AJP - Renal Physiology, Vol 248, Issue 3 444-F448, Copyright © 1985 by American Physiological Society
An alternate method for estimating efferent arteriolar plasma colloid osmotic pressure
A. I. Wolfert, L. A. Laveri and D. E. Oken
The colloid osmotic pressure (COP) of efferent arteriolar plasma in
glomerular dynamic studies generally is estimated from the measured protein
concentration (CE) while the nephron filtration fraction (SNFF) is derived
from CE and the systemic plasma protein concentration (CA) according to the
equation SNFF = (1 - CA/CE). Estimates of both SNFF and COPE are quite
sensitive to small errors in protein measurement, however, with a putative
coefficient of variation of +/- 5% in protein measurement at a typical SNFF
of 0.33, for example, providing an uncertainty (i.e., +/- SD) of +/- 14% in
the SNFF estimate and +/- 2.4 mmHg in the estimated COPE value. In this
study, we evaluated in vitro the precision with which the COP of plasma
samples can be estimated after ultrafiltration by coupling direct oncometry
of native plasma with isotopically measured filtration fractions derived
employing nanoliter and microliter volumes and applying a modification of
the equation of Ladegaard-Pedersen (Scand. J. Clin. Lab. Invest. 23:
153-158, 1969). The measured and estimated oncotic pressures were then
compared. The mean differences between theoretic and measured COP values at
filtration fractions of less than 0.1, 0.1-0.2, 0.2-0.3 and greater were:
-0.4 +/- 0.8 (SE) (n = 22); 1.8 +/- 1.1; 3.9 +/- 1.0; and 6.0 +/- 1.7%,
respectively. It is concluded that the coupling of direct oncometric
measurement of arterial plasma colloid osmotic pressure with isotopically
determined filtration fractions provides a satisfactory estimate of COPE
that is suitable for studies of glomerular dynamics.
