Estimating plasma volume from red cell volume.

Following the withdrawal of the only licensed supply of 125I-HSA in 1997, most UK centres now simply estimate plasma volume from a knowledge of the red cell volume and venous haematocrit. We compared measured and estimated plasma volume in 107 consecutive patients who had had red cell and plasma volume measured independently in the conventional way. In only 40% of cases was the estimated value accurate to within +/- 5%, and discrepancies of more than +/- 10% were seen in 35%.

Appropriate corrections to glomerular filtration rate and volume of distribution based on the bolus injection and single-compartment technique.

The bolus injection, single-compartment technique for measuring GFR overestimates the true value. Nevertheless, assuming that for a given indicator the area under the first exponential of the plasma clearance curve is constant from subject to subject, the observed (uncorrected) value can be corrected by multiplication with a 'sliding' factor, the value of which is a nonlinear function of GFR. Several second-order polynomials, based on pre-determined relationships between simultaneously determined two-compartment and one-compartment GFR, have been described for correcting GFR (GFR correction).