Blasts 101 — what they are
and where they belong
Your body constantly makes new blood cells in the bone marrow, a process called hematopoiesis. Blasts are early, still-developing cells that are in a transitionary stage between stem cells and fully mature blood cells. There are two main types of blast cells that make up the blood cell production pipeline:
- Myeloblasts: mature into red cells, platelets, and several types of white cells
- Lymphoblasts: mature into white blood cells called lymphocytes
In healthy conditions, blasts stay inside the bone marrow while they mature. They are generally not seen circulating in the bloodstream, sometimes called peripheral blood, on routine testing. Seeing blasts in the peripheral blood, or seeing rising blast levels in the marrow, prompts doctors to look closer.
Why blasts in the blood matter in MPNs
In MPNs, a sustained rise in blasts can mean the disease is more seriously disrupting the production of blood cells and moving out of its chronic phase. Doctors use blast cutoffs to label these changes:
- Accelerated phase (AP): 10–19% blasts in blood or marrow
- Blast phase (BP): 20% or more blasts, often described as acute myeloid leukemia (AML) that has evolved from an MPN
These thresholds are widely used as criteria in current classifications and care pathways.
Developing MPN-BP or AML from an MPN is called leukemic transformation. In each of the three different classic MPNs, how often new cases of AML develop in each disease state, called the incidence estimates. Tefferi et al. averaged between multiple studies which checked patient progress over the course of 3 to 10 years, and reported the incidence estimates for each disease state as (2023):
- essential thrombocythemia (ET): 1–4%
- polycythemia vera (PV): 3–7%
- primary myelofibrosis (pMF): 9–13%
Signs of increased risk of blast phase transformation
Several features raise the chance of leukemic transformation over time, including:
- certain high-risk mutations
- ET: TP53, SRSF2, EZH2, U2AF1, RUNX1
- PV: SRSF2, IDH2, RUNX1
- pMF: IDH1, IDH2, SRSF2, ASXL1
- abnormal chromosome, also called karyotype, changes
- the number of circulating blasts at or above roughly 3%
Transformation to a blast-phase MPN from chronic-phase is rare and carries a much tougher outlook. Large clinical studies have reported survival rates under 20% at 1 year and under 5% at 3 years when transplant is not part of the treatment plan.
If a high number of risk factors is present, care teams often monitor more closely and consider referring a patient to receive an allogeneic hematopoietic stem cell transplant (allo-HSCT).
How doctors measure and monitor blasts
Expect a mix of tests including:
- blood counts and blood smears: checking blood cell production and looking for blasts in the bloodstream
- bone marrow biopsy and aspirate: tallying blasts in the marrow and assessing fibrosis and chromosomes
- molecular testing: finding mutations that can guide treatment choices if the disease is accelerating or in blast phase
Trends matter. Healthcare teams look at blast percentages over time alongside symptoms, spleen size, transfusion needs, and response to medicines.
When an MPN enters accelerated or blast phase: treatment goals and options
Goals include lowering blasts, controlling symptoms and complications, and, when feasible, preparing for allogeneic hematopoietic stem cell transplant (allo-HSCT).
Common approaches your healthcare team may discuss:
- Hypomethylating agent plus venetoclax (HMA + venetoclax): Many centers favor this combination for MPN-BP because it can reduce blasts and serve as a bridge to transplant. Responses are more likely in some genetic settings and less likely with extremely abnormal karyotypes.
- Hypomethylating agent plus ruxolitinib: Ongoing studies suggest this pairing in accelerated and blast phase may help with symptom and spleen size control while treating disease.
- CPX-351 or IDH-targeted therapy: When certain mutations are present, doctors may use specific inhibitors alone or combine them with HMA and sometimes venetoclax.
- Clinical trials: Because outcomes remain limited with current options, trial enrollment is strongly encouraged when available.
Chemotherapy on its own rarely provides durable control in MPN-BP. Transplant planning — if a patient is a candidate — should begin early.
Where transplant fits and how blasts affect eligibility
Allogeneic stem cell transplant is the only strategy consistently linked with substantially better long-term survival in advanced-phase disease, with 3-year survival exceeding 30% in aggregate reports.
Two practical points often surprise patients and caregivers:
- You do not have to reach absolute zero blasts before transplant. Reducing blasts before transplant is desirable, but repeatedly chasing complete clearance with extra cycles can add complications and, in some cases, erode transplant eligibility without improving outcomes. Many experts favor proceeding to transplant once a safe window is achieved rather than delaying for maximal response.
- Bridging therapy in accelerated phase is not one-size-fits-all. In AP, the benefit of pre-transplant chemotherapy is uncertain and individualized. In BP, blast-lowering therapy before transplant is typically advised. Either way, early referral, HLA typing, and a donor search help avoid delays.
Teams may also continue or add a JAK inhibitor for problematic splenomegaly as part of a bridge strategy.
Important questions to ask healthcare teams
If you’re an MPN patient or caretaker, here’s a short list of questions to discuss with your healthcare team:
- Based on my current blood and marrow results, are we in chronic phase, accelerated phase, or blast phase?
- What is my mutation profile and karyotype, and how does that influence treatment choices and transplant timing?
- Should we start HLA typing and a donor search now, and when should I meet a transplant team?
- Which bridge therapy best positions me for transplant given my specific risks?
- What clinical trials are open that fit my situation?
Living with advanced-phase MPNs
Alongside disease-directed therapy, supportive care matters. This can include transfusions, infection prevention, symptom control, and early palliative care input to manage stress, fatigue, and quality of life — for both patients and caregivers.
If your results show rising blasts or concern for accelerated or blast phase, ask for a transplant evaluation early and explore clinical trials that may apply to you. Follow MPN Research foundation for trusted education and research updates.
This article is for education only and is not a substitute for medical advice. Always discuss your individual situation with your care team.
Next steps
While leukemic transformation is uncommon, especially in ET and PV, your care team watches for early signals so that, if needed, the right steps can be taken quickly. Ongoing research is also working toward better treatments for all phases of MPNs
The MPN PROGRESSion Registry™ is a longitudinal, observational study developed by MPN Research Foundation to address the research questions surrounding MPN disease progression and improve patient care. With the help of real-world electronic medical records and patient reported outcomes gathered over 5-10 years, it’s our hope that researchers might one day identify and prevent blast-phase MPN before it happens.
Glossary
Blasts: immature blood cells that usually mature in the bone marrow before entering the bloodstream
Accelerated phase: 10–19% blasts in blood or marrow in an mpn
Blast phase: 20% or more blasts in blood or marrow in an mpn
Hypomethylating agent: a chemotherapy class used in myeloid cancers that can be paired with venetoclax
Allogeneic stem cell transplant: a procedure that replaces diseased marrow with donor cells to re-establish healthy blood formation
Sources
Kröger, N., Bacigalupo, A., Barbui, T., Ditschkowski, M., Gagelmann, N., Griesshammer, M., Gupta, V., Hamad, N., Harrison, C., Hernandez-Boluda, J. C., Koschmieder, S., Jain, T., Mascarenhas, J., Mesa, R., Popat, U. R., Passamonti, F., Polverelli, N., Rambaldi, A., Robin, M., … Barosi, G. (2024). Indication and management of allogeneic haematopoietic stem-cell transplantation in myelofibrosis: Updated recommendations by the EBMT/ELN International Working Group. The Lancet Haematology, 11(1), e62–e74. https://doi.org/10.1016/S2352-3026(23)00305-8
Palmer, J., Kosiorek, H. E., Wolschke, C., Fauble, V. D. S., Butterfield, R., Geyer, H., Scherber, R. M., Dueck, A. C., Gathany, A., Mesa, R. A., & Kroger, N. (2019). Assessment of Quality of Life following Allogeneic Stem Cell Transplant for Myelofibrosis. Biology of Blood and Marrow Transplantation, 25(11), 2267–2273. https://doi.org/10.1016/j.bbmt.2019.07.001
Polverelli, N., Farina, M., D’Adda, M., Damiani, E., Grazioli, L., Leoni, A., Malagola, M., Bernardi, S., & Russo, D. (2022). How We Manage Myelofibrosis Candidates for Allogeneic Stem Cell Transplantation. Cells, 11(3), 553. https://doi.org/10.3390/cells11030553
National Comprehensive Cancer Network. (2025). NCCN clinical practice guidelines in oncology: Myeloproliferative Neoplasms (Version 1.2025). https://www.nccn.org
Schäfersküpper, M., Simon, A., Yorgan, T. A., von Brackel, F. N., Delsmann, M. M., Baranowsky, A., Gagelmann, N., Ayuk, F., Schinke, T., Amling, M., Kröger, N., & Rolvien, T. (2025). Reconstructing skeletal homeostasis through allogeneic hematopoietic stem cell transplantation in myelofibrosis. Nature Communications, 16(1), 741. https://doi.org/10.1038/s41467-025-55915-w
Tamari, R., Rapaport, F., Zhang, N., McNamara, C., Kuykendall, A., Sallman, D. A., Komrokji, R., Arruda, A., Najfeld, V., Sandy, L., Medina, J., Litvin, R., Famulare, C. A., Patel, M. A., Maloy, M., Castro-Malaspina, H., Giralt, S. A., Weinberg, R. S., Mascarenhas, J. O., … Rampal, R. K. (2019). Impact of High-Molecular-Risk Mutations on Transplantation Outcomes in Patients with Myelofibrosis. Biology of Blood and Marrow Transplantation: Journal of the American Society for Blood and Marrow Transplantation, 25(6), 1142–1151. https://doi.org/10.1016/j.bbmt.2019.01.002
What Are Blast Cells? (11/21/2024). Cleveland Clinic. Retrieved September 15, 2025, from https://my.clevelandclinic.org/health/body/blast-cells
