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Diagnostic Tests – Leukocyte Alkaline Phosphatase (LAP) Score
Leukocyte alkaline phosphatase (LAP) is an enzyme found inside mature white blood cells, particularly neutrophils. The LAP score reflects the activity level of this enzyme within these cells. When neutrophils are increased in number and functioning normally, the LAP score rises proportionately. This means that a high LAP score suggests that the elevated white blood cell count is due to a reactive, healthy response—such as an infection or inflammation—rather than a malignant or dysfunctional process. Conversely, when neutrophils are increased in number but function abnormally, the LAP score remains low.
LAP becomes an important diagnostic tool when there is concern about chronic myelogenous leukemia (CML) versus a leukemoid reaction. Both conditions can present with extremely high white blood cell counts dominated by neutrophils. However, in CML, the neutrophils are part of a malignant proliferation and do not function normally, leading to a low LAP score. A patient with CML often has additional findings such as a markedly enlarged spleen, causing left upper quadrant abdominal discomfort or early satiety. In contrast, a leukemoid reaction—typically triggered by infection or severe stress—produces very high white counts as part of a normal physiological response, resulting in a high LAP score. Therefore, LAP is the answer when distinguishing between these two conditions in the setting of significant neutrophilia.
Leukocyte alkaline phosphatase (LAP) is an enzyme found inside mature white blood cells, particularly neutrophils. The LAP score reflects the activity level of this enzyme within these cells. When neutrophils are increased in number and functioning normally, the LAP score rises proportionately. This means that a high LAP score suggests that the elevated white blood cell count is due to a reactive, healthy response—such as an infection or inflammation—rather than a malignant or dysfunctional process. Conversely, when neutrophils are increased in number but function abnormally, the LAP score remains low.
LAP becomes an important diagnostic tool when there is concern about chronic myelogenous leukemia (CML) versus a leukemoid reaction. Both conditions can present with extremely high white blood cell counts dominated by neutrophils. However, in CML, the neutrophils are part of a malignant proliferation and do not function normally, leading to a low LAP score. A patient with CML often has additional findings such as a markedly enlarged spleen, causing left upper quadrant abdominal discomfort or early satiety. In contrast, a leukemoid reaction—typically triggered by infection or severe stress—produces very high white counts as part of a normal physiological response, resulting in a high LAP score. Therefore, LAP is the answer when distinguishing between these two conditions in the setting of significant neutrophilia.
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