Leucocyte Alkaline Phosphatase (LAP) Score

What is the LAP score test?

The LAP score test measures the activity of the enzyme leukocyte alkaline phosphatase in neutrophils. It helps differentiate between different types of blood disorders, especially distinguishing chronic myeloid leukemia (CML) from leukemoid reactions.


Why is this test ordered?

The test is useful when there is an abnormal increase in white blood cell count. A low LAP score is suggestive of CML, while a high score might indicate a leukemoid reaction or other inflammatory conditions. It helps guide hematologic diagnosis and treatment.


What does a high or low LAP score mean?

A low LAP score supports a diagnosis of CML, while a high score may suggest a leukemoid reaction, polycythemia vera, or infection. The LAP score is one part of a larger diagnostic workup and should not be used in isolation.


How is the test done?

The test is performed on a peripheral blood smear. Neutrophils are stained, and the enzyme activity is evaluated microscopically. A numerical score is then calculated based on the intensity of staining in several cells.


Are there any limitations to the test?

The LAP score can be subjective as it depends on microscopic examination. It is also influenced by factors like pregnancy, infections, and certain medications. Modern tests like BCR-ABL molecular assays have largely replaced LAP scores in the diagnosis of CML.