Determination of possible contamination of interstitial fluid. (A) A tracer that does not pass intracellularly is equilibrated in the extracellular fluid phase. (B) If undiluted interstitial fluid (IF) is isolated, the concentration in IF and plasma (P) will be equal, i.e., IF/P = 1.0. If, however, intracellular fluid (with dissolved proteins) not containing tracer is added to the interstitial fluid during the isolation process (e.g., centrifugation), the IF will become diluted and as a consequence IF/P <1.0. (C) Tissue-to-plasma distribution ratios (IF/P) of the extracellular tracer 51Cr-ethylenediaminetetraacetic acid (EDTA) as a function of G-force for interstitial fluid isolated by centrifugation from intact tumor and skin (means ± SE). For g < 424, the IF/P ratio was not significantly different from 1.0, suggesting negligible dilution and thus contamination of IF. An IF/P <1.0 for g > 424 (*P < 0.05 for tumor as well as skin) suggested contamination of IF with intracellular water and proteins (Data in Figure 2C modified from ).