Easier of access than an artery and more generous in yield than a capillary, the vein is a major source of diagnostic clues. Laboratory data on blood chemistry and serology derive largely from venous blood.
"True" mixed venous blood is obtainable only from the pulmonary artery or the outflow tract of the right ventricle—sources made accessible by the cardiac catheter, but hardly lending themselves to routine exploitation. For ordinary diagnostic tests, the superficial vein provides blood which adequately approximates the composition of its ideally mixed counterpart.
Paradoxically, true mixed venous blood may not be always the ideal source of diagnostic data. There are times when relevant information is best obtained from a vein closest to the area which it subtends before its blood becomes admixed at more distant points of confluence. Such a situation is exemplified by the regional venous blood analysis currently used in the diagnosis of renovascular hypertension.