IgG versus albumin for measurement of plasma volume in normal and hypertensive men

Labeled IgG was evaluated versus labeled albumin for measurement of plasma volume, to see whether it would give a smaller initial dilution volume (indicating a smaller premixing extravascular loss of label) and a smaller difference between initial dilution volume and dilution volume at time 10 min (indicating a negligible extravascular loss in the first 10 min after mixing). IgG was found to have a lower transcapillary escape rate than albumin (P<0.01) in eight normal volunteers and in eight hypertensive subjects, hypertension being associated with an increased transcapillary escape of both proteins. Despite this, the dilution volumes obtained were indistinguishable and the need for a correction through a retropolation procedure was the same with both proteins. Albumin and IgG dilution volumes were highly correlated in 21 subjects (r=0.977) so that use of labeled IgG is a proper alternative to use of labeled albumin for plasma volume determination. However, since IgG brings no consistent advantage, labeled albumin remains the best available tracer for that purpose.