Primary Myelofibrosis

Primary myelofibrosis (PMF) is a chronic myeloproliferative neoplasm in which the bone marrow becomes progressively replaced by fibrous scar tissue, disrupting its ability to produce blood cells normally. As the marrow fails, the spleen and liver take over blood cell production — a process called extramedullary hematopoiesis — leading to significant organ enlargement and a range of debilitating symptoms.

PMF is driven by acquired genetic mutations, most commonly JAK2 V617F (present in approximately 50–60% of cases), CALR mutations (approximately 25%), or MPL mutations (approximately 5–10%). Around 10% of cases carry none of these three driver mutations (triple-negative PMF).

Why this matters for your blood test results

PMF often produces an abnormal CBC — anaemia is common, and white blood cell and platelet counts can be elevated, low, or normal depending on disease stage
A leukoerythroblastic blood film — immature white and red blood cells appearing in the peripheral blood — is a characteristic finding
JAK2, CALR, and MPL mutation testing identifies the genetic driver in approximately 90% of cases
LDH is frequently elevated, reflecting increased cell turnover
MXD% may be elevated due to increased monocytes in some subtypes

Symptoms

PMF produces a wide range of symptoms, many driven by progressive anaemia and massive splenomegaly:

Fatigue and weakness — often severe, from anaemia
Enlarged spleen (splenomegaly) causing left-sided abdominal pain, early satiety, and discomfort
Enlarged liver (hepatomegaly)
Night sweats, fever, and unintentional weight loss — constitutional symptoms reflecting high disease burden
Bone pain
Increased susceptibility to infection from impaired immune function
Easy bruising or bleeding from thrombocytopenia in advanced disease

Causes and diagnosis

PMF is caused by acquired mutations in haematopoietic stem cells that activate the JAK-STAT signalling pathway, driving abnormal proliferation of megakaryocytes (platelet-producing cells) and secondary fibrosis of the bone marrow. The exact trigger for the mutation is unknown — it is not inherited.

Diagnosis requires bone marrow biopsy demonstrating megakaryocytic proliferation and reticulin or collagen fibrosis, alongside exclusion of other myeloid disorders. WHO diagnostic criteria incorporate mutation status, bone marrow morphology, and CBC findings. Risk stratification systems (DIPSS, MIPSS70) guide prognosis and treatment decisions.

Treatment

Treatment depends on risk category and symptom burden.

Low-risk, asymptomatic disease — observation with regular monitoring
Anaemia management — erythropoiesis-stimulating agents, danazol, or transfusions depending on the cause and severity
Ruxolitinib — a JAK1/JAK2 inhibitor that reduces splenomegaly and constitutional symptoms; first-line for intermediate and high-risk symptomatic disease
Fedratinib and pacritinib — second-generation JAK inhibitors for ruxolitinib-resistant or intolerant patients
Allogeneic stem cell transplantation — the only potentially curative treatment; considered for eligible patients with intermediate-2 or high-risk disease

FAQ

Is primary myelofibrosis curable? For most patients it is not curable with standard treatments, which aim to control symptoms and slow progression. Allogeneic stem cell transplantation offers the possibility of cure for eligible patients but carries significant risks and is not appropriate for everyone.

Can myelofibrosis develop from another condition? Yes. Myelofibrosis can arise de novo (primary myelofibrosis) or develop as a progression from polycythemia vera or essential thrombocythaemia — in which case it is called post-PV or post-ET myelofibrosis.

Related biomarkers JAK2 V617F Mutation, JAK2 Exon 12 Mutation, CALR Mutation, MPL Mutation, Hemoglobin, White Blood Cells, Platelets, LDH, MXD%, Erythropoietin (EPO).

Primary Myelofibrosis

Why this matters for your blood test results

Symptoms

Causes and diagnosis

Treatment

FAQ

All Health Conditions