For the use of Hemastix, a drop of water is applied to the suspicious stain. After a few moments (to allow some of the blood to dissolve), the reactive area at one end of the strip is brought in contact with the water. A green coloration of the test surface is indicative for the presence of blood.
 
The strips are 8 cm long with a reactive area of 5 x 5 mm at one end. The reactive area is yellow but colors green in the presence of blood.