Occasionally, there is concern whether the blood is arterial or venous. 

• This query comes up when ever the blood is dark or the results are close to the usual venous values.
•  In general, venous blood tends to have a higher PCO2 and lower pH. 
• The values cannot be used in individual case to say that it is definitely venous blood.

A decision as to whether it is arterial or venous blood should be made at the time of specimen collection. 

• Venous blood never has enough pressure to fill a syringe spontaneously. 
• When a syringe fills by itself, particularly in a pulsatile manner, it is arterial blood.

REF: Scamman FC. Percutaneous sample for blood gas analysis: Arterial or venous blood. Anesthesiology 1979; 51:474-479.

Lichter JL, et al. Another method for distinguishing arterial from venous puncture. Anesthesiology 1980; 52:448-449