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
Criteria
Points
high nuclear-to-cytoplasmic ratio in >= 75% of leukemic cells
+1
low nuclear-to-cytoplasmic ratio in >= 25% of leukemic cells
-1
0 or 1 small nucleoli in >= 75% of cells
+1
1 or more prominent nucleoli in >= 25% of cells
-1
irregular nuclear membranes in >= 25% of cells
-1
large cells > 50% of cells
-1
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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