Acute nonlymphocytic leukemias of childhood. Inter-observer variability and problems in the use of the FAB classification.

The French-American-British (FAB) classification system and some recent modifications were applied to 486 children with a diagnosis of acute nonlymphocytic leukemia (ANLL) to determine the distribution of the subtypes in children, to document the extent of inter-observer variation in assigning subtypes, and to examine the reasons for the differences. The distribution of FAB subtypes of childhood ANLL was similar to that reported for adults. In the initial year of the study, the inter-observer concurrence between the institutional diagnosis and the reviewing pathologists was 50%, but in the more recent years, concurrence between institutions and the review pathologist has approached 80%, averaging around 73% for the entire study. Many problems remain to be solved with this classification system, including the imprecision in wording, the subjectiveness of the interpretation, errors due to the random distribution of cells, and the current lack of evidence that certain FAB subtypes, such as M1 and M2, differ significantly in terms of biological behavior and prognosis.