Leukocyte Alkaline Phosphatase (LAP) Score
| Leukocyte Alkaline Phosphatase (LAP) Score | | | |
| Number | | 001966 |
| CPT | | 85540 |
| Synonyms | | LAP Score |
| Test Includes | | Blood films are stained and 100 neutrophilic leukocytes are scored from 0 to 4+ on the basis of the intensity of the precipitated dye in their cytoplasm. The values of the 100 cells are added and the total score reported. |
| Specimen | | Smears |
| Volume | | Six properly labeled blood films (slides) |
| Minimum Volume | | Two freshly made blood films |
Container | | Slides
For Raritan only use green-top (heparin) tube. |
| Collection | | Prepare smears on six (frosted end) slides from either fingerstick blood or immediately after drawing into heparin Vacutainer® and mixing. Air dry the slides and label with patient's name. Submit all. Note: Do not use EDTA anticoagulant. |
| Storage Instructions | | Smears are to be maintained at room temperature for up to 48 hours. |
| Causes for Rejection | | Improper labeling; specimen more than 48 hours old |
| Reference Interval | | 25-130 |
| Use | | Aids in the differential diagnosis of chronic myelocytic leukemia (CML) versus leukemoid reaction; aids in the evaluation of polycythemia vera, myelofibrosis with myeloid metaplasia, and paroxysmal nocturnal hemoglobinuria |
| Limitations | | Pregnancy, increased number of immature forms of neutrophils, and postoperative or “stressful” states are associated with increased scores. The differential must have adequate numbers of mature neutrophilic granulocytes to perform the LAP. |
| Methodology | | Enzyme reaction with leukocyte alkaline phosphatase liberating naphthol or a substituted naphthol compound which then couples with fast blue RR or other chromogen to form an insoluble precipitate. Color of the precipitate relates to the type of substituted naphthol substrate and diazonium dye used (color is reagent dependent). Cells are scored as to the degree of phosphatase activity present, 0 to 4+. One hundred cells are counted and the score totaled. |
| Additional Information | | Low scores have been associated with CML, PNH, thrombocytopenic purpura, and hereditary hypophosphatasia. In CML regardless of the total white count, the score remains low. In CML, it has been demonstrated that the mRNA for leukocyte alkaline phosphatase by Northern blotting is undetectable.1 This suggests either rapid degradation of the message or no transcription of the LAP gene. In nonleukemic neutrophilia, the LAP rises as the WBC rises. High scores have been seen in polycythemia vera, myelofibrosis, aplastic anemia, mongolism, hairy cell leukemia, leukemoid reactions, and neutrophilia either physiological or secondary to infection. It is also increased in Hodgkin disease. Serial LAP activity can be a useful adjunct in evaluating the activity of Hodgkin disease as well as its response to therapy. Increase in LAP does not occur in cases of sickle cell crisis, possibly due to zinc deficiency (leukocyte alkaline phosphatase is a zinc metalloenzyme) but more likely relating to a mild defect in the hypothalamic-pituitary-adrenal axis with decreased plasma cortisol response in patients in sickle cell crisis.2 |
| Footnotes | | - Rambaldi A, Terao M, Bettoni S, et al, “Differences in the Expression of Alkaline Phosphatase mRNA in Chronic Myelogenous Leukemia and Paroxysmal Nocturnal Hemoglobinuria Polymorphonuclear Leukocytes,” Blood, 1989, 73(5):1113-5.
- Rosenbloom BE, Odell WD, and Tanaka KR, “Pituitary-Adrenal Axis Function in Sickle Cell Anemia and Its Relationship to Leukocyte Alkaline Phosphatase,” Am J Hematol, 1980, 9(4):373-9
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| References | | Miale JB, Laboratory Medicine, Hematology, 6th ed, St Louis, MO: CV Mosby Company, 1982, 207-9, 871. |
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