This prospective ICU cohort study (AlbuS1P) investigated how human albumin replacement affects the distribution of sphingosine 1-phosphate (S1P) between HDL and serum albumin and its impact on immune-cell dynamics and vascular integrity in critically ill patients with hypoalbuminemia. Flow Induced Dispersion Analysis (FIDA) was used to measure S1P binding to serum albumin versus HDL, showing albumin treatment increased the serum-albumin–S1P complex size by 5% and reduced the HDL–S1P complex size by 12% — an HDL-to-albumin shift in S1P binding without a change in total plasma S1P. It is an example of using FIDA to measure how a small molecule redistributes between carrier proteins in solution.