This step-by-step video clearly demonstrates the correct technique for the execution of the fundamental manual technique that constitutes the basis for the most commonly used assays in the diagnosis of bleeding disorders: the activated partial thromboplastin time (APTT). Supplementing the Lab Manual written protocol (Section 13), it also explains the theory behind the assay, details the important reagents, discusses the challenges posed by limited resources, and works through the interpretation of a series of typical diagnostic cases. The accompanying searchable transcript links to the individual sections of the video.