Alkaline phosphatase (ALP or AKP for short) is a class of phosphatase, widely distributed in mammalian tissues, and its activity requires an optimal pH of 9.2~9.8. This enzyme is mainly found in the place of active material exchange (cell membrane), such as the intestinal epithelium and the brush border of the proximal tubule of the kidneys, the static cilia of the epithelium of the epididymis, the capillary bile ducts of the liver, as well as the endothelium of the arteries and capillaries. The endothelium of arterioles and capillary arterioles, endoplasmic reticulum, Golgi complex, vesicles, lysosomes of intestinal epithelium, neutrophil neutrophil granules, and cell membranes of smooth muscle.

Neutrophil Alkaline Phosphatase Stain (NAP) does not use the metal precipitation method to show alkaline phosphatase activity, but uses the azo-coupling method (also known as the simultaneous coupling method), which is based on the principle that intracellular alkaline phosphatase hydrolyzes AS-BI phosphate under alkaline conditions of pH 9.2-9.8, releasing phosphoric acid and naphthol, which generates a colored product with the coupled diazonium salt that becomes fixed in the cytoplasm, and is used in blood or bone marrow. This stain is specifically used for alkaline phosphatase staining of neutrophils in blood or bone marrow cell smears, the active site of alkaline phosphatase is blue and located in the cytoplasm, and the results are more reliable than those of the metal salt precipitation method.