A High Hemoglobin Is Not Always Dehydration: Reading Polycythemia Vera
This one usually turns up on a routine panel. The hemoglobin is high. Someone says you were probably dehydrated, or it is the smoking, or the altitude. Check it again in a few months. A few months later it is high again, and the same sentence gets said again. A number explained away is a number nobody investigated, and this one carries a clot risk while it is being explained away. Healz's root-cause technology refuses the first plausible reason and keeps searching.
This post takes the finding from the top: what a high hemoglobin measures, how a real excess of red cells is told apart from a shrunken plasma volume, what makes the diagnosis of polycythemia vera, the clues people mention last, and why treatment aims at the clot rather than the number. Most high hemoglobins are not polycythemia vera. All of them are worth one honest look.
Why a high hemoglobin gets explained away
The explanations are not stupid. They are usually right. Dehydration shrinks plasma volume, so the red cells left behind look more concentrated even though there are no extra ones. That is relative or spurious polycythemia, and it is why a single high hematocrit on a hot day means very little (per StatPearls, NIH). Chronic smoking, altitude, untreated obstructive sleep apnea, chronic lung disease, and testosterone replacement are all documented causes of a genuine rise; the SGLT2 inhibitor association is the newest of them (Cardiovascular Diabetology, 2026).
The failure is not the explanation. It is that the explanation ends the search. Nobody rechecks after rehydration. Nobody asks whether the smoker also has an enlarged spleen. The number gets a story attached and stops being a question. Our guide on how to read a complete blood count covers why one high line is always read against the others.
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Primary or secondary: the question the number cannot answer
A hemoglobin of 17.4 looks identical whichever mechanism produced it, and there are three. Relative is the one above: red cell mass normal, plasma low, value falls after rehydration. The other two are a real excess, and they split on one question.
Secondary. There genuinely are extra red cells, because something is telling the marrow to make them. The messenger is erythropoietin, so its level runs high. Chronic hypoxia is the usual driver, and drugs are a large and growing share. Rarely an erythropoietin-secreting tumor is behind it, including hepatocellular carcinoma, uterine leiomyoma, and cerebellar hemangioblastoma (per StatPearls, NIH). Rare, but it is why a persistent unexplained erythrocytosis is not something to shrug at.
Primary. The marrow is making red cells on its own, without being asked. Because nothing is asking, the erythropoietin level is low or low-normal. This is polycythemia vera, a chronic blood cancer of the myeloproliferative neoplasm family.
Serum erythropoietin is the test that splits those two apart, and it is genuinely useful: levels run low in primary erythrocytosis and high in secondary. It is not perfect. A meaningful share of confirmed polycythemia vera patients have an erythropoietin sitting inside the normal range. A normal erythropoietin narrows the field. It does not close it.
What actually makes the diagnosis
The WHO criteria are three major criteria plus one minor. The first is the red cell measure itself: hemoglobin above 16.5 g/dL in men or above 16.0 g/dL in women, or hematocrit above 49% in men or above 48% in women (the 5th edition dropped the older red cell mass measurement). The second is a bone marrow biopsy showing hypercellularity for age with growth across all three lines, red cells, white cells, and platelets together. The third is a JAK2 mutation. The minor criterion is a below-normal serum erythropoietin. The diagnosis is made either on all three major criteria, or on the first two plus the minor one. WHO adds one exception: the marrow biopsy may be skipped when the erythrocytosis is sustained and marked (hemoglobin above 18.5 g/dL with hematocrit 55.5% in men, or above 16.5 g/dL with hematocrit 49.5% in women) and both the JAK2 mutation and the subnormal erythropoietin are present.
JAK2 is the anchor. The V617F mutation is detectable in roughly 95% of people with polycythemia vera, and most of the remainder carry a JAK2 exon 12 mutation instead, so a full JAK2 workup covers nearly the whole disease. It is a blood test, not a marrow procedure, and it can be ordered long before anyone goes near a biopsy needle.
The thresholds look low on purpose. WHO lowered them in 2016 from the older 18.5 g/dL in men and 16.5 g/dL in women to catch masked polycythemia vera: real disease in people whose counts never reach the old numbers, who were being diagnosed late. A hemoglobin sitting a little over the range for years, with nobody impressed by it, describes the exact population the change was made for. Our piece on reading a blood test beyond the reference range covers why a value parked at the edge for years is a different signal from one that touched it once.
The clues people mention last
These rarely make it into the visit, because they do not sound like blood problems.
Itching after a shower. Aquagenic pruritus is itching, tingling, or burning that starts after contact with water, classically a warm shower, usually on the trunk. In a cohort of 441 people with polycythemia vera, about 68% had it, and among those affected, 64.8% had it before the diagnosis, by an average of 2.9 years. Nearly 15% called it unbearable. That is years of lead time, routinely reported to a dermatologist rather than to whoever is reading the CBC.
Burning red hands or feet. Erythromelalgia is burning pain in the extremities with redness and warmth, worsened by heat and standing, relieved by cooling or elevation. Pain that responds to aspirin but not to other platelet inhibitors is characteristic enough to be used as a diagnostic clue. Underneath it is arteriolar inflammation with small thrombotic occlusions, a clotting symptom nobody reads as one.
The quieter set. Headache, dizziness, tiredness, and blurred vision are common, and over 30% of patients have an enlarged spleen, which causes fullness under the left ribs (per the Merck Manual). In some people the first event is a clot in an unusual place, such as the veins draining the liver, which is one situation where a hematologist tests for JAK2 even when the counts look unremarkable.
What the treatment is actually aimed at
Not the number. The clot.
Arterial and venous thrombosis are the most common complications of polycythemia vera and its leading causes of illness and death. That is the whole reason it is managed, and why the two foundations of treatment are so plain.
Phlebotomy to a hematocrit target. Blood is removed to bring the hematocrit down and keep it there, and the number is not arbitrary. The CYTO-PV randomized trial compared a target under 45% against 45% to 50%, and the looser target carried roughly four times the rate of cardiovascular death or major thrombosis (NEJM 2013). Before that trial the 45% line rested on consensus. After it, it rests on evidence.
Low-dose aspirin. In the ECLAP randomized trial, low-dose aspirin cut the combined rate of nonfatal heart attack, nonfatal stroke, pulmonary embolism, major venous thrombosis, and cardiovascular death (relative risk 0.40, NEJM 2004), without a significant rise in major bleeding. Typical dosing is 40 to 100 mg daily.
Beyond that, people are sorted by risk, with age over 60 or any previous thrombosis marking the higher-risk group considered for cytoreductive medication on top of phlebotomy and aspirin. Long term, a minority progress, and the numbers depend on how long people are followed: post-polycythemia myelofibrosis runs around 10% at ten years and about 16% at twenty, while transformation to acute leukemia runs around 3% at ten years and about 4% at twenty (Tefferi and Barbui, American Journal of Hematology). Real numbers, and for most people not the story. The clot is, and it is what the treatment is built to prevent.
How Healz reads a high hemoglobin
Healz is equipped with root-cause technology. It treats "probably dehydration" as a hypothesis rather than a verdict, cross-checks your case against more than a million rare cases, and drills past the easy explanation toward the mechanism: relative, secondary, or a marrow producing red cells on its own. That is the difference between a plausible story and a cause.
The rest of the workup lives in the same chat. Frontier AI works your whole picture, so a hemoglobin is read next to the hematocrit, the platelets, the white count, the erythropoietin, and the JAK2 result instead of alone. As a blood test ai analyzer, Healz flags the pattern that does not fit the offered explanation. Healz has memory that keeps every result you upload and connects them over time, so a value that has quietly sat above the range for four years shows up as a line, not a surprise. Everything in one place, one chat, not ten apps and three portals. When you want a human in the loop, you can bring a board-certified doctor into the same chat for a second opinion.
Frequently asked questions
- Can dehydration really cause a high hemoglobin?
Yes, and it is common. Dehydration shrinks plasma volume, so the same number of red cells is measured in less fluid and the hemoglobin and hematocrit read high. That is relative or spurious polycythemia: the red cell mass itself is normal (per StatPearls, NIH). The distinguishing step is straightforward: the value is rechecked after rehydration. A value that stays high is no longer explained by dehydration.
- What is the difference between polycythemia vera and secondary polycythemia?
The driver. In secondary polycythemia something outside the marrow is signalling for more red cells, usually low oxygen or a drug, so the erythropoietin level runs high. In polycythemia vera the marrow makes them on its own, so erythropoietin is low or low-normal and a JAK2 mutation is present in roughly 95% of cases.
- Does a negative JAK2 V617F test rule out polycythemia vera?
Not by itself. About 95% of cases carry JAK2 V617F, and most of the rest carry a JAK2 exon 12 mutation, a separate test that has to be specifically ordered. If the picture still fits after a negative V617F, exon 12 testing and a marrow biopsy are the next steps, not the end of the workup.
- Why does polycythemia vera make you itch after a shower?
Aquagenic pruritus is one of the most common symptoms, reported by about 68% of patients in a 441-person cohort. It matters because it usually arrives first: among affected patients, 64.8% had the itching before the diagnosis, by an average of 2.9 years. Persistent post-shower itching alongside a high hemoglobin is worth naming out loud.
A high hemoglobin is usually not polycythemia vera, and saying so is not the same as dismissing it. The difference is whether the alternative was tested or only asserted. The standard path is a recheck after rehydration, an erythropoietin level, and JAK2 testing if the value stays high, and the post-shower itch and the burning hands belong in that conversation even though they sound unrelated. The risk here is not the number on the page. It is the clot that arrives while everyone is still explaining the number away.
Written by Healz Team · Filed under Health Insights