Diagnosis of Acute Lymphoblastic Leukemia (ALL) is established through the patient's physical examination, symptoms, and subsequent laboratory tests, as previously mentioned. An algorithmic approach would begin with a complete blood count (CBC), assessing white blood cell (leukocyte), hemoglobin, and platelet counts. If these values suggest acute leukemia, a peripheral blood smear is examined to identify the presence of blast cells. Whether or not blast cells are seen, if acute leukemia is still suspected, a bone marrow biopsy is performed. This allows for both pathological examination and genetic/molecular testing from the aspirated bone marrow sample, which may identify features associated with ALL. Flow cytometry, which identifies surface antigens on leukemia cells, is also performed. The diagnosis is confirmed by the results of all these tests.

A bone marrow blast percentage >20% and confirmation of these cells as lymphoblasts by flow cytometry establishes the diagnosis. Genetic testing helps determine the genetic subtype of ALL and identifies prognostic genetic features to guide treatment. At the time of diagnosis, blast percentages exceed 90% in more than half of patients.