Leukocyte Alkaline Phosphatase (LAP) Score: Key Clinical Distinctions
The LAP score primarily distinguishes chronic myeloid leukemia (CML), which shows markedly decreased or absent LAP activity, from leukemoid reactions and other myeloproliferative disorders (particularly polycythemia vera and myelofibrosis), which demonstrate elevated LAP scores. 1
Primary Hematologic Differentiations
CML vs. Leukemoid Reaction vs. Other Myeloproliferative Disorders
CML patients consistently show low LAP scores (range 0.07-1.75, median 0.79 × 10⁻⁵ mV·s/cell), which is significantly lower than healthy individuals (range 0.61-8.49, median 2.04 × 10⁻⁵ mV·s/cell). 1
Leukemoid reactions demonstrate markedly elevated LAP scores (range 3.84-47.24, median 9.58 × 10⁻⁵ mV·s/cell), making this the most reliable way to distinguish reactive leukocytosis from CML. 1
The LAP deficiency in CML results from LAP messenger RNA deficiency and is not directly dependent on p210 tyrosine kinase activity, representing a fundamental characteristic of the disease related to precocious release of immature cells from bone marrow. 2
Plasma Cell Disorders
LAP score differentiates multiple myeloma from monoclonal gammopathy of undetermined significance (MGUS), with MGUS patients showing LAP scores comparable to normal controls while multiple myeloma patients demonstrate significantly elevated scores (p < 0.001). 3
Within multiple myeloma, LAP scores correlate with disease activity: scores increase with tumor burden, are higher in active disease versus plateau phase, and rise during relapse compared to stable disease. 3
Secondary Clinical Applications
Metastatic Cancer Detection
LAP scores are significantly elevated in metastatic breast and colorectal cancer (mean 244) compared to healthy controls (mean 61), with no overlap in 95% confidence intervals between groups. 4
LAP demonstrates superior sensitivity compared to serum alkaline phosphatase for detecting metastatic disease, as considerable percentages of metastatic patients have normal serum alkaline phosphatase levels. 4
Brain Pathology Differentiation
- LAP scores distinguish cerebrovascular disease from brain tumors: CVD patients show mean scores of 96 ± 87, primary brain tumor patients 155 ± 40 (p < 0.03 vs. CVD), and metastatic brain tumor patients 251 ± 55 (p < 0.001 vs. both groups). 5
Neonatal Infection
LAP scores differentiate bacterial from viral infections in newborns: control newborns show mean LAP of 87.5, viral illness 160.3, and bacterial infection 232.7, with statistically significant differences between each group. 6
An LAP score greater than 200 is highly suggestive of bacterial infection in neonates, demonstrating higher sensitivity than band count or bands/segs ratio for distinguishing infection types. 6
Critical Pitfalls to Avoid
Do not rely on LAP scoring alone for definitive diagnosis of CML—molecular confirmation with BCR-ABL1 testing by RT-PCR or FISH is mandatory per current guidelines. 7
LAP is a marker of terminally differentiated neutrophils (CD11b-bright/CD16-bright positive cells), so interpretation requires adequate numbers of mature granulocytes in the sample. 2
G-CSF exposure increases LAP expression in both normal and CML granulocytes, potentially confounding results if patients have received growth factor support. 2