Leukocyte Alkaline Phosphatase (LAP)

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Subject: Leukocyte Alkaline Phosphatase (LAP)

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Definition

  • LAP, or neutrophil alkaline phosphatase, refers to a staining reaction of peripheral blood smears. It reflects the presence of LAP in neutrophils and their precursors. Normally, about 20% of mature neutrophils show stainable leukocyte LAP activity.

  • Normal range: score of 11–95. The scoring is based on counting 100 neutrophils and grading the stained granules from 0 to 4 on the basis of the intensity and appearance of the precipitated dye in the cytoplasm.

Use

  • LAP stain helps differentiate a severe neutrophilia (leukemoid reaction) and myeloproliferative neoplasms, where it is increased, from chronic myeloid leukemia, in which case it is decreased or absent.

  • With the advent of modern diagnostic technologies, the use of LAP stains has diminished.

Interpretation

Increased In

  • Leukemoid reaction

  • Polycythemia vera and essential thrombocythemia (occasionally it may be normal)

  • Idiopathic myelofibrosis

  • Pregnancy

  • Trisomy 21 and Klinefelter syndrome

Decreased In

  • Chronic myeloid leukemia

  • PNH and pernicious anemia

  • Congenital hypophosphatasia

Limitations

  • Blood that is not processed soon after drawing may cause low LAP scores.

  • There is observer-dependent variability.

 
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