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In human hematopoietic tissue, alkaline phosphatase activity is present in neutrophil granulocytes, osteoblasts, vascular endothelial cells and sometimes lymphocytes. Leukocyte alkaline phosphatase (LAP) activity in the neutrophils varies widely in activity in different hematological and non-hematological disorders, and may be helpful diagnostically. Assessment of LAP activity in the neutrophils is most useful in differentiating chronic myeloid leukemia (CML), where it is typically decreased, from granulocytic leukemoid reactions, where it is typically increased. In CML, about 90% of cases show decreased LAP activity. When the disease is in remission, the LAP score may become normal; when CML is in blast crisis, LAP activity is often normal or increased. LAP activity in the neutrophils is usually increased in polycythemia vera, whereas it is normal in secondary polycythemia. The LAP score is variable in the other myeloproliferative disorders such as essential thrombocythemia and agnogenic myeloid metaplasia. Low LAP activity is not unique to CML, but may also be found in diseases such as paroxysmal nocturnal hemoglobinuria (PNH), sickle cell anemia and sideroblastic anemia. High LAP activity is frequently seen in bacterial infections, pregnancy, and in patients receiving corticosteroids. Reference value is 20 - 137. Specimen requirement is one 10 mL green top (heparin) tube or 4 well made peripheral blood smears. All slides should be air-dried, unfixed, and unstained. |
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