What is hematocrit?
Blood is cells in fluid. Haematocrit is the share of those cells, in the Netherlands as a fraction: 0.45 means that 45 percent of your blood consists of red blood cells and the rest of plasma. Because the red cells carry the oxygen, haematocrit is a second measure of your oxygen transport, beside haemoglobin. The two almost always run in step: three times the haemoglobin in mmol/L comes out at about the haematocrit in percent. It is a ratio, not a count, and that is the core of reading it. If your plasma shrinks, because you drank little, sweated a lot or just trained hard, haematocrit rises without a single cell being added. If your plasma grows, as in endurance athletes and in pregnancy, it falls while your number of cells stays the same or even grows. More than with any other blood value, how you were doing when you had blood drawn counts here. Your report shows haematocrit in L/L. Normal is about 0.40 to 0.50 L/L for men and 0.35 to 0.45 L/L for women. Thick blood flows more slowly and strains the heart, and that is the reason the upper end counts here as much as the lower end.
Why is hematocrit relevant?
The first reason to know your haematocrit is that it confirms and complements haemoglobin. A low haemoglobin with a low haematocrit is anaemia; a low haemoglobin with a normal haematocrit is a sign that the cells are there but hold too little haemoglobin, and that points at iron. Together with the cell size, haematocrit shows what kind of anaemia it is, and so where to look. The second reason is sport. Endurance training enlarges your plasma volume within weeks; haematocrit then drops a few hundredths while your oxygen transport is actually better. That is why well-trained endurance athletes often come out below the band with nothing wrong. Altitude does the opposite: thin air sets the marrow to making more red cells, and after a few weeks at altitude the haematocrit sits higher. In cycling, haematocrit was for decades the line on which blood doping was read for that reason, because thick blood flows more slowly and strains the heart. The third reason is your fluid balance itself. A high haematocrit in someone healthy is nine times out of ten too little drunk, and that is the most common explanation for a blood count that suddenly looks different from last year. Smoking and sleep apnoea are the other two reasons that keep a high value up for years, and both can be reversed.
Hematocrit reference ranges
What counts beside the number:
- Haemoglobin from the same tube, which should tell the same story
- The red cell count and MCV, which say whether it is fewer cells or smaller cells
- Ferritin, because iron deficiency is the most common reason for a low value
- How much you drank and whether you trained hard before the draw
- Whether you do endurance sport, were at altitude, smoke or snore
- With a high value: white cells and platelets, which rise with it in a bone marrow matter
In the Netherlands haematocrit is given as a fraction in L/L; 0.45 is 45 percent red cells. The band for women sits lower than that for men, as with haemoglobin. More important than the band is how you were doing when you had blood drawn: drinking too little or hard exertion lifts the value, and the large blood volume of endurance athletes pushes it down, without anything being wrong with your cells. Always read haematocrit beside haemoglobin.
What Dutch labs actually use
0.35 – 0.45L/L
Outside this range every lab calls it abnormal.
every lab calls 0.35 – 0.45 L/L normal. Outside that range every lab calls it abnormal.
Every age, per lab
| Laboratory | Group | Reference range |
|---|---|---|
| Unilabs(Saltro, Medlon, SHO, Atalmedial)2026 | Men from 16 yrs | 0.40–0.50 L/L |
| Women from 16 yrs | 0.35–0.45 L/L | |
| Star-shl2026 | Men | 0.40–0.50 L/L |
| Women | 0.35–0.45 L/L | |
| Certe2026 | Men from 18 yrs | 0.4–0.5 L/L |
| Women from 18 yrs | 0.35–0.45 L/L | |
| Clinical Diagnostics2026 | Men from 16 yrs | 0.45–0.50 L/L |
| Women from 16 yrs | 0.35–0.45 L/L |
Each band as the lab publishes it, retrieved in 2026, unless marked otherwise.
Hematocrit high or low: what it means
A low haematocrit means proportionally fewer red cells or more plasma. In order of how often it occurs: anaemia, usually from iron deficiency; a larger plasma volume in endurance athletes and in pregnancy; blood loss; and reduced kidney function, because the kidney makes the hormone that drives the marrow. Look at the haemoglobin beside it and at ferritin: if those fall with it, it is a deficiency; if ferritin is normal and you train a lot, it is your blood volume. A high haematocrit means thickened blood or more cells. Dehydration is the most common reason, and a repeat after drinking normally settles that. If the value stays high, time at altitude, smoking and sleep apnoea are the usual explanations. Only a persistently high haematocrit without one of those reasons, especially with plenty of white cells and platelets too, belongs to a rare bone marrow condition and is investigated further. What you do about it: with a low value from a deficiency you top up iron, B12 or folate, and with a low value from training you do nothing. With a high value you drink normally and have blood drawn again; if you smoke, stopping is the lever, and if you snore and wake tired, sleep apnoea is the question. Always read haematocrit beside haemoglobin and the red cell count, and beyond that beside MCV and ferritin. Have blood drawn normally hydrated, not after hard exertion, and at the same time of day if you follow a trend.
What lowers it
Proportionally fewer red cells, or more plasma.
| Cause | How often |
|---|---|
| AnaemiaUsually from iron deficiency, and after that from a deficiency of B12 or folate; haemoglobin falls with it and ferritin or the cell size says which | Often |
| Expanded plasma volumeEndurance training and pregnancy let the plasma grow faster than the cells; the share falls while the transport is fine | Often |
| Blood lossHeavy periods, or unnoticed from stomach or gut; the body refills the plasma faster than the cells, so the share falls | Often |
| Reduced kidney functionThe kidney makes the hormone that drives the marrow; if it works less, fewer red cells are added | Sometimes |
What raises it
Thickened blood, or a body making more cells because it is short of oxygen.
| Cause | How often |
|---|---|
| DehydrationToo little drunk, plenty sweated or just trained hard; the plasma shrinks and the share of cells rises without a cell being added | Often |
| Time at altitudeThin air sets the marrow to making more red cells; after a few weeks at altitude the haematocrit sits higher, and that lasts for weeks | Sometimes |
| SmokingCarbon monoxide occupies part of the oxygen carrier and the body makes more cells to compensate | Sometimes |
| Sleep apnoeaNight-time oxygen dips drive production up; snoring and waking tired belong with it | Sometimes |
| Bone marrow diseaseSuch as polycythaemia; rare, and then the white cells and platelets usually rise too | Rare |
How does a hematocrit blood test work?
- Referral
- Not needed. Haematocrit is in every blood count you can have measured yourself.
- Fasting
- Not needed, but drink normally, because thickened blood gives a higher value.
- When
- Not just after hard exertion, a sauna or a day of drinking little, and not in the weeks after time at altitude if you want to know your usual value.
- The draw
- One tube of blood from a vein in your arm, the same tube as for the rest of your blood count.
- Repeating
- With a high value after a few days of drinking normally; with a low value together with haemoglobin and ferritin, a month after starting iron.
- Which test
- Hematocrit is in the Iron & fatigue test (€99) and in .
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
What is haematocrit?
The share of your blood that consists of red blood cells. At 0.45 L/L, 45 percent of your blood is cells and the rest plasma. Because the red cells carry the oxygen, it is a second measure of your oxygen transport, beside haemoglobin. It is a ratio and not a count, and that is why the value also moves when only your amount of fluid changes.
What is a normal haematocrit level?
About 0.40 to 0.50 L/L for men and 0.35 to 0.45 L/L for women. The value should run in step with your haemoglobin: three times the haemoglobin in mmol/L comes out at about the haematocrit in percent. If that deviates, it sits in the filling of the cells.
What does a low haematocrit mean?
Proportionally fewer red cells or more plasma. If haemoglobin falls with it, it is anaemia, usually from an iron deficiency, and beyond that from blood loss, a deficiency of B12 or folate, or kidneys working less. If you train a lot or are pregnant, it is often your blood volume that has grown, and that is not a deficiency. Ferritin and the cell size tell the difference.
What does a high haematocrit mean?
Usually thickened blood: too little drunk, plenty sweated or drawn after hard exertion. Drink normally and have blood drawn again. If the value stays high, it is a body making more red cells because it is short of oxygen: time at altitude, smoking or sleep apnoea. Rare is a bone marrow condition, and that stands out because the white cells and platelets rise too. Thick blood flows more slowly and strains the heart, so a high value that stays is one to resolve.
Why do endurance athletes sit lower?
Because endurance training enlarges the plasma volume within weeks: more fluid is added than cells, and the share of cells drops a few hundredths while the total oxygen transport is actually better. That is an adaptation, not anaemia. If your ferritin is normal, nothing is wrong; if it is empty, a deficiency does sit under that low value.
What does altitude do to your haematocrit?
Thin air holds less oxygen, and your body responds by making more red cells. After a few weeks at altitude your haematocrit sits higher, and that lasts for weeks after returning. That is exactly the effect altitude camps aim for. If you want to know your usual value, do not have blood drawn in the weeks after time at altitude.
What does my fluid balance have to do with haematocrit?
Everything, because it is a ratio between cells and plasma. Drinking too little, sweating, a sauna or a hard effort just before the draw let the plasma shrink and the value rise without a cell being added. That is the most common reason a blood count suddenly looks different from last year. Have blood drawn normally hydrated and at the same time of day if you follow a trend.
Do I need to fast for a haematocrit test?
No, but drink normally and do not have blood drawn after hard exertion, because thickened blood gives a higher value. Haematocrit comes from the same tube as your whole blood count, so haemoglobin and the red cell count sit beside it automatically.
Which tests include hematocrit?
Hematocrit is in the Iron & fatigue test (€99, also as an add-on for €15), in the Biological age test (€69, also as an add-on for €15), in the comprehensive test (€229) and in .
Related biomarkers
Read on
Sources
- 1.Unilabs, test catalogue (Dutch): haematocrit. 2026. bepalingenklapper.nl
- 2.Star-shl, test catalogue (Dutch): haematocrit. 2026. star-shl.nl
- 3.Certe, test catalogue (Dutch): haematocrit. 2026. bepalingenwijzer.certe.nl
- 4.Clinical Diagnostics, lab guide (Dutch): haematocrit. 2026. clinicaldiagnostics.nl
- 5.LESA Laboratory diagnostics (Dutch primary-care laboratory agreement). 2026. richtlijnen.nhg.org
- 6.NHG guideline on anaemia (M76), Dutch College of General Practitioners. 2024. richtlijnen.nhg.org
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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