Leukocyte Alkaline Phosphatase

Synonym/Acronym:
LAP, LAP score, LAP smear.

Rationale
To monitor response to therapy in Hodgkin disease and diagnose other disorders of the hematological system such as aplastic anemia.

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.

Normal Findings
(Method: Microscopic evaluation of specially stained blood smears) 25 to 130 (score based on 0 to 4+ rating of 100 neutrophils).

Critical Findings and Potential Interventions
N/A

Overview

(Study type: Blood collected in a lavender-top [EDTA] tube; related body system: Circulatory/Hematopoietic and Immune systems.) Alkaline phosphatase is present in the cytoplasm of neutrophilic granulocytes from the metamyelocyte to the segmented stage of development. The study involves counting 100 neutrophils from a stained smear. Segmented and band forms are counted; eosinophils, basophils, and other immature neutrophil forms are excluded. The reaction is subjectively scored from 0 to 4 based on the number of stained granules observed in the counted cells as well as the intensity of the staining. The number of cells counted is then multiplied by the score to arrive at the result. Leukocyte alkaline phosphatase (LAP) concentrations may be altered by the presence of infection, stress, chronic inflammatory diseases, Hodgkin disease, and hematological disorders. Low levels are associated with the presence of leukemic leukocytes, and high levels are present in normal white blood cells (WBCs). The test has been used for many years as a supportive test in the differential diagnosis of leukemia.

As time from its inception has passed and other technologies such as polymerase chain reaction (PCR) and fluoresence in-situ hybridization (FISH) have emerged, the LAP assay is used less often. The current World Health Organization classification of chronic myeloproliferative tumors does not use the LAP score; rather, by consensus, experts have developed international criteria such as morphology, immunophenotype, genetics, and clinical features. PCR and FISH technologies are used to evaluate blood or bone marrow specimens collected in a sodium heparin collection tube. The testing can be used to identify BCR/ABL-1 or JAK2 gene rearrangements that assist in the classification and treatment of hematologic conditions such as chronic myelogenous leukemia (CML), polcythemia vera, and other myeloproliferative tumors.

Indications

  • Differentiate chronic myelocytic leukemia from other disorders that increase the WBC count.
  • Monitor response of Hodgkin disease to therapy.

Interfering Factors

Factors that may alter the results of the study

Drugs that may increase the LAP score include steroids.

Potential Medical Diagnosis: Clinical Significance of Results

Increased In:

Conditions that result in an increase in leukocytes in all stages of maturity will reflect a corresponding increase in LAP.

  • Aplastic leukemia
  • Chronic inflammation
  • Down syndrome
  • Hairy cell leukemia
  • Hodgkin disease
  • Leukemia (acute and chronic lymphoblastic)
  • Myelofibrosis with myeloid metaplasia
  • Multiple myeloma
  • Polycythemia vera (increase in all blood cell lines, including leukocytes)
  • Pregnancy
  • Stress
  • Thrombocytopenia

Decreased In:

  • Chronic myelogenous leukemia
  • Hereditary hypophosphatemia (insufficient phosphorus levels)
  • Idiopathic thrombocytopenia purpura
  • Nephrotic syndrome (excessive loss of phosphorus)
  • Paroxysmal nocturnal hemoglobinuria (possibly related to the absence of LAP and other proteins anchored to the red blood cell wall, resulting in complement-mediated hemolysis)
  • Sickle cell anemia
  • Sideroblastic anemia

Nursing Implications

Before the Study: Planning and Implementation

Teaching the Patient What to Expect

  • Inform the patient this test can assist in evaluating for blood disorders.
  • Explain that a blood sample is needed for the test.

After the Study: Potential Nursing Actions

Treatment Considerations

  • Instruct the patient to avoid exposure to infection if WBC count is decreased.

Followup Evaluation and Desired Outcomes

  • Recognizes the value of this study in relation to disease management and monitoring.

Leukocyte Alkaline Phosphatase is a sample topic from the Davis's Lab & Diagnostic Tests.

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