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Overview :
The French-American-British (FAB) classification of acute
lymphoblastic leukemia (ALL) separates out 3 types (L1, L2, L3) based on the
morphologic appearance of the blastic cells. Separation of L1 and L2 types can
be done by using a simple scoring system.
Features used to classify:
(1) nuclear to cytoplasmic ratio
(2) presence, prominence and frequency of nucleoli
(3) regularity of the nuclear membrane outline
(4) cell size
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Criteria
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Points
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high nuclear-to-cytoplasmic ratio in >= 75% of leukemic
cells
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+1
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low nuclear-to-cytoplasmic ratio in >= 25% of leukemic
cells
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-1
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0 or 1 small nucleoli in >= 75% of cells
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+1
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1 or more prominent nucleoli in >= 25% of cells
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-1
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irregular nuclear membranes in >= 25% of cells
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-1
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large cells > 50% of cells
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-1
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FAB score for L1 and L2 variants =
= SUM(points for all criteria)
Interpretation:
• minimum score -4
• maximum score +2
• score of 0 to +2 indicates L1
• score of -1 to -4 indicates L2
NOTE: The
1997 WHO Classification of Hematologic Malignancies felt that the FAB terms L1,
L2 and L3 should no longer be retained. (Modern Pathology. 2000; 13: 193-207,
page 198-199)
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