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In addition to cell morphology and cytogenetics, immunophenotyping, a laboratory technique used to identify proteins that are expressed on their cell surface, is a key component in the diagnosis of ALL. The preferred method of immunophenotyping is through flow cytometry. In the malignant lymphoblasts of ALL, expression of terminal deoxynucleotidyl transferase (TdT) on the cell surface can help differentiate malignant lymphocyte cells from reactive lymphocytes, white blood cells that are reacting normally to an infection in the body. On the other hand, myeloperoxidase (MPO), a marker for myeloid lineage, is typically not expressed. Because precursor B cell and precursor T cells are morphologically identical, immunophenotyping can help differentiate the subtype of ALL and the level of maturity of the malignant white blood cells. The subtypes of ALL as determined by immunophenotype and according to the stages of maturation.^[1]

An extensive panel of monoclonal antibodies to cell surface markers, particularly CD or cluster of differentiation markers, are used to classify cells by lineage. Below are immunological markers associated with B cell and T cell ALL.^[2]

Cytogenetic analysis has shown different proportions and frequencies of genetic abnormalities in cases of ALL from different age groups. This information is particularly valuable for classification and can in part explain different prognosis of these groups. In regards to genetic analysis, cases can be stratified according to ploidy, number of sets of chromosomes in the cell, and specific genetic abnormalities, such as translocations. Hyperdiploid cells are defined as cells with more than 50 chromosomes, while hypodiploid is defined as cells with less than 44 choromosomes. Hyperdiploid cases tend to carry good prognosis while hypodiploid cases do not.^[2]

Below is a table with the frequencies of some cytogenetic translocations and molecular genetic abnormalities in ALL.

12;21 is the most common translocation and portends a good prognosis. 4;11 is the most common in children under 12 months and portends a poor prognosis.^{[citation needed]}

French-American-British

Historically, prior to 2008, ALL was classified morphologically using the French-American-British (FAB) system that heavily relied on morphological assessment. The FAB system takes into account information on size, cytoplasm, nucleoli, basophilia (color of cytoplasm), and vacuolation (bubble-like properties).^{[10] [11]}

While some clinicians still use the FAB scheme to describe tumor cell appearance, much of this classification has been abandoned because of limited impact on treatment choice and prognostic value.^[12]: 491

World Health Organization In 2008, the World Health Organization classification of acute lymphoblastic leukemia was developed in an attempt to create a classification system that was more clinically relevant and could produce meaningful prognostic and treatment decisions. This system recognized differences in genetic, immunophenotype, molecular, and morphological features found through cytogeneticand molecular diagnostics tests.^{[13]: 1531–1535 [2]} This subtyping helps determine the prognosis and the most appropriate treatment for each specific case of ALL. The WHO subtypes related to ALL are as follows.^[14]