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WBCs in peripheral blood samples retain enzymatic activity and can alter substrates added in the laboratory. The presence of enzymatic activity is useful in studying cells that are so morphologically abnormal on stained smear that it is difficult to determine their cell line of origin (see Fig. 1-2). The two most common WBC enzyme tests are the test for leukocyte alkaline phosphatase, an enzyme found in neutrophils, and the periodic acid-Schiff stain, which tests for enzymes found in granulocytes and erythrocytes. Both tests are used to diagnose hematologic disorders, especially leukemias. Specific causes of alterations in WBC enzymes are presented in Table 1-12. Another WBC enzyme test, tartrate-resistant acid phosphatase (TRAP), is performed to diagnose hairy cell leukemia, because this enzyme activity is present in the lymphocytic cells of this type of leukemia. Additional details for each test are briefly discussed subsequently.

Leukocyte Alkaline Phosphatase

Leukocyte alkaline phosphatase (LAP) is an enzyme found in neutrophils. This enzyme is completely independent of serum alkaline phosphatase, which reflects osteoblastic activity and hepatic function. The LAP content of neutrophils increases as the cells mature; therefore, the LAP study is useful in assessing cellular maturation and in evaluating departures from normal differentiation.

The LAP study is used to distinguish among various hematologic disorders. For example, LAP increases in polycythemia vera, myelofibrosis, and leukemoid reactions to infections, but decreases in chronic granulocytic leukemia. Because all of these conditions have increased numbers of immature circulating neutrophils, LAP scores can be helpful in differentiating among them.

Periodic Acid-Schiff Stain

In the periodic acid-Schiff (PAS) stain, compounds that can be oxidized to aldehydes are localized by brilliant fuschia staining. Many elements in many tissues are PAS-positive, but in blood cells the PAS-positive material of diagnostic importance is cytoplasmic glycogen. Early granulocytic precursors and normal erythrocytic precursors are PAS-negative. Mature RBCs remain PAS-negative, but granulocytes acquire increasing PAS positivity as they mature.³⁷

[Section Outline]

• Leukocyte Alkaline Phosphatase
• Periodic Acid-Schiff Stain

• Leukocyte alkaline phosphatase
  • 13-130 U
• Periodic acid-Schiff stain
  • Granulocytes–positive
  • Agranulocytes–negative
  • Granulocytic precursors–negative
  • Erythrocytes–negative
  • Erythrocytic precursors–negative
• Tartrate-resistant acid phosphatase
  • Activity absent

• Identification of morphologically abnormal WBCs on stained smear
• Suspected leukemia or other hematologic disorders (see Table 1-12, p. 37)

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

A capillary sample is generally preferred for these tests. The sample is spread on a slide, fixed, and stained.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample (see Appendix I).