What Is the Difference Between JAK2 and CALR in ET?

If you have been diagnosed with essential thrombocythemia (ET) and learned you have a CALR mutation instead of a JAK2 mutation, it generally means you are likely to have a lower risk of experiencing a blood clot, but your platelet counts may run noticeably higher ^{[1] [2]}. For most patients, a CALR mutation points toward a statistically lower risk of cardiovascular events, but it often requires a careful, individualized approach to managing bleeding risks and medications ^{[3] [4]}.

Understanding Your Mutation

In ET, blood cells in the bone marrow acquire genetic changes (called driver mutations) that cause them to overproduce platelets. These are not inherited traits passed down through families, but rather changes that happen during your lifetime. The two most common driver mutations in ET are JAK2 (specifically JAK2 V617F) and CALR. A smaller number of patients have an MPL mutation or no detectable driver mutation at all (called “triple-negative” ET).

It is important to remember that mutation status is just one piece of the puzzle. Your diagnosis and monitoring also depend heavily on your exact blood counts, your symptoms, your age, cardiovascular health, and sometimes bone marrow findings ^{[5] [6]}.

Blood Counts and Clinical Presentation

While both JAK2 and CALR mutations drive the overproduction of blood cells, they show different patterns in routine blood work and demographics:

Note: These are population-level patterns, not strict rules for your individual experience.

Blood Clots, Bleeding Risk, and Aspirin

The most critical difference between these two driver mutations is how they affect your blood’s tendency to clot (thrombosis) or bleed.

URGENT SAFETY WARNING: Regardless of your mutation, seek emergency medical care if you experience signs of a blood clot (sudden weakness or numbness, trouble speaking, chest pain, severe shortness of breath, or a painful swollen limb) or severe bleeding (uncontrolled bleeding, coughing up blood, or blood in your stool).

A JAK2 mutation is a strong, independent risk factor for both arterial blood clots (such as strokes or heart attacks) and venous blood clots (such as deep vein thrombosis) ^{[1] [6]}. Current medical scoring systems use the presence of a JAK2 mutation, along with age and prior clot history, to classify a patient’s clotting risk ^[5].

In contrast, patients with a CALR mutation have a statistically lower average risk of forming blood clots ^[1]. However, this changes how doctors approach preventative medications like daily low-dose aspirin.

Aspirin decisions in ET are highly individualized based on age, prior clots, symptoms, and bleeding risk:

• Because of the higher clotting risk, low-dose daily aspirin is commonly considered for many patients with a JAK2 mutation ^[4].
• However, CALR mutations often cause extreme thrombocytosis (excessively high platelets, typically over 1,000 × 10^9/L). When platelets are this high, the sheer number of cells can cause the accelerated clearance or loss of a crucial clotting protein called von Willebrand factor. This leads to acquired von Willebrand syndrome, a condition that paradoxically increases your risk of bleeding despite having too many platelets ^{[8] [9]}.
• Observational studies show that for “low-risk” ET patients (typically defined as under age 60 with no prior blood clots) who have a CALR mutation, taking daily aspirin may not significantly reduce clot risk but could increase the risk of bleeding ^[4].
• Therefore, your doctor might carefully observe your condition, check for acquired von Willebrand syndrome, or prescribe platelet-lowering therapies (like hydroxyurea or interferon) instead of automatically starting aspirin ^[9].

Important: Never start, stop, or change an aspirin regimen or any other medication without direct instruction from your treating hematologist.

Note: Your personal risk of blood clots is also heavily influenced by other cardiovascular risk factors like high blood pressure, diabetes, obesity, or a history of smoking. Managing these factors is a crucial part of your overall care ^{[10] [2]}.

Long-Term Outlook and CALR Subtypes

When looking at long-term overall survival, patients with a CALR mutation are often observed to have a slightly better overall survival rate compared to those with a JAK2 mutation, particularly among younger patients ^{[11] [12]}. The risk of ET transforming into acute leukemia is relatively uncommon and generally comparable between the two groups, though exact rates vary by study ^{[10] [1]}.

When it comes to the risk of ET progressing to myelofibrosis (scarring of the bone marrow), the specific subtype of CALR mutation you have provides additional prognostic clues:

• CALR Type 1 (or Type 1-like): This mutation pattern is often associated with a greater risk of bone marrow scarring over time (a myelofibrosis-like phenotype) compared to Type 2 ^{[3] [7]}.
• CALR Type 2 (or Type 2-like): This mutation pattern is more often associated with extremely high platelet counts and a more typical ET phenotype ^{[3] [13]}.

It is important to understand that CALR subtypes are statistical associations, not individual guarantees. Many laboratories do not routinely report the exact subtype, and treatment decisions are ultimately driven by your age, symptoms, history of clots, and complete blood counts rather than mutation status alone.