The Morphological Classification of Acute Lymphoblastic Leukaemia: Concordance among Observers and Clinical Correlations

The degree of concordance in the morphological classification of ALL was assessed by the FAB group after two successive reviews of 200 and 100 slides respectively. As a result, a simple scoring system for types L1 and L2 is proposed based on the following four features: (1) nuclear cytoplasmic ratio, (2) presence, prominence and frequency of nucleoli, (3) regularity of nuclear membrane outline, and (4) cell size. By this method, the overall concordance by seven observers (agreement of 7:0 or 6:1 only) increased from 63% to 84%. A significant difference in the incidence of the ALL morphological types in children (less than or equal to 15 years) and adults (greater than 15 years) was found: 74% of L1 cases were children while 66% of L2 cases were adults (P less than 0.001). No significant differences were observed in the incidence of L3 in children and adults or between L1 and L2 according to the membrane phenotype of the blast cells. All L3 cases had B-cell characteristics. A better prognosis for L1 and a higher relapse rate for L2 has been found in several recent reports; The present study may facilitate the morphological analysis of ongoing clinical trials in ALL by improving the reproducibility of the FAB classification.