Leukocyte Alkaline Phosphatase (LAP) Score
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
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. 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.
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)
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.
Smears can be maintained at room temperature for seven days.
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