Comparison of radio-iodinated serum albumin and blue dextran as indicators to measure rate of formation of cerebrospinal fluid

Abstract The characteristics of two nondiffusible indicators, 125 I-labeled albumin (RISA-125) and blue dextran, were compared by using them simultaneously to measure rate of formation of cerebrospinal fluid (CSF) in in vitro experiments and in a series of ventriculocisternal perfusion experiments in rhesus monkeys. Mean CSF formation calculated from the dilution of RISA-125 was 37.6 μl/min (±0.74 sem ); the CSF formation calculated from the corresponding dilution of blue dextran was 37.1 μl/min (±0.70 sem ). The difference is not statistically significant. As measured by counts per minute, the concentration of RISA-125 in aliquots of a dilute RISA-125 solution varied considerably. In the in vitro experiments, simulated CSF formation calculated from these counts varied around the mean by more than ± 10% at the 95% confidence limits. Counting variability is probably related to the low energy levels of the 125 I isotope. Cerebrospinal fluid formation calculated from the dilution of blue dextran had one-tenth the coefficient of variation of that calculated from the dilution of RISA-125. We have concluded that blue dextran is preferable to RISA-125 to measure the rate of formation of cerebrospinal fluid in the experimental setting because it is easier to detect accurately small changes of CSF formation with blue dextran.