Leukocyte Alkaline Phosphatase (LAP) Score

CPT: 85540
Print Share

Test Details

Test Includes

Blood films are stained and 100 neutrophilic (segmented and band forms only) 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.


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


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.


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.

Reference Interval


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

Specimen Requirements


Smears made from fingerstick or heparinized specimen; heparin (green-top) tube − slides must be made within 24 hours of collection.


Six properly labeled blood films (slides)

Minimum Volume

Two freshly made blood films


Slides; green-top (heparin) tube − slides must be made within 24 hours of 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. Do not fix slides with any type of fixative prior to submission.

Storage Instructions

Smears can be maintained at room temperature for seven days.

Causes for Rejection

Improper labeling; heparin tube specimen more than 24 hours old; frozen slides; slides more than seven days old; slides made from any anticoagulant other than heparin

Clinical Information


1. 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 Apr; 73(5):1113-1115. 2930836
2. Rosenbloom BE, Odell WD, Tanaka KR. Pituitary-adrenal axis function in sickle cell anemia and its relationship to leukocyte alkaline phosphatase. Am J Hematol. 1980; 9(4):373-379. 7011001


Miale JB. Laboratory Medicine, Hematology. 6th ed. St Louis, Mo: CV Mosby Co;1982: 207-209, 871.


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
001966 Leukocyte Alkaline Phos Score 4659-9 001966 Leukocyte Alkaline Phos Score 4659-9

For Providers

Please login to order a test.


© 2018  Laboratory Corporation of America® Holdings and Lexi-Comp Inc. All Rights Reserved.

CPT Statement/Profile Statement

The LOINC® codes are copyright © 1994-2018, Regenstrief Institute, Inc. and the Logical Observation Identifiers Names and Codes (LOINC) Committee. Permission is granted in perpetuity, without payment of license fees or royalties, to use, copy, or distribute the LOINC® codes for any commercial or non-commercial purpose, subject to the terms under the license agreement found at https://loinc.org/license/. Additional information regarding LOINC® codes can be found at LOINC.org, including the LOINC Manual, which can be downloaded at LOINC.org/downloads/files/LOINCManual.pdf