This study adapted Flow Induced Dispersion Analysis (FIDA) to rapidly quantify how the signaling lipid sphingosine 1-phosphate (S1P) partitions between its two blood carriers, human serum albumin (HSA) and high-density lipoprotein (HDL), in plasma from healthy controls and patients with surgical trauma, sepsis, or septic shock. Using fluorescein-labelled S1P on the Fida 1 instrument, FIDA was used to measure the hydrodynamic radii of S1P-HSA and S1P-HDL complexes and thereby resolve carrier binding. Patients showed a shift toward more S1P bound to HDL and less to HSA, the FIDA-derived complex sizes correlated with LC-MS/MS, and very low S1P-HDL formation tracked with ICU mortality. More broadly, this is an example of using FIDA to measure how a small molecule distributes between carrier proteins directly in human plasma or serum, label-free and without separating the carriers.