] • Labelled; transported immediately (ice if not POC): [Y] 📋 Result: • pH __ H⁺ __ pCO2 __ kPa • pO2 not correlated with arterial pO2 📌 Plan: • [ABG if critically ill / shock / SBP <90] • [repeat VBG at __] 👤 [Your Name], [Role] IMC: _______
Educational reference only. Medical disclaimer