Previous article
Foreign Body Ingestion in Children
Apr 2026
Next article
Vaccine Adverse Effects: An Overview

Thrombocytosis: Rapid Evidence Review

Pamela M. Williams, MD
Alan L. Williams, MD, MPH

American Family Physician. 2026;113(4):332-338.

Author disclosure: Dr. Pamela Williams and Dr. Alan Williams disclosed stock and bond holdings for Cardinal Health Technologies and Pfizer Inc. All relevant financial relationships have been mitigated.

This clinical content conforms to AAFP criteria for CME.

Thrombocytosis is defined by guidelines as a platelet count of 450 × 103/μL or greater. It most often results from secondary causes and should resolve with treatment of the underlying condition. Primary thrombocytosis in adults is most commonly due to myeloproliferative neoplasms, including essential thrombocythemia. Thrombocytosis may be identified incidentally in asymptomatic patients or when performing blood tests to investigate symptoms or signs that could indicate microvascular conditions (eg, headaches, paresthesia, erythromelalgia), splenomegaly, thrombosis, and hemorrhage, which are presenting features of essential thrombocythemia. A repeat complete blood cell count with differential and peripheral blood smear should be performed to confirm persistent thrombocytosis and assess other blood cell lines for findings suggestive of myeloproliferative neoplasms. Additional testing, including C-reactive protein levels and iron studies with ferritin levels, should be performed based on history and examination findings. If essential thrombocythemia is suspected in patients with persistent thrombocytosis without an identified cause, testing for Janus kinase 2 gene sequence variations should be performed and, if negative, testing for calreticulin and myeloproliferative leukemia gene sequence variations; a bone marrow biopsy should also be performed. The revised International Prognostic Score of Thrombosis for Essential Thrombocythemia should be calculated to stratify thrombosis risk and guide management, including when to use low-dose aspirin and cytoreductive therapy. Cardiovascular risk factors should also be addressed. Hydroxyurea is the first-line agent in cytoreductive therapy. No treatments have been shown to increase survival or prevent progression to myelofibrosis or leukemia in patients with essential thrombocythemia.

Thrombocytosis is defined by guidelines as a platelet count of 450 × 103/μL (450 × 109/L) or greater.1,2 Elevated platelet counts result from a primary condition or secondary cause. Most episodes of thrombocytosis are secondary (reactive), resulting from external factors that elevate platelet count.3–5 Primary thrombocytosis in adults is most commonly due to myeloproliferative neoplasms, including essential thrombocythemia.3 This article summarizes current evidence to guide the diagnostic approach to a patient with undifferentiated thrombocytosis and the diagnosis and treatment of essential thrombocythemia.

SORT: KEY RECOMMENDATIONS FOR PRACTICE

Clinical recommendationEvidence ratingComments
A repeat complete blood cell count with differential and peripheral blood smear should be performed to confirm persistent thrombocytosis and assess other blood cell lines.19,26 CExpert opinion and limited quality, patient-oriented evidence
Testing for Janus kinase 2 gene sequence variations and, if negative, testing for calreticulin and myeloproliferative leukemia gene sequence variations, and bone marrow biopsy should be performed in patients with persistent thrombocytosis without an identified cause.19,25–27 CObservational study and consensus guidelines
The revised IPSET-thrombosis score should be used to determine the patient's risk category at the time of diagnosis of essential thrombocythemia to guide treatment.25,26 BValidated decision rule and consensus guidelines
Low-dose aspirin therapy (81–100 mg once daily) should be considered in patients with essential thrombocythemia and prescribed based on symptoms and thrombosis risk, as categorized by their revised IPSET-thrombosis score.25–27,31 CConsensus guidelines

IPSET = International Prognostic Score of Thrombosis for Essential Thrombocythemia.

A = consistent, good-quality patient-oriented evidence; B = inconsistent or limited-quality patient-oriented evidence; C = consensus, disease-oriented evidence, usual practice, expert opinion, or case series. For information about the SORT evidence rating system, go to https://www.aafp.org/afpsort.

PAMELA M. WILLIAMS, MD, is a professor in the Department of Family Medicine at the Uniformed Services University of the Health Sciences, Bethesda, Maryland.

ALAN L. WILLIAMS, MD, MPH, is an associate professor in the Department of Family Medicine at the Uniformed Services University of the Health Sciences, Bethesda.

Address correspondence to Pamela M. Williams, MD, at pamela.williams@usuhs.edu.

Author disclosure: Dr. Pamela Williams and Dr. Alan Williams disclosed stock and bond holdings for Cardinal Health Technologies and Pfizer Inc. All relevant financial relationships have been mitigated.

  1. 1.Loghavi S, Kanagal-Shamanna R, Khoury JD, et al. WHO 5th Edition Classification Project. Fifth edition of the World Health Classification of tumors of the hematopoietic and lymphoid tissue: myeloid neoplasms. Mod Pathol. 2024;37(2):100397.
  2. 2.Arber DA, Orazi A, Hasserjian RP, et al. International consensus classification of myeloid neoplasms and acute leukemias: integrating morphologic, clinical, and genomic data. Blood. 2022;140(11):1200-1228.
  3. 3.Edahiro Y, Kurokawa Y, Morishita S, et al. Causes of thrombocytosis: a single-center retrospective study of 1,202 patients. Intern Med. 2022;61(22):3323-3328.
  4. 4.Hsieh RW, Ravindran A, Hook CC, et al. Etiologies of extreme thrombocytosis: a contemporary series. Mayo Clin Proc. 2019;94(8):1542-1550.
  5. 5.Rose SR, Petersen NJ, Gardner TJ, et al. Etiology of thrombocytosis in a general medicine population: analysis of 801 cases with emphasis on infectious causes. J Clin Med Res. 2012;4(6):415-423.
  6. 6.Giannakeas V, Narod SA. Incidence of cancer among adults with thrombocytosis in Ontario, Canada. JAMA Netw Open. 2021;4(8):e2120633.
  7. 7.Indolfi G, Catania P, Bartolini E, et al. Incidence and clinical significance of reactive thrombocytosis in children aged 1 to 24 months, hospitalized for community-acquired infections. Platelets. 2008;19(6):409-414.
  8. 8.Schattner A, Kadi J, Dubin I. Reactive thrombocytosis in acute infectious diseases: prevalence, characteristics and timing. Eur J Intern Med. 2019;63:42-45.
  9. 9.Eder AF, Yau YY, West K. The effect of iron balance on platelet counts in blood donors. Transfusion. 2017;57(2):304-312.
  10. 10.Hultcrantz M, Ravn Landtblom A, Andréasson B, et al. Incidence of myeloproliferative neoplasms - trends by subgroup and age in a population-based study in Sweden. J Intern Med. 2020;287(4):448-454.
  11. 11.Matsubara K, Fukaya T, Nigami H, et al. Age-dependent changes in the incidence and etiology of childhood thrombocytosis. Acta Haematol. 2004;111(3):132-137.
  12. 12.Titmarsh GJ, Duncombe AS, McMullin MF, et al. How common are myeloproliferative neoplasms? A systematic review and meta-analysis. Am J Hematol. 2014;89(6):581-587.
  13. 13.Moulard O, Mehta J, Fryzek J, et al. Epidemiology of myelofibrosis, essential thrombocythemia, and polycythemia vera in the European Union. Eur J Haematol. 2014;92(4):289-297.
  14. 14.Smith CJ, Thomas JW, Ruan G, et al. A population-based study of outcomes in polycythemia vera, essential thrombocythemia, and primary myelofibrosis in the United States from 2001 to 2015: Comparison with data from a Mayo Clinic single institutional series. Am J Hematol. 2021;96(12):E464-E468.
  15. 15.Gangat N, Karrar O, Al-Kali A, et al. One thousand patients with essential thrombocythemia: the Mayo Clinic experience. Blood Cancer J. 2024;14(1):11.
  16. 16.Babacan A, Şenol FF. Thrombocytosis in children. Rev Assoc Med Bras (1992). 2023;69(6):e20230020.
  17. 17.Zandecki M, Genevieve F, Gerard J, et al. Spurious counts and spurious results on haematology analysers: a review. Part I: platelets. Int J Lab Hematol. 2007;29(1):4-20.
  18. 18.Stockklausner C, Duffert CM, Cario H, et al. THROMKID-Plus Studiengruppe der Gesellschaft für Thrombose- und Hämostaseforschung (GTH) and of Gesellschaft für Pädiatrische Onkologie und Hämatologie (GPOH). Thrombocytosis in children and adolescents–classification, diagnostic approach, and clinical management. Ann Hematol. 2021;100(7):1647-1665.
  19. 19.Almanaseer A, Chin-Yee B, Ho J, et al. An approach to the investigation of thrombocytosis: differentiating between essential thrombo cythemia and secondary thrombocytosis. Adv Hematol. 2024:3056216.
  20. 20.Kucine N, Chastain KM, Mahler MB, et al. Primary thrombocytosis in children. Haematologica. 2014;99(4):620-628.
  21. 21.Godfrey AL, Khan AA, McGregor A, et al. BSH Committee. Investigation and management of thrombocytosis without JAK2, CALR or MPL mutations: a British Society for Haematology guideline [published online December 4, 2025]. Br J Haematol. Accessed January 19, 2026. https://b-s-h.org.uk/guidelines/guidelines/investigation-and-management-of-thrombocytosis-without-jak2-calr-or-mpl-mutations-guideline
  22. 22.Harrison CN, Bareford D, Butt N, et al.; British Committee for Standards in Haematology. Guideline for investigation and management of adults and children presenting with a thrombocytosis. Br J Haematol. 2010;149(3):352-375.
  23. 23.Ianotto JC, Curto-Garcia N, Lauermanova M, et al. Characteristics and outcomes of patients with essential thrombocythemia or polycythemia vera diagnosed before 20 years of age: a systematic review. Haematologica. 2019;104(8):1580-1588.
  24. 24.Loscocco GG, Gesullo F, Capecchi G, et al. One thousand patients with essential thrombocythemia: the Florence-CRIMM experience. Blood Cancer J. 2024;14(1):10.
  25. 25.Tefferi A, Vannucchi AM, Barbui T. Essential thrombocythemia: 2024 update on diagnosis, risk stratification, and management. Am J Hematol. 2024;99(4):697-718.
  26. 26.National Comprehensive Care Network. Myeloproliferative neoplasms. Accessed September 1, 2025. https://www.nccn.org/professionals/physician_gls/pdf/mpn.pdf
  27. 27.Barbui T, Tefferi A, Vannucchi AM, et al. Philadelphia chromosome–negative classical myeloproliferative neoplasms: revised management recommendations from European LeukemiaNet. Leukemia. 2018;32(5):1057-1069.
  28. 28.Barbui T, Finazzi G, Carobbio A, et al. Development and validation of an International Prognostic Score of thrombosis in World Health Organization–essential thrombocythemia (IPSET-thrombosis). Blood. 2012;120(26):5128-5133.
  29. 29.Barbui T, Vannucchi AM, Buxhofer-Ausch V, et al. Practice-relevant revision of IPSET-thrombosis based on 1019 patients with WHO-defined essential thrombocythemia. Blood Cancer J. 2015;5(11):e369.
  30. 30.Stuckey R, Ianotto JC, Santoro M, et al. Validation of thrombotic risk factors in 1381 patients with essential thrombocythaemia: a multicentre retrospective real-life study. Br J Haematol. 2022;199(1):86-94.
  31. 31.Gill H, Leung GMK, Ooi MGM, et al. Management of classical Philadelphia chromosome–negative myeloproliferative neoplasms in Asia: consensus of the Asian Myeloid Working Group. Clin Exp Med. 2023;23(8):4199-4217.
  32. 32.Connelly-Smith L, Alquist CR, Aqui NA, et al. Guidelines on the use of therapeutic apheresis in clinical practice – evidence-based approach from the writing committee of the American Society for Apheresis: the ninth special issue. J Clin Apher. 2023;38(2):77-278.
  33. 33.Griesshammer M, Sadjadian P, Wille K. Contemporary management of patients with BCR-ABL1-negative myeloproliferative neoplasms during pregnancy. Expert Rev Hematol. 2018;11(9):697-706.
  34. 34.Kucine N. Myeloproliferative neoplasms in children, adolescents, and young adults. Curr Hematol Malig Rep. 2020;15(2):141-148.
  35. 35.Högberg C, Gunnarsson U, Jansson S, et al. Diagnosing colorectal cancer in primary care: cohort study in Sweden of qualitative faecal immunochemical tests, haemoglobin levels, and platelet counts. Br J Gen Pract. 2020;70(701):e843-e851.
  36. 36.Clarke C, Hamilton W, Price S, et al. Association of non-malignant diseases with thrombocytosis: a prospective cohort study in general practice. Br J Gen Pract. 2020;70(701):e852-e857.
  37. 37.Gofrit ON, Shapiro A, Rund D, et al. Thrombocytosis accompanying urinary tract infection suggests obstruction or abscess. Am J Emerg Med. 2006;24(1):118-121.
  38. 38.Tchebiner JZ, Nutman A, Boursi B, et al. Diagnostic and prognostic value of thrombocytosis in admitted medical patients. Am J Med Sci. 2011;342(5):395-401.
  39. 39.Chiba N, Sugita A, Mizuochi M, et al. Clinical significance of reactive thrombocytosis in the course of acute pancreatitis. BMC Gastroenterol. 2023;23(1):206.
  40. 40.Bailey SE, Ukoumunne OC, Shephard EA, et al. Clinical relevance of thrombocytosis in primary care: a prospective cohort study of cancer incidence using English electronic medical records and cancer registry data. Br J Gen Pract. 2017;67(659):e405-e413.
  41. 41.Skeith L, Carrier M, Robinson SE, et al. Risk of venous thromboembolism in pregnant women with essential thrombo cythemia: a systematic review and meta-analysis. Blood. 2017;129(8):934-939.
  42. 42.How J, Leiva O, Bogue T, et al. Pregnancy outcomes, risk factors, and cell count trends in pregnant women with essential thrombocythemia. Leuk Res. 2020;98:106459.
  43. 43.Gangat N, Tefferi A. Myeloproliferative neoplasms and pregnancy: overview and practice recommendations. Am J Hematol. 2021;96(3):354-366.
  44. 44.Tefferi A, Loscocco GG, Rokach L, et al. Triple A Plus (AAA+) survival prediction model for essential thrombocythemia: analysis involving 7308 patients. Am J Hematol. 2025;100(11):2017-2027.
Previous article
Foreign Body Ingestion in Children
Apr 2026
Next article
Vaccine Adverse Effects: An Overview

Current issue

July 2026

July 2026

Copyright © 2026 by the American Academy of Family Physicians.

This content is owned by the AAFP. A person viewing it online may make one printout of the material and may use that printout only for his or her personal, non-commercial reference. This material may not otherwise be downloaded, copied, printed, stored, transmitted or reproduced in any medium, whether now known or later invented, except as authorized in writing by the AAFP. See permissions for copyright questions and/or permission requests.