A high hematocrit means your blood contains a larger-than-normal proportion of red blood cells. Normal hematocrit is 41% to 50% for men and 36% to 44% for women. When your percentage climbs above those ranges, your blood becomes thicker, which makes it harder for your heart to pump and raises your risk of blood clots. The cause can be as simple as dehydration or as serious as a bone marrow disorder, so the number itself is just the starting point.
What a High Hematocrit Feels Like
Many people with mildly elevated hematocrit feel nothing unusual. As levels climb higher, the increased blood thickness starts producing symptoms that are easy to mistake for other problems: headaches, dizziness, fatigue, shortness of breath when lying down, and blurred or double vision. Some people notice heavy night sweats or unusual bleeding from the gums or nose.
One distinctive sign is skin-related. Itching after a warm shower or bath, facial redness, and a burning sensation in the hands or feet point specifically toward too many red blood cells rather than a general illness. If you’re experiencing a combination of these symptoms alongside a high hematocrit reading, that pattern helps your doctor narrow the cause.
Three Categories of Causes
Doctors group the causes of high hematocrit into three buckets, and the distinction matters because each one is managed differently.
Relative Erythrocytosis
This is the most common and least alarming explanation. Your body isn’t actually making extra red blood cells. Instead, you’ve lost plasma (the liquid portion of blood), so red blood cells make up a bigger share of what’s left. Dehydration is the classic trigger. If you were dehydrated when your blood was drawn, a repeat test after proper hydration may come back normal.
Secondary Erythrocytosis
Here, something outside the bone marrow is driving your body to produce more red blood cells. The mechanism is a hormone called erythropoietin (EPO). When your tissues don’t get enough oxygen, EPO levels rise and signal the bone marrow to ramp up red blood cell production. Conditions that trigger this include:
- Chronic lung disease or heart disease, which reduce how much oxygen reaches your tissues
- Sleep apnea, which causes repeated drops in oxygen overnight
- Smoking, which introduces carbon monoxide and displaces oxygen in the blood
- Living at high altitude, where the air contains less oxygen
- Testosterone therapy or anabolic steroids, which directly stimulate red blood cell production
Smokers typically show hematocrit levels about 1.6% to 2.3% higher than non-smokers, based on data from the Copenhagen General Population Study. That sounds modest, but for someone already at the upper end of normal, it can push them over the threshold. People living at high altitude see a more dramatic shift. Hematocrit rises within a few weeks of moving to altitude and stabilizes at a new, higher steady state for as long as you remain there. Andean populations living around 4,000 meters routinely carry hemoglobin levels of 17 to 19 g/dL, well above sea-level norms.
Rarely, certain tumors (including some kidney, liver, and brain tumors) secrete excess EPO on their own, pushing hematocrit up without an oxygen-related trigger.
Primary Erythrocytosis
In this category, the problem originates in the bone marrow itself. A genetic defect causes the bone marrow to overproduce red blood cells regardless of what the body actually needs. The most serious form is polycythemia vera, a slow-growing blood cancer. Under WHO diagnostic guidelines, polycythemia vera is suspected when hematocrit exceeds 49% in men or 48% in women, alongside other findings like a specific gene mutation and characteristic bone marrow changes. At very high levels (above 55.5% in men or 49.5% in women), the diagnosis becomes even more straightforward.
One way doctors distinguish primary from secondary causes is by measuring EPO. In primary erythrocytosis, EPO is low because the bone marrow is acting on its own. In secondary erythrocytosis, EPO is high because something is actively signaling for more red blood cells.
Why Thick Blood Is Dangerous
Red blood cells carry oxygen, so having more of them sounds like it should be beneficial. Up to a point, it is. But past a certain concentration, the blood becomes viscous enough to slow circulation. Thick, sluggish blood is more likely to form clots, and those clots can block arteries supplying the brain, heart, or lungs. The result is an increased risk of stroke, heart attack, and deep vein thrombosis. Without management, severely thickened blood can reduce flow to vital organs enough to cause organ damage.
This is why even people who feel fine with a high hematocrit reading need follow-up. The complications of hyperviscosity tend to strike suddenly rather than building gradually, so the absence of symptoms doesn’t mean the risk isn’t there.
How High Hematocrit Is Managed
Treatment depends entirely on the underlying cause. If dehydration explains the result, rehydration is all that’s needed. If sleep apnea is driving the elevation, treating the apnea (usually with a CPAP machine) allows oxygen levels to normalize and red blood cell production to slow down naturally. Smokers who quit often see their hematocrit drift back toward normal over weeks to months.
For polycythemia vera and other primary causes, the standard intervention is therapeutic phlebotomy, which is essentially a controlled blood draw. It’s similar to donating blood: a set volume is removed at regular intervals to bring hematocrit below 45%, which is the target most hematologists aim for. Early on, phlebotomy sessions may happen every day or every other day until levels come down. Once stable, the interval stretches to every few weeks or months, depending on how quickly your levels climb back up.
For people on testosterone replacement therapy or anabolic steroids, adjusting or stopping the medication is often enough to resolve the problem. Your doctor will likely monitor your hematocrit periodically if you stay on testosterone, since it reliably pushes red blood cell counts upward.
What to Expect After an Abnormal Result
A single high hematocrit reading on a routine blood test doesn’t automatically mean something is wrong. Your doctor will typically want to confirm the result with a repeat test, ideally when you’re well-hydrated and haven’t just exercised intensely. If the number stays elevated, the next steps usually include checking EPO levels, iron stores, oxygen saturation, and sometimes a specific gene mutation associated with polycythemia vera.
Context matters a lot in interpreting the result. A hematocrit of 51% in a man who just ran a marathon in the heat has a very different meaning than the same number in a sedentary non-smoker with no obvious explanation. The goal of the workup is to figure out which of the three categories your elevation falls into, because that determines whether you need ongoing treatment, a lifestyle change, or simply a glass of water.