From the Guidelines

Venous Blood Gas (VBG) has limited utility.

It is not recommended as the initial blood gas measurement for critically ill patients or those with shock or hypotension, as an arterial sample is preferred in these cases 1.
The British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings does not specifically mention the use of VBG in its recommendations for blood gas sampling 1.
However, VBG may be useful in certain situations, such as in patients with breathlessness who are thought to be at risk of metabolic conditions like diabetic ketoacidosis or metabolic acidosis due to renal failure, where it can provide information on pH and bicarbonate levels 1.
In general, arterial blood gases (ABGs) are preferred over VBG for obtaining an accurate measure of pH, PCO2, and PO2, especially in critically ill patients or those requiring close monitoring of their oxygenation status 1.

From the Research

VBG analysis can be used to screen for acute acid/base disturbances, diagnose diabetic keto-acidosis, and measure lactate as an independent prognostic marker for mortality 2.
VBG can also be used to diagnose carbomonoxide intoxication or methaemoglobinaemia, but it is limited in assessing oxygenation and ventilatory compensation 2.
Central VBG parameters of pH and Pco2 are potentially good surrogates for determining arterial pH and Pco2 in stable patients without severe acid-base disturbances 3.
An adjustment formula for ABG conversion from central VBG can be used: arterial pH = venous pH + 0.05 units and arterial Pco2 = venous Pco2 - 5 mm Hg 3.

The mean arterial minus venous difference for pH, Pco2, and bicarbonate was found to be 0.027, -3.8, and -0.80, respectively, indicating a close agreement between central VBG and ABG measurements 4.
Regression equations can be derived to predict arterial values from venous values, allowing for the use of VBG in many clinical contexts encountered in the ICU 4.
Arteriovenous agreement for pH is such that values are clinically interchangeable, and agreement for bicarbonate is also close, but agreement for pCO2 is poor 5.
A venous pCO2 ≤45 mm Hg (6 kPa) can reliably exclude clinically significant hypercarbia, making VBG a useful screening tool 5.

VBG analysis can replace ABG analysis in many clinical contexts, especially in emergency care, where the results can be used to guide management decisions 5.
The use of VBG as an alternative to ABG has been explored in various acid-base states, with some studies finding a strong correlation between the two, but others highlighting significant disparities 6.
The appropriateness and utilization of VBG as an alternative to ABG depend on the clinical context and the specific parameters being measured, with consideration of physiological differences between venous and arterial blood values and sampling procedures 6.