A simple method is presented for estimating extracellular fluid volume from the plasma clearance of [99Tc]DPTA or [169Yb]DTPA. Two plasma samples are required, at 1 hr and 3 hr, following a single intravenous injection. (The same plasma samples can be used for measurement of glomerular filtration rate.) Using the complete plasma clearance curve as a reference (eight samples at 10 to 240 min), the error of the two-sample method in 40 patients was 1.5 l for [99mTc]DPTA, 2.1 l for [169Yb]DTPA (residual standard deviation.
