Some artifacts in measuring single nephron glomerular filtration rate.

The existence of many artifacts in measuring single nephron glomerular filtration rate (SNGFR) by micropuncture technique has been pointedI out by several investigators. One of the more important sources of error is "retrograde contamination." This artifact was initially suggested by Rector and co-workers in 1 966 tnd(ler conclitions of partial uireteral obstrtuction( 1). Subse(quently Brenner and co-workers(2) in their study of the effect of furosemidle on proximal reabsorption, claimed to have evidence for retrograde contamination wlhen a short oil block was usecl. It is asstumedl that (lutring sampling a pressure gradient is created between the two ends of the oil coltumn usecd to block the tubule distal to the princttire site. Stuclh a gradient, greater in con(litions of hiigli intratubular pressure (suclh as partial ureteral obstruction or fturosemide (liuresis) wouild be sufficient to force the (listal tubular fluid between the tubular wall and the oil column. Suclh retrograde flow will spturiotusly raise the inulini concentration and the volume of the collected sample witlh consequent erroneously higlh values of TF/P inulin ratio aindI SNGFR. In view of these considerations it lhas become conventional to (liscard unexpectedly hiighi valuies of these pararneters. This would seem to be an unjustifiecd selection of (lata since no (lirect (lenonstration of retrograde contaminiation has yet leen reported. We have recently obtained evidence, by direct demonstration, that retrograde contamination does not take place in eitlher hydropenia or saline diuresis when mineral oil is used to b)lock the proximal tubule and a conventional collection teclhnique is employed. Neither didI we find objective evidence for such an artifact when a higlh pressure gradient was deliberately created across the oil block during the collection, even in the presence of partial ureteral obstruction. Two original methods have been used: the split-droplet technique and the distal perfusion technique. The split-droplet technique (Fig. 1) involves intra-