As with AML, ALL is first suspected when blasts are seen on the peripheral blood smear. Twenty percent or greater blasts in the blood or bone marrow constitute a diagnosis of acute leukemia, and then establishing that they are lymphoblasts will give an initial diagnosis of ALL. Techniques, as described in the section on cytochemical stains, can help distinguish myeloblasts from lymphoblasts.
In ALL, lymphoblasts can sometimes be very numerous, as much as 90% of the total white blood count. Bone marrow smears and biopsies are usually done next; the bone marrow can typically show between 25–95% lymphoblasts. To further establish a definitive diagnosis, immunophenotyping using flow cytometry and cytogenetic studies can be performed. More information on this will follow under laboratory diagnosis.
A lumbar puncture to analyze the CSF is usually performed upon initial diagnosis. If the central nervous system is involved, further testing might be done. Coagulation studies and baseline chemistries are also performed.
The image at the right shows a smear with numerous lymphoblasts.
