The most commonly used heme-based reagents are phenolphthalein (Kastle-Mayer), ortho-tolidine, tetramethylbenzidine, leucocrystal violet, fluorescein, and luminol. Fluorescein has a positive result in the form of a product that fluoresces but must be visualized using an ALS. With luminol, a positive test is indicated by luminescence rather than a visual color formation. A positive reaction, as indicated by the formation of a colored, fluorescent, or illuminating product, merely indicates the possibility of the presence of blood (see Figure M.2). There are many substances that will also catalyze this same reaction such as plant peroxidase or strong chemical oxidants. It is essential for the crime scene investigator to make sure that the testing reagents have been tested on known standards of blood prior to testing the suspected stains; the lack of a reaction on the unknown stain can be deemed to mean that blood is absent.