Suspected blasts on peripheral blood smear

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Author: Mikael Häggström [note 1]

Cytology of a precursor (blast) cell, with features often seen even after partial differentiation into any of the more specific cell types. Wright's stain.

When a tech or equivalent lets you know that there are suspected blasts on a peripheral blood smear, the following needs to be determined:

  • If it is a new finding or a known condition in the patient, generally by a medical records lookup.
  • Whether the cells are definite blast cells, suspected blast cells, or another cell type.
  • The percentage of blasts compared to all white blood cells.

New finding of blast cells

If it is a new finding (not already being part of a known diagnosis), generally proceed as follows:

  • During regular work hours, confirm that the person who reports on peripheral blood smears is notified about the case, so that the case is prioritized. Generally, the procedure is to make a complete peripheral blood smear report, in addition to calling the clinician about the presence of blasts. Further information: Peripheral blood smear
  • During on call hours, ask the technician to perform a manual count if not already done.
  • If you are close to the lab, generally look at the slide yourself as well to estimate the blast percentage.
  • If you are far from the lab, ask a technician if images can be sent to you by micrography and telepathology.

Suspect or exclude acute promyelocytic leukemia (APL)

Main features of acute promyelocytic leukemia (APL).[1]
Although cells with irregular nuclei may look similar to benign monocytes, there should be precursor cell (or "blast") features as shown.[2]

APL typically shows blast cells with ample amount of cytoplasmic granules and Auer rods. However, it may also have a hypogranular variant with very scant Auer rods.

If there is even a suspicion of APL, consult commensurately with any readily available colleague(s), and ensure that the following will be done, without waiting for any additional workup, and without waiting for the regular work hours of any people with more expertise:

  • That any outpatient seeks the emergency department to be admitted (such as by having the ordering physician or another clinician call the patient, or call yourself).
  • That the patient will be treated by a clinician who knows your suspicion of APL, as well as the urgency and drug of choice for initial treatment (such as an oncologist or hematologist on call, but it may not be an unprepared emergency room team).[note 2]


Consult with a senior or hematopathologist commensurately, before conveying a preliminary report to the lab technician and/or clinician. At least during on call hours, it is not worth speculating about further sub-classification of non-APL blast cells.


  1. For a full list of contributors, see article history. Creators of images are attributed at the image description pages, seen by clicking on the images. See Patholines:Authorship for details.
  2. The reason for the urgent reporting and treatment of suspected APL is that even a suspected APL mandates immediate initiation of ATRA treatment before confirming or disproving the diagnosis by further workup. It is much better to make a false positive call and have a patient treated with a vitamin A-like drug in vain, rather than delaying or failing to initiate the treatment in a patient who actually has APL, which can quickly have fatal consequences. After all, the initial treatment only costs about $40 as a first dose.
    - Indication for treatment: Sanz MA, Fenaux P, Tallman MS, Estey EH, Löwenberg B, Naoe T (2019). "Management of acute promyelocytic leukemia: updated recommendations from an expert panel of the European LeukemiaNet. ". Blood 133 (15): 1630-1643. doi:10.1182/blood-2019-01-894980. PMID 30803991. PMC: 6509567. Archived from the original. . 
    - Cost of treatment: Dosage of ATRA in APL is 45 mg/m2/day administered as two evenly divided doses, which for a typical adult is about 40 mg as a first dose. The cost per 10mg capsule is about $10.
    - Dosage reference: Osman AEG, Anderson J, Churpek JE, Christ TN, Curran E, Godley LA (2018). "Treatment of Acute Promyelocytic Leukemia in Adults. ". J Oncol Pract 14 (11): 649-657. doi:10.1200/JOP.18.00328. PMID 30423270. Archived from the original. . 
    - Cost per 10mg capsule: . Vesanoid Prices. Retrieved on 2022-11-09.
    The treatment must be started immediately to counteract severe coagulopathy of untreated APL.

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  1. Image by Mikael Häggström, MD. Reference for findings: Syed Zaidi, M.D.. APL with PML-RARA. APL with PML-RARA. Last author update: 1 February 2013
    Source image: File:Faggot cell in AML-M3.jpg from PEIR Digital Library (Pathology image database) (Public Domain)
  2. Image by Mikael Häggström, MD. Source for findings: Syed Zaidi, M.D.. Bone marrow neoplastic, APL with PML-RARA. Pathology Outlines. Last author update: 1 February 2013 Last staff update: 29 November 2022

Image sources