Medically Reviewed by: Dr. Dipak Ladda, M.D.
Expertise: Consultant Pathologist
Last Updated: July 16, 2026
Medical Analysis
Understanding the Leukocyte Alkaline Phosphatase (LAP) Score and Diagnostic Testing
The Leukocyte Alkaline Phosphatase (LAP) score, also frequently referred to as the Neutrophil Alkaline Phosphatase (NAP) score, is a critical diagnostic measurement used to assess the activity of the alkaline phosphatase enzyme within white blood cells [1, 2]. This laboratory metric provides essential insights into the functional health of an individual’s bone marrow and is a staple in the clinical evaluation of various hematological and blood-related disorders [7, 15].
Clinical Indications and Applications of LAP Testing
Medical professionals utilize the LAP score to investigate and differentiate between several significant clinical conditions [9, 12]. Key indications for ordering this test include:
Chronic Myeloid Leukaemia (CML) [6, 8]
Leukemoid reaction [4, 9]
Severe infection [2, 10]
Paroxysmal Nocturnal Haemoglobinuria (PNH) [3, 12]
Polycythemia Vera [5, 10]
The primary clinical utility of this test lies in its ability to distinguish between the cells of a leukemoid reaction, which show increased enzymatic activity, and those associated with Chronic Myeloid Leukaemia (CML), which typically exhibit decreased activity [6, 9]. The biological principle behind this is that alkaline phosphatase activity is present in varying degrees within the neutrophil and band forms of granulocytes, and occasionally in B lymphocytes [1, 14].
Staining Methodology and Laboratory Procedure
To accurately determine the LAP score, a specific staining procedure is followed in the laboratory [1, 15]:
Prepare a blood smear from a finger prick and allow it to air dry.
Fix the smear using 4% formalin or methanol for 30 seconds [1, 15].
Rinse with tap water and allow it to air dry.
Apply the working solution (consisting of Citric acid and sodium citrate at 53.25 Gms/Lit) to the smear and incubate for 30 minutes [1].
Rinse with tap water and allow it to air dry.
Counterstain the smear with Neutral Red or Safranin for 5 minutes [1].
Perform a final rinse with tap water and allow it to air dry.
Scoring System and Grading Standards
The quantitative assessment requires counting 100 neutrophils and assigning them a score ranging from 0 to 4+ based on the intensity of the stain [1, 15]. The final LAP score is calculated by summing the individual scores of these cells [1].
(0): No stain observed
(+1): Faint stain observed
(+2): Moderate stain intensity
(+3): Strong stain intensity
(+4): Strong stain without cytoplasmic background
The normal reference range for a healthy individual is typically between 30 and 185 [2, 15].
Interpretation: Factors Affecting LAP Scores
Various clinical conditions influence whether the LAP score is elevated or decreased [9, 12].
| LAP Decreased In | LAP Elevated In |
| CML (Chronic Myeloid Leukaemia) [6, 8] | Leukaemoid reaction [4, 9] |
| Paroxysmal Nocturnal Hemoglobinuria [12] | Sickle cell anemia [12] |
| Hypophosphatasia [15] | Multiple myeloma [15] |
| Hodgkin’s disease [13] | |
| Pregnancy [15] | |
| Polycythemia vera [5, 10] | |
| Aplastic anemia [15] | |
| Obstructive jaundice [15] |
Additionally, certain conditions do not significantly impact the LAP result, including untreated hemolytic anemia, lymphosarcoma, viral hepatitis, and secondary polycythemia [15].
Clinical Utility Table
The following table summarizes the clinical applications and expected outcomes of the LAP scoring system [7, 15].
| Clinical Utility | Score Interpretation | Associated Condition |
| Differentiate CML | Decreased enzyme activity | Chronic Myeloid Leukemia (CML) [6, 8] |
| Differentiate Leukemoid Reaction | Increased activity | Severe infection/inflammation [2, 10] |
| Differentiate Myelofibrosis or PV from CML | Elevated or normal | Primary Myelofibrosis, Polycythemia Vera [5, 10] |
| Distinguish PV from Secondary Erythrocytosis | Elevated levels | Reflects clonal proliferation [5] |
| Monitor CML Therapy | Rising score | CML in remission / therapeutic response [8] |
For Non-Medicos
A Simple Guide to the LAP Score Test
If you have been told that your doctor is ordering an “LAP Score” test, this guide will help you understand what it is and why it matters in simple terms.
What is an LAP Score?
The LAP (Leukocyte Alkaline Phosphatase) score is a blood test that looks at a specific enzyme—a type of protein—found inside your white blood cells [1, 2]. By measuring how active this enzyme is, doctors can learn more about how your bone marrow is functioning and gain clues about blood disorders [7, 15].
Why do doctors use this test?
The most important reason for this test is to help doctors tell the difference between two very different conditions:
Leukemoid Reactions: This is usually a temporary reaction your body has to a severe infection or inflammation. In this case, the LAP score is typically high [4, 9].
Chronic Myeloid Leukaemia (CML): This is a type of blood cancer where the white blood cells behave differently. In this case, the LAP score is typically low [6, 8].
By looking at the “score” (the amount of enzyme activity), the doctor can quickly see if your body is responding to an infection or if there is a more serious underlying blood condition that needs further investigation [9, 11].
What happens during the test?
The process is straightforward [15]:
A small amount of blood is taken, usually from a finger prick.
The blood is spread on a slide to create a “smear.”
The lab technician adds special stains to the slide [1].
The technician then looks through a microscope to count and grade 100 white blood cells based on how dark they appear under the stain. The more intense the color, the higher the “score” [1].
Things that change your score
Elevated (High) Scores: Can be seen during pregnancy, severe infections, or other conditions like polycythemia vera [5, 15].
Decreased (Low) Scores: Can be seen in specific blood disorders like CML or Paroxysmal Nocturnal Hemoglobinuria [6, 12].
In summary: The LAP score is a highly reliable “clue” that helps doctors decide which path to take when diagnosing blood disorders [7, 15]. It is not a final diagnosis, but rather a vital part of your overall blood health evaluation [15]. Always discuss your specific results with your doctor to understand what they mean for your personal health journey.
References:
Kaplow, L. S. (1963). Cytochemistry of leukocyte alkaline phosphatase: Use of complex naphthol AS phosphates in azo dye-coupling technics. American Journal of Clinical Pathology, 39(5), 439-449.
Hayhoe, F. G., & Quaglino, D. (1958). Cytochemical demonstration and measurement of leucocyte alkaline phosphatase activity in normal and pathological states. British Journal of Haematology, 4(4), 375-389.
Rosner, F., & Lee, S. L. (1972). Leukocyte alkaline phosphatase. Annals of Internal Medicine, 77(4), 659-660.
Scott, R. B., & Cheng, D. C. (1971). Leukocyte alkaline phosphatase: A clinical evaluation. Southern Medical Journal, 64(4), 481-484.
Tefferi, A. (2010). Polycythemia vera and essential thrombocythemia: 2010 update on diagnosis, risk stratification and management. American Journal of Hematology, 85(11), 869-875.
Swirsky, D. M., & Li, Y. (1985). Leukocyte alkaline phosphatase in the diagnosis of chronic myeloid leukemia. Clinical and Laboratory Haematology, 7(3), 221-228.
Bain, B. J. (2015). Blood Cells: A Practical Guide. 5th Edition. Wiley-Blackwell.
Basso, G., & Foa, R. (1982). Leukocyte alkaline phosphatase in the monitoring of patients with chronic myeloid leukemia. Cancer, 50(9), 1774-1778.
Wiernik, P. H., & Serpick, A. A. (1972). Clinical significance of leukocyte alkaline phosphatase activity in leukemia and other hematologic disorders. American Journal of the Medical Sciences, 264(3), 197-204.
McFarland, W., & Dameshek, W. (1963). Relationship of neutrophilic alkaline phosphatase to myeloid proliferation. Blood, 22(4), 466-470.
Arber, D. A., et al. (2016). The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood, 127(20), 2391-2405.
Gulati, G. L., et al. (1986). Neutrophil alkaline phosphatase: A review. Laboratory Medicine, 17(1), 16-20.
Rappaport, H., & Johnson, F. B. (1955). Intracellular alkaline phosphatase activity of the leukocytes in malignant lymphomas. Blood, 10(9), 920-927.
Yam, L. T. (1974). Diagnostic significance of lymphocyte alkaline phosphatase in leukemias. Journal of Histochemistry & Cytochemistry, 22(7), 808-812.
Miale, J. B. (1982). Laboratory Medicine: Hematology. 6th Edition. C.V. Mosby Company.
FAQ’s:
What is the LAP score?
The LAP score measures alkaline phosphatase enzyme activity within white blood cells to evaluate bone marrow health.Why is the LAP test essential?
It helps distinguish between leukemoid reactions with high activity and Chronic Myeloid Leukaemia with decreased activity.What is the normal range?
The normal reference range for the Leukocyte Alkaline Phosphatase score is between 30 and 185.How is the test performed?
A finger-prick blood smear is fixed, stained with a working solution, counterstained, and then microscopically evaluated.How is the score calculated?
Technicians count 100 neutrophils, assign a grade (0 to +4) to each, and sum the total scores.What conditions elevate LAP levels?
Leukemoid reactions, pregnancy, Polycythemia vera, Hodgkin’s disease, and multiple myeloma can cause elevated LAP levels.Which diseases decrease LAP?
Chronic Myeloid Leukaemia, Paroxysmal Nocturnal Hemoglobinuria, aplastic anemia, and hypophosphatasia are associated with decreased LAP levels.Are there unaffected conditions?
Conditions like viral hepatitis, lymphosarcoma, and secondary polycythemia do not affect the patient’s LAP score results.What does a +4 grade mean?
A grade of +4 represents a strong stain without any cytoplasmic background during the neutrophil evaluation.How is CML therapy monitored?
A rising LAP score typically indicates the return of normal hematopoiesis and a positive therapeutic response.
