Polycythemia vera (PV) is diagnosed through a combination of blood tests, genetic testing, and sometimes a bone marrow biopsy. The process typically starts when routine bloodwork reveals an abnormally high red blood cell count, then moves through a specific sequence of tests to confirm PV and rule out other causes. The 2022 WHO diagnostic guidelines rely on three major criteria and one minor criterion to make the diagnosis.
The First Clue: Abnormal Blood Counts
Most people with PV are first flagged through a complete blood count (CBC), often ordered for an unrelated reason. The key numbers doctors look at are hemoglobin and hematocrit, which measure how much of your blood is made up of red blood cells. Under current WHO guidelines, the first major diagnostic criterion is a hemoglobin above 16.5 g/dL in men or above 16 g/dL in women, or a hematocrit above 49% in men or above 48% in women.
These thresholds alone don’t confirm PV. Plenty of conditions can push red blood cell counts higher, from chronic lung disease to dehydration to living at high altitude. But crossing these numbers is what triggers the deeper workup. One simple early step is checking oxygen saturation: if it’s below 92% on room air, that points toward a secondary cause like heart or lung disease rather than PV.
JAK2 Mutation Testing
The single most important test in the PV workup is a blood test for mutations in the JAK2 gene. About 95% of people with PV carry a specific mutation called JAK2 V617F, and another 5% carry a mutation in exon 12 of the same gene. Together, JAK2 mutations account for roughly 98% of all PV cases.
The JAK2 gene provides instructions for a protein that tells your bone marrow how many blood cells to produce. In PV, the mutation creates a permanently “on” signal, causing the marrow to churn out too many red blood cells regardless of what the body actually needs. Finding this mutation is one of the three major WHO criteria and is often the test that clinches the diagnosis. It’s done through a simple blood draw, not a bone marrow sample.
If the V617F test comes back negative but suspicion for PV remains high (especially if erythropoietin levels are low), doctors will order testing for the less common exon 12 mutations. A person who tests negative for all JAK2 mutations and has normal or elevated erythropoietin levels almost certainly has secondary erythrocytosis, not PV.
Erythropoietin Levels
Erythropoietin (EPO) is a hormone your kidneys produce to signal the bone marrow to make more red blood cells. In secondary erythrocytosis, EPO is high because the body is responding appropriately to something like low oxygen levels. In PV, the bone marrow is overproducing on its own, so the body dials EPO down. A serum EPO level below the normal reference range is the minor criterion in the WHO diagnostic system.
This test is especially useful when blood counts are elevated but the picture isn’t clear-cut. A low EPO combined with a positive JAK2 mutation can sometimes be enough to diagnose PV without a bone marrow biopsy, particularly when hemoglobin is very high (above 18.5 g/dL in men or 16.5 g/dL in women).
Bone Marrow Biopsy
A bone marrow biopsy is the second major WHO criterion. During this procedure, a needle is inserted into the back of the hip bone to extract a small core of marrow tissue, which is then examined under a microscope. In PV, the marrow shows a characteristic pattern called panmyelosis: it’s packed more densely than expected for the person’s age, with overgrowth of all three blood cell lines (red cells, white cells, and platelets). The platelet-producing cells, called megakaryocytes, appear in unusual clusters and vary noticeably in size.
Not everyone needs a biopsy. The WHO guidelines allow it to be skipped when red blood cell counts are very high (hemoglobin above 18.5 g/dL in men or above 16.5 g/dL in women) and the person also has a JAK2 mutation plus low EPO. In those cases the diagnosis is clear enough without it. But when counts are only moderately elevated, a condition sometimes called “masked” PV, the biopsy becomes essential. It’s the only way to confirm what’s happening inside the marrow and distinguish early PV from other blood disorders.
Putting the Criteria Together
The WHO system requires either all three major criteria or the first two major criteria plus the minor criterion:
- Major criterion 1: Hemoglobin or hematocrit above the sex-specific thresholds
- Major criterion 2: Bone marrow biopsy showing hypercellularity with panmyelosis
- Major criterion 3: Presence of a JAK2 V617F or JAK2 exon 12 mutation
- Minor criterion: Serum EPO below normal
So a person with high blood counts, a positive JAK2 mutation, and a characteristic bone marrow biopsy meets all three major criteria. Alternatively, someone with high counts and a biopsy showing panmyelosis who tests negative for JAK2 (very rare) could still be diagnosed if their EPO is subnormal. The flexibility in these criteria reflects the reality that not every case looks the same on paper.
Symptoms That Prompt Testing
PV is sometimes caught before symptoms appear, through routine bloodwork. But many people are tested because of symptoms driven by thickened, sluggish blood. Headaches, dizziness, blurred vision, and a flushed or reddish complexion are common. One distinctive symptom is aquagenic pruritus, an intense itching triggered by warm water, particularly after a bath or shower.
An enlarged spleen is considered a hallmark of PV, though its frequency at diagnosis varies widely, reported in anywhere from 20% to 75% of patients depending on the study and how spleen size is measured. When a doctor feels an enlarged spleen during a physical exam in someone with high red blood cell counts, it raises suspicion for PV specifically, since secondary causes of erythrocytosis don’t typically cause spleen enlargement.
Ruling Out Secondary Causes
Before confirming PV, doctors need to make sure high red blood cell counts aren’t being driven by something else entirely. The most common secondary causes include chronic obstructive pulmonary disease, sleep apnea, smoking, kidney tumors that produce excess EPO, and testosterone supplementation. The combination of a normal or high EPO level with a negative JAK2 test effectively rules out PV and redirects the workup toward these secondary causes.
Dehydration can also create a false impression of high red blood cell counts by reducing plasma volume, making the blood appear more concentrated than it actually is. This is sometimes called relative polycythemia. When counts are borderline, repeating the CBC after adequate hydration can help clarify whether the elevation is real.