Besremi and Blood Count Monitoring: Why CBC Tests Matter During Polycythemia Vera Treatment?

Polycythemia vera (PV) is a chronic blood disorder characterized by an increased number of blood cells. Managing it well depends on tracking those numbers closely over time, even when you are feeling well. BESREMi®, a type of immunotherapy, is the only FDA-approved treatment indicated for adults with PV that targets the source of the disease and can be used regardless of the risk category or previous treatment history. ^[1-4]

If you are being treated with BESREMi®, you may be asked to have regular complete blood count (CBC) tests. But why are these tests needed so often? What do those numbers actually mean? And how do they help your doctor understand whether your treatment is working as expected?

Let’s take a closer look.

What is Polycythemia Vera?

Polycythemia vera (PV) is a type of blood cancer in which the bone marrow produces too many blood cells, particularly red blood cells. This can make the blood thicker than normal.

Symptoms of PV often develop slowly and may be nonspecific. They can include headache, tiredness, excessive sweating (especially at night), itchy skin, stomach problems, and an enlarged spleen.

PV can increase the risk of blood clots, leading to serious complications such as stroke or heart attack.

What is BESREMi® (Ropeginterferon)?

BESREMi is a type of interferon, also called ropeginterferon alfa-2b. ^[6] Interferons are signalling proteins naturally produced by the body. They play an important role in the immune system and help the body respond to infections and certain cancers. Manufactured interferons can act on cancer cells to help control their growth and division. ^[7]

Thus, BESREMi works by helping reduce the production of blood cells in the bone marrow. By helping control blood cell production, BESREMi® can help normalize blood counts and may help reduce disease-related complications, including blood clots and an enlarged spleen. ^[6,8] BESREMi is also linked to a substance called polyethylene glycol (PEG), which helps the medicine remain in the body for a longer period of time, of upto 2 weeks. ^[6,9]

Why are blood tests important in Polycythemia Vera?

Regular blood tests are an important part of PV management because changes in blood counts can be associated with disease-related complications.

PV is associated with thrombotic (blood clotting), fibrotic (tissue scarring), and leukemic (development of blood cancer) events, which are major causes of morbidity and mortality. At 20 years, the incidence of these events has been reported to be 26%, 16%, and 4%, respectively. ^[3]

The risk of developing thrombotic events (TEs) or blood clots in people with PV is assessed using several factors. Age and a history of previous blood clots are traditionally considered important risk factors. In addition, hematocrit (HCT) levels are closely linked to the risk of blood clots. HCT levels tell us what percentage of the blood is made up of red blood cells. When HCT is elevated, the blood can become thicker, which may increase the risk of blood clots. ^[3] Studies have found that patients with an HCT level above 45% have a significantly higher risk of developing TEs than those with an HCT level below 45%. ^[10]

Thus, for many years, PV management has focused on keeping HCT within the desired range of 45% and controlling symptoms to improve quality of life. However, PV can affect more than just red blood cells.

Recent studies have also highlighted the importance of white blood cell (WBC) and platelet (PLT) counts in assessing disease control and thrombotic risk. ^[3] For example, in clinical trials, an elevated WBC count was significantly associated with the first occurrence of a thrombotic event in patients with PV. Even when HCT was controlled at ≤45%, a WBC count above 12 × 10⁹/L was significantly associated with the occurrence of TEs. ^[10]

Thus, treatment of PV not only aims to maintain HCT below 45%, but also to achieve a complete hematologic response (CHR), with the goal of reducing thrombotic risk and helping limit disease progression to blood cancers, including myelofibrosis (MF) and acute myeloid leukemia (AML). ^[3]

Achieving CHR means that ^[11]:

 - All 3 blood cell counts, red blood cells, white blood cells, and platelets, have returned to normal levels

 - You have not needed a phlebotomy during the past 3 months

Therefore, managing PV involves regular monitoring of blood counts, including HCT, WBCs, and platelets, to help assess disease control and thrombotic risk.

Why are CBC tests important in patients being treated with BESREMi®?

BESREMi® is given every 2 weeks. BESREMi treatment usually starts with a 100 µg dose. For people taking hydroxyurea (HU), treatment starts at 50 µg. The dose can then be gradually increased by 50 µg at a time, up to a maximum of 500 µg.

Because the dose is increased gradually, it may take approximately 6 months or longer to reach the appropriate maintenance dose for an individual patient. During this dose-titration period, blood counts may not yet be optimally controlled. As a result, patients may remain at risk of thrombotic or hemorrhagic events due to inadequate drug exposure. ^[3]

Additionally, as we know BESREMi® can also lower the levels of different types of blood cells. This effect is known as myelosuppression. If BESREMi® is used along with other medicines that can also suppress the bone marrow, blood cell counts may become too low. This can lead to: ^[5]

 - Anemia: a low red blood cell count, which can reduce the blood’s ability to carry oxygen ^[5]

 - Neutropenia: a low level of certain white blood cells that help fight infections ^[5]

 - Thrombocytopenia: a low platelet count, which can affect the blood’s ability to clot ^[5]

This is why regular CBC testing is important during BESREMi® treatment. It allows your doctor to monitor your blood cell counts and make treatment or dose adjustments when needed.

How routinely would I need to do blood tests when using BESREMi?

CBC monitoring is particularly important during the dose-titration and dose-modification phases.

 - During the titration phase: CBC should be performed every 2 weeks.

 - During the maintenance phase: Once the optimal dose has been established, CBC should generally be performed every 3–6 months.

When clinically indicated: Your doctor may recommend CBC testing more frequently based on your individual condition or treatment needs.

Conclusion:

When you are living with PV, blood counts can provide important information about how well your disease is being controlled. Monitoring HCT, WBC, platelet counts, and other relevant parameters help your doctor assess treatment response and identify changes that may require attention.

If you are receiving BESREMi®, regular CBC tests are especially important during dose titration. These tests help your healthcare team monitor your response to treatment, watch for blood cell counts that become too high or too low, and make appropriate treatment adjustments when needed.

Medical Disclaimer: 

This information is provided for educational purposes only and is not a substitute for professional medical advice. Medicine should only be taken under the guidance of a qualified healthcare provider. Patients should always consult their doctor for advice on diagnosis, treatment, and medication use, and should not make changes to their prescribed therapy without medical supervision.

References:

 - Grossi G. FDA approves treatment for rare blood disease polycythemia vera. HCPLive. 2021 Nov 15.

 - PharmaEssentia Corporation. U.S. FDA approves BESREMi® (ropeginterferon alfa-2b-njft) as the only interferon for adults with polycythemia vera. Press release. 2021 Nov 12.

 - Suo SS, Fu RF, Qin A, et al. Effective management of polycythemia vera with ropeginterferon alfa-2b treatment. J Hematol. 2024;13(1-2):12-22. doi:10.14740/jh1245.

 - BESREMi®. Polycythemia vera treatment. BESREMi PV website

 - Elste J. Besremi for treatment of polycythemia vera. WebMD. Medically reviewed October 2, 2025.

 - National Cancer Institute. Besremi. NCI Dictionary of Cancer Terms.

 - Cleveland Clinic. Interferons (IFNs). Medically reviewed February 19, 2024.

 - HealthTree Foundation for Myeloproliferative Neoplasms. What is Besremi for polycythemia vera (PV)? 2024 Nov 4.

 - How does Besremi work? Mechanism of action explained in plain English [Internet]. Medfinder; 2026 Mar 12 [cited 2026 Sep 26]

 - Gerds AT, Mesa R, Burke JM, et al. Association between elevated white blood cell counts and thrombotic events in polycythemia vera: analysis from REVEAL. Blood. 2024;143(16):1646-1655. doi:10.1182/blood.2023020232.

 - Piece Together PV. Manage PV: Complete hematologic response (CHR). Piece Together PV resources

 - PharmaEssentia Corporation. BESREMi® (ropeginterferon alfa-2b-njft) prescribing information. Updated 2026