Overview
The Leukocyte Alkaline Phosphatase (LAP) score is a cytochemical test used primarily to evaluate neutrophil activity in patients with elevated white blood cell counts. It is most commonly employed to differentiate a reactive leukemoid reaction from chronic myelogenous leukemia. In leukemoid reactions, the LAP score is usually normal or increased, whereas in chronic myelogenous leukemia, the score is characteristically low. The test is based on staining neutrophils for alkaline phosphatase activity using a diazotization-coupling principle, which produces a colored precipitate within the cytoplasm of enzyme-containing cells.
Symptoms
The LAP score itself does not produce symptoms; instead, it reflects underlying clinical conditions associated with abnormal leukocyte activity. Patients undergoing LAP score testing typically present with leukocytosis, which may be associated with symptoms of infection, inflammation, hematological malignancy, or other systemic disorders. Clinical features depend on whether the cause is reactive, such as infection or inflammation, or neoplastic, such as chronic myelogenous leukemia.
Causes
Alkaline phosphatase activity is predominantly present in mature neutrophils. Increased LAP activity is seen in conditions associated with reactive or inflammatory neutrophilia, including leukemoid reactions, infections, inflammation, polycythemia vera, myelofibrosis, Hodgkin’s disease, and pregnancy. Decreased LAP activity occurs in neoplastic and marrow failure conditions, most notably chronic myelogenous leukemia, some myelogenous leukemias, paroxysmal nocturnal hemoglobinuria, and aplastic anemia. The reduced enzyme activity in these disorders reflects abnormal neutrophil maturation and function.
Risk Factors
Risk factors associated with abnormal LAP scores depend on the underlying condition. Elevated LAP scores are associated with reactive states such as severe infections, inflammatory disorders, and myeloproliferative conditions like polycythemia vera and myelofibrosis. Reduced LAP scores are strongly associated with chronic myelogenous leukemia, particularly in its chronic phase, and may also be seen in paroxysmal nocturnal hemoglobinuria and aplastic anemia. The presence of a low LAP score, along with features such as splenomegaly, increased basophils, and the Philadelphia chromosome, supports a diagnosis of chronic myelogenous leukemia.
Prevention
There are no specific preventive measures to alter LAP scores directly. Prevention focuses on early recognition and appropriate management of the underlying cause of leukocytosis. Accurate interpretation of the LAP score requires proper sample collection, staining technique, and scoring methodology. Whole blood samples should be collected in EDTA or heparin tubes and processed promptly to maintain enzyme activity. The LAP score is calculated by assessing alkaline phosphatase staining intensity in one hundred neutrophils, assigning scores from zero to four, and deriving a cumulative score. When used alongside clinical findings, peripheral smear examination, and cytogenetic studies, the LAP score remains a valuable tool for differentiating reactive leukocytosis from chronic myelogenous leukemia and guiding further diagnostic evaluation.
