The test principle applied in this kit is Sandwich enzyme immunoassay. The microtiter plate provided in this kit has been pre-coated with an antibody specific to Activin Receptor Like Kinase 1 (ALK1). Standards or samples are then added to the appropriate microtiter plate wells with a biotin-conjugated antibody specific to Activin Receptor Like Kinase 1 (ALK1). Next, Avidin conjugated to Horseradish Peroxidase (HRP) is added to each microplate well and incubated. After TMB substrate solution is added, only those wells that contain Activin Receptor Like Kinase 1 (ALK1), biotin-conjugated antibody and enzyme-conjugated Avidin will exhibit a change in color. The enzyme-substrate reaction is terminated by the addition of sulphuric acid solution and the color change is measured spectrophotometrically at a wavelength of 450nm ± 10nm. The concentration of Activin Receptor Like Kinase 1 (ALK1) in the samples is then determined by comparing the O.D. of the samples to the standard curve.