What laboratory finding is diagnostic for Acute Lymphoblastic Leukemia (ALL)?

The definitive diagnostic criterion for Acute Lymphoblastic Leukemia (ALL) is the identification of more than 25% lymphoblasts in the bone marrow on biopsy. In ALL, the proliferation of these immature white blood cells leads to a significant increase in lymphoblasts, which distinguishes this type of leukemia from other hematological disorders. The presence of these lymphoblasts disrupts normal hematopoiesis, often resulting in symptoms such as anemia, bleeding, and infections due to the reduced production of healthy blood cells.

Other findings, while often associated with leukemia, are not specific enough to make a diagnosis of ALL. For instance, a low platelet count can occur in many conditions, including other types of leukemia, vascular disorders, and bone marrow involvement from various diseases. Auer rods are cytoplasmic inclusions found in certain subtypes of Acute Myeloid Leukemia (AML), thus they are not relevant for diagnosing ALL. A positive PAS stain indicates the presence of glycogen accumulation in cells but is not a definitive indicator for ALL. Therefore, the clear diagnostic marker for ALL is the significant presence of lymphoblasts in the bone marrow.