The clearance concept with special reference to determination of glomerular filtration rate in patients with fluid retention

In subjects without fluid retention, the total plasma clearance of a renal filtration
indicator (inulin, 99mTc-DTPA, 51Cr-EDTA) is close to the urinary plasma clearance.
Conversely, in patients with fluid retention (oedema, pleural effusions, ascites),
there is a substantial discrepancy between the total plasma clearance and the
urinary plasma clearance. This is owing to delayed indicator distribution to smaller
or larger parts of the interstitial space, which in patients with ascites may simulate
a peritoneal dialysator. In patients with fluid retention, urinary plasma
clearance should be assessed to obtain a correct measurement of the glomerular
filtration rate (GFR). In theory, total plasma clearance with late samples (24-h,
48-h) may be applied in patients with fluid retention, but validation hereof has
not been performed. Until such studies are completed, it is recommended that
patients with fluid retention have their GFR measured by a urinary plasma clearance
technique with controlled quantitative urinary sampling within a few hours
after indicator injection.