LAP Test (Leukocyte Alkaline Phosphatase)

Leukocyte Alkaline Phosphatase (LAP) Test: LAP Score in Leukemia Diagnosis

LAP test

Synonyms

LAP, LAP Score, LAP Smear

Abstract

A cytochemical test that helps differentiate **chronic myeloid leukemia (CML)** from **leukemoid reactions**, especially in the diagnosis of **myeloproliferative disorders**.

Specimen

Whole blood (via fingerstick)

Container

Long glass slides with blood smears

Collection Instructions

• Smear blood from a fingerstick onto **six long slides**.
• Allow slides to **air dry immediately**.
• Transport to hematology within **30 minutes**, ideally.

Storage Instructions

• Fix slides within **8 hours** (preferably within 30 minutes) using:
  • Cold 10% formalin methanol, or
  • Citrated buffered acetone
• After fixation, smears may be stored for **up to 8 weeks** before staining.

Reason to Reject Sample

• EDTA-anticoagulated blood used
• Transport delay beyond 30 minutes
• Neutrophil count < 1000/mm³

Reference Range

Normal LAP Score: 11–95

Use

• Differentiating **chronic granulocytic leukemia (CML)** from **leukemoid reaction**
• Evaluating **polycythemia vera**, **myelofibrosis**, and **Hodgkin’s disease**

Limitations

• **Elevated LAP** seen in pregnancy, infections, postoperative states, and Hodgkin’s disease.
• Test requires a sufficient number of **mature neutrophils** in peripheral blood.
• **Low LAP scores** do not vary with total WBC count in CML.

Methodology

The LAP enzyme reacts with **naphthol substrates** and **diazonium dyes** (e.g., fast blue RR), producing a **colored precipitate**. Based on staining intensity, neutrophils are scored from **0 to 4+**. The **LAP score** is the sum of the scores for **100 neutrophils**.

Additional Information

• Low LAP scores are seen in:
  • Chronic myeloid leukemia (CML)
  • Paroxysmal nocturnal hemoglobinuria (PNH)
  • Hereditary hypophosphatasia
  • Thrombocytopenic purpura
• High LAP scores observed in:
  • Polycythemia vera
  • Myelofibrosis
  • Hairy cell leukemia
  • Leukemoid reaction
  • Hodgkin’s lymphoma
• LAP does **not increase** during **sickle cell crisis**, possibly due to hypothalamic-pituitary-adrenal axis dysfunction.
• **Northern blotting** shows absent LAP mRNA expression in CML, indicating no or defective gene transcription.

References

1. Jacobs, Demott, Finley, Horvat, Kasten.JR, & Tilzer. “Laboratory Test Handbook.” Lexi-Comp Inc, 1994.
2. Catovsky D. “Leukocyte Cytochemical and Immunological Techniques.” Practical Haematology, 7th ed, 1991.
3. Cline MJ. “Benign Quantitative Granulocyte and Monocyte Disorders.” Hematology: Clinical and Laboratory Practice, Vol 2, 1993.
4. Kaplow LS. “Cytochemistry of Leukocyte Alkaline Phosphatase.” Am J Clin Pathol, 1963; 39:439–49.