This step-by-step video clearly demonstrates the correct technique for the execution of the most commonly used manual technique in the diagnosis of bleeding disorders: the one-stage assay for FVIII. Supplementing the Lab Manual written protocol (Section 23), it also explains the theory behind the assay, details the important reagents, discusses the challenges posed by limited resources, and works through the plotting and interpretation of a series of typical diagnostic cases. The accompanying searchable transcript links to the individual sections of the video. A blank one-stage FVIII assay record sheet, log-log graph paper, and log-linear graph paper are also provided for free download.