Your patient is in the resus bay and the nurse asks, “ABG or VBG?”
Does it matter?
Let’s answer that with some evidence.
Why do we need a blood gas?
Blood gas analysis is used in the emergency department to rapidly assess ventilation, oxygenation, and acid–base status.
In the ER, we usually order a blood gas for a few key reasons:
- Suspected sepsis or septic shock
- Hemorrhagic shock
- Respiratory distress or respiratory failure
- Suspected hypercapnia, especially in COPD
- Diabetic ketoacidosis (DKA)
- Overdoses and poisonings
- As a rapid (though imperfect) snapshot of electrolytes and hemoglobin
…and a few other selected clinical situations.
What values are we looking at?
Now that we’re clear why we ask for blood gases, let’s list the values that actually matter to us:
- pH
- PCO₂
- PO₂
- HCO₃⁻
- Anion gap
- Lactate
- Base excess
Anion gap is not directly measured on a blood gas. It is calculated from serum electrolytes.
VBG or ABG?
Now that we know why we ask for blood gases and what we’re looking at, let’s answer the million-dollar question:
Does it matter if it’s a VBG or an ABG?
A venous blood gas is taken from venous blood, and an arterial blood gas from an artery.
I wanted to address this question because ABG is not a harmless test.
ABG is:
- Very painful
- Associated with an increased risk of bleeding and hematoma
- Risk of pseudoaneurysm and AV fistula
- Risk of infection
- Risk of nerve injury
- Risk of digital ischemia
- Can delay care
- Costly
In contrast, a peripheral VBG can be obtained when IV access is placed on arrival—no additional pricks.
So if a VBG can give us the information we need, it’s worth asking:
Do we really need an ABG every time?
Let’s break it down, component by component
pH
Venous and arterial pH values correlate closely and accurately reflect the severity of acidosis.
On average, VBG pH is about 0.03–0.04 lower than ABG pH.
Kelly et al. (2001) demonstrated excellent correlation with narrow limits of agreement — a finding repeatedly replicated across multiple studies (Razi 2012, McCanny 2012, Byrne 2014).
Even in critically ill patients, this correlation largely holds (Zeserson et al., 2018).
That said, correlation can worsen in shock, hemodynamic instability, or extreme acid–base disturbances, likely due to impaired perfusion and tissue-level hypercapnia and acidemia.
For assessing the presence and severity of acidosis, VBG is sufficient.
PCO₂
It’s intuitive to assume that PCO₂ cannot be reliably assessed using a VBG—and you wouldn’t be wrong.
But in the ER, we usually only need to answer one question:
Is this patient hypercapnic?
Kelly et al. (2005) showed that a venous PCO₂ >45 mmHg had 100% sensitivity and 100% negative predictive value for detecting arterial hypercapnia. So if venous PCO₂ ≤45 mmHg, arterial hypercapnia is very unlikely.
That’s enough for emergency decision-making.
The exact number rarely changes management. Follow-up ABGs can be done later during inpatient care if needed.
Bicarbonate, base excess, and lactate
These parameters show excellent agreement between venous and arterial samples (Kelly et al. 2006)
- Bicarbonate differences are small (≈0.5–1.5 mmol/L) -
- Lactate correlates closely and can be reliably trended using venous samples
- Base excess/base deficit differences are not clinically significant
Tourniquet use and VBGs
A common concern is whether how VBGs are drawn affects their reliability.
Cengiz et al. (2009) studied blood gases drawn with a tourniquet applied and at intervals after release in healthy volunteers. They found no clinically significant difference in blood gas values with the tourniquet applied or after its removal.
PO₂
This is where ABG still matters.
The PaO2 level does not correlate between the venous and arterial blood gases (Malatesha 2007, Byrne 2014).
But first, isn’t pulse oximetry enough?
Systemic oxygen delivery is determined by:
O₂ delivery = Cardiac output × Hemoglobin × O₂ saturation
- Oxygen content depends far more on saturation than on PaO₂
- SpO₂ is continuous, whereas PaO₂ is just a moment in time
That said, there are situations where ABG is needed:
- Poor arterial waveform
- Dyshemoglobinemia (methemoglobinemia or sulfhemoglobinemia)
- P/F ratio, especially in ARDS
But it's worthwhile to mention here SpO2/FiO2 ratio (SFR) can be a surrogate for PaO2/FiO2 ratio (Reddy M 2025).
- A–a gradient, if you’re calculating it
- IV dyes (e.g., methylene blue)
- Motion artefact
- Venous pulsation (e.g., severe TR with hypotension)
What Matters
Before ordering a blood gas, ask three questions:
1️⃣ Do I actually need a blood gas?
What decision will this test change?
If you’re not using it to assess:
- Acid–base status
- Lactate or base excess
- Ventilation (CO₂)
- Oxygenation
…you probably don’t need a blood gas at all.
2️⃣ Is my question about acid–base status or CO₂?
If you’re asking:
- Is there acidosis and how severe is it?
- Is lactate elevated?
- Is hypercapnia present?
👉 VBG is enough.
3️⃣ Is there a compelling reason that overrides everything else?
Situations where physiology or monitoring cannot be trusted:
- Need for an accurate PaO₂ (ARDS (P/F ratio), A–a gradient)
- Unreliable SpO₂ or poor waveform
- Severe shock or circulatory failure
- Suspected dyshemoglobinemia (e.g., methemoglobinemia)
👉 This is when an ABG is justified.
The next time you’re pushed for an ABG, don’t argue—ask:
What are we trying to measure that the VBG didn’t already answer?
What are we trying to measure that the VBG didn’t already answer?
Want to Read More?
- Kelly, Anne-Maree, Ross McAlpine, and Elizabeth Kyle. "Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department." Emergency Medicine Journal 18.5 (2001): 340-342.
Razi, Ebrahim, et al. "Correlation of arterial blood gas measurements with venous blood gas values in mechanically ventilated patients." Tanaffos 11.4 (2012): 30.
- McCanny P, Bennett K, Staunton P, McMahon G. Venous vs arterial blood gases in the assessment of patients presenting with an exacerbation of chronic obstructive pulmonary disease. American Journal of Emergency Medicine 2012;30:896–900
- Byrne AL, Bennett M, Chatterji R, Symon R, Pace NL, Thomas PS. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systemic review and metaanalysis. Respirology 2014;19:168-175.
- Zeserson, Eli, et al. "Correlation of venous blood gas and pulse oximetry with arterial blood gas in the undifferentiated critically ill patient." Journal of intensive care medicine 33.3 (2018): 176-181.
- Shirani, Farhad, et al. "The effects of hypotension on differences between the results of simultaneous venous and arterial blood gas analysis." Journal of research in medical sciences: the official journal of Isfahan University of Medical Sciences 16.2 (2011): 188.
- Kelly, AM, Kerr, D, Middleton, P. Validation of venous pCO2 to screen for arterial hypercarbia in patients with chronic obstructive airways disease. J Emerg Med. 2005;28(4):377-379
- Middleton, P, Kelly, AM, Brown, J, et al. Agreement between arterial and central venous values for pH, bicarbonate, base excess, and lactate. Emerg Med J. 2006;23:622-624.
- Arnold, Timothy DW, et al. "Base deficit from the first peripheral venous sample: a surrogate for arterial base deficit in the trauma bay." Journal of Trauma and Acute Care Surgery 71.4 (2011): 793-797.
Cengiz, Melike, et al. "Influence of tourniquet application on venous blood sampling for serum chemistry, hematological parameters, leukocyte activation and erythrocyte mechanical properties." Clinical chemistry and laboratory medicine 47.6 (2009).
- Malatesha, G., et al. "Comparison of arterial and venous pH, bicarbonate, PCO2 and PO2 in initial emergency department assessment." Emergency Medicine Journal 24.8 (2007): 569-571.
- Reddy, Madhura, et al. "Non-invasive SpO2/FiO2 ratio (SFR) as surrogate for PaO2/FiO2 ratio (PFR): A scoping review." J. Crit. Care Med 11 (2025): 221-232.
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