Leukocyte (neutrophil) alkaline phosphatase is a cytoplasmic enzyme present in myeloid cells from the metamyelocyte to mature segmented stages. The enzyme can be detected by conversion of a substrate to a colored product at an alkaline pH. A score based on staining intensity in 100 neutrophils can help in the diagnosis of some hematologic disorders.
Specimen:
(1) fresh blood smears are made and air dried
(2) if staining cannot be done immediately (within 30 minutes of smear preparation), the slides must be fixed (10% buffered formalin-methanol, citrated buffered acetone) or frozen
(3) testing should not be done if the absolute neutrophil count is less than 1000 per µL
Staining procedure:
(1) The substrate is a naphthol phosphate.
(2) Staining occurs in the presence of a diazonium salt at an alkaline pH (9.5)
(3) The alkaline phosphatase converts the naphthol phosphate to arylnaphtholamide and phosphate, with the arylnaphtholamide coupling to the azo diazonium salt with the formation of an insoluble azo dye which can be seen on light microscopy
(4) The slide is then counterstained.
Staining intensity scoring
(1) 0 to 4+, with 0 negative and 4+ very strong
leukocyte alkaline phosphatase score =
= SUM (staining intensity scores for 100 neutrophils)
Interpretation:
• minimum score 0
• maximum score 400
• normal range varies with each laboratory; typically 20-100
Elevated LAP score occurs in
(1) myeloproliferative disorders (polycythemia vera, myelofibrosis)
(2) infection
(3) Hodgkin's disease
Decreased LAP score:
(1) CML
(2) AML
(3) aplastic anemia
(4) viral infections, including infectious mononucleosis
(5) paroxysmal nocturnal hemoglobinuria
(6) aplastic anemia
(7) hereditary hypophosphatasia
Diagnostic Utility:
(1) differential diagnosis of leukocytosis: leukemoid reactions vs CML
(2) diagnosis of polycythemia vera and myelofibrosis
