An improved apparatus and method for dispersion of a labeling conjugate in a diagnostic assay, the result being a one-step assay. By eliminating a conjugate pad as in conventional lateral diagnostic devices, and forming a frazil ice pellicle (FIP), rehydration and flow are improved resulting in better reproducibility, improved sensitivity, and reduced costs of individual assay devices. The formation of a frazil ice film formed on a super cooled surface of a sample receiving means simplifies assay assembly. Lyophilization of the FIP improves the release of a sample/analyte/label matrix into a macro channel as in a direct flow assay, while at the same time allowing reagents to mix and flow, thereby optimizing the assay performance. The reagents of the conjugate and the formation of the FIP stabilize the conjugate proteins and provide extended shelf life to the diagnostic assay device.