Determination of extraction fraction by a double tracer method

A relatively simple dual tracer method for measuring renal extraction fraction was compared with the direct method. In the dual tracer method an extracted and nonextracted tracer are mixed together and administered into the arterial circuit and samples of venous blood are obtained. The ratio of recovered to injected tracer then determines the extraction fraction. In the direct method tracer is administered directly into the renal artery and the total venous effluent is collected. Extraction fraction is then calculated as arteriovenous difference divided by the quantity injected. In the experiments the extraction fractions for ²⁰¹Tl and ²⁰³Hg chlormerodrin were determined. The extraction fraction for ²⁰¹Tl averaged 81±5% by the direct method and 81±6% by the dual tracer technique (r=0.94). The extraction fraction for ²⁰³Hg chlormerodrin was found to change as a function of the renal blood flow. At normal flow the extraction fraction averaged 48% by the direct method and 45% by the dual tracer technique. At reduced flow, in contrast, it averaged 78% by the direct method and 71% by the dual tracer technique.