Evaluation of methods for estimating renal medullary blood flow

Although only 10-15% of renal blood flow enters the medulla, renal medullary hemodynamics have been implicated as a key factor for the ability of the kidney to excrete concentrated or diluted urine and to be involved in sodium homeostasis. Due to the obvious importance of medullary hemodynamics, several methods have been developed or modified for estimating regional renal blood flow. The aim of the present paper is to evaluate critically the most commonly used methods available today. Those which are considered in this paper are: (1) albumin accumulation (Lilienfield), (2) rubidium-86 uptake, (3) transit time-indicator dilution, (4) red cell velocity tracking, (5) red cell flux (fluorescently labeled red cells) and (6) laser-Doppler flowmetry. Depending on the method of choice, the reported control values for renal medullary blood flow differ up to tenfold. Overall, it appears that the discrepancies in values obtained from the different methods will not be resolved until the methods are compared directly in the same animal and during different experimental conditions. Furthermore, one needs to understand and describe accurately the involved errors so that proper corrections can be made. It seems likely, however, that the majority of the techniques can be used to study relative changes, but attempts should not be made to draw conclusions about the absolute flow value. In this regard, the laser-Doppler method seems to be superior in its simplicity of use and its large advantage of continuous and relatively noninvasive measurement.