Differentiating Leukemoid Reaction from CML using LAP Score

The Leukocyte alkaline phosphatase (LAP) test is crucial for distinguishing between a leukemoid reaction and chronic myeloid leukemia (CML).

• LAP Test Principle: LAP is an enzyme present in mature neutrophils. The test measures the activity of this enzyme.
• Chronic Myeloid Leukemia (CML): In CML, the neoplastic cells (granulocytes) typically have very low or absent LAP activity. Therefore, the LAP score is usually low.
• Leukemoid Reaction: This is a reactive condition causing a high white blood cell count. The neutrophils involved are mature and healthy, exhibiting normal or even increased LAP activity. Consequently, the LAP score is typically high.

This difference in enzyme activity allows for effective differentiation between the two conditions.

Choosing the Correct Test

Based on the enzymatic activity differences:

• LAP score is characteristically low in CML and high in leukemoid reactions.
• LCA, MPO, and TRAP tests are less specific for this particular differentiation.

Therefore, the LAP test is the key laboratory test used to differentiate between a leukemoid reaction and CML.