Neutrophil Alkaline Phosphatase (NAP)

Abstract:
The neutrophil alkaline phosphatase (synonym: leukocyte alkaline phosphatase, LAP) is present in the cytoplasm and secondary granules of neutrophils. Activity increases with cell maturity. Lymphocytes and monocytes are negative. Since EDTA reduces the activity of the enzyme, this staining may not be perfomed with EDTA blood. Control blood films of a healthy person and of a patient with leukocytosis induced by a bacterial infection must be prepared at the same time.

Assessment:
Based on the staining, 5 activity grades ranging from 0 to 4 (see image on the left). Some laboratories use a range from 0 to 5. 100 segmented neutrophils are counted (in the case of neutropenia, 10% band neutrophils may be included) and an activity grade is estimated. Each activity grade is multiplied by the number of neutrophils pertaining to that activity grade. By adding up all activity grades, a total score is calculated (see example below). The possible scores range from 0 to 400. This value is called total NAP score. It normally ranges from 30 to 100. Each laboratory has to determine its own range of reference. A score below 20 is considered decreased and above 150 increased.

Indication:
Increased scores are found in reactive neutrocytosis (e.g. in leukemoid reaction) and in polycythemia vera, decreased scores in chronic myeloid leukemia in the chronic phase and in paroxysmal nocturnal hemoglobinuria (PNH).
The introduction of molecular biological methods has diminished the value of the neutrophil alkaline phosphatase assay. Especially for the discrimination of a leukemoid reaction from a chronic myeloid leukemia, the RT-PCR assay (reverse transcriptase-polymerase chain reaction) of BCR-ABL is a much more specific test.