A low sed rate is rarely a cause for concern. Most of the time, it simply means your blood shows little or no sign of inflammation, which is exactly what you want to see. The sed rate (also called ESR, short for erythrocyte sedimentation rate) measures how quickly red blood cells settle to the bottom of a thin tube over one hour. A slower settling time produces a lower number, and most healthy people naturally fall well below the upper limits of the normal range.
What the Numbers Actually Mean
The sed rate is reported in millimeters per hour (mm/hr), and the “normal” ceiling depends on your age and sex:
- Males under 50: less than 15 mm/hr
- Males over 50: less than 20 mm/hr
- Females under 50: less than 20 mm/hr
- Females over 50: less than 30 mm/hr
- Newborns: 0 to 2 mm/hr
- Children before puberty: 3 to 13 mm/hr
These are upper limits, not targets. A result of 2 or 5 mm/hr in a healthy adult is perfectly normal. Even a result of 0 mm/hr can be normal in a person who has no symptoms and no other abnormal lab values. The test is designed to flag inflammation when the number goes up, not when it stays low.
How the Test Works
When there’s inflammation somewhere in the body, the liver produces more of certain proteins that coat red blood cells. Those coated cells tend to clump together in stacks (a formation called rouleaux), and those stacks are heavier, so they sink faster. A high sed rate reflects that faster sinking, signaling that something inflammatory may be going on, such as an infection, autoimmune condition, or tissue injury.
A low sed rate means the opposite: your red blood cells aren’t clumping much, so they settle slowly. In most cases, that’s simply a sign that your body isn’t dealing with significant inflammation.
When a Low Sed Rate Could Signal Something
In certain situations, a very low sed rate occurs not because you’re healthy, but because something about your blood is keeping those cells from settling normally. These causes are uncommon, and they almost always show up alongside other symptoms or abnormal lab results rather than appearing as a surprise on routine bloodwork.
Polycythemia is one example. This is a condition where your body makes too many red blood cells, thickening the blood. When the tube is packed with extra cells, they physically can’t fall as fast, and the sed rate drops. People with polycythemia often have other signs like headaches, dizziness, itching after a warm shower, or a ruddy complexion.
Abnormal red blood cell shapes can also keep the sed rate low. In sickle cell disease, the crescent-shaped cells can’t stack into those clumps the way normal disc-shaped cells do. The same is true for spherocytosis, where cells are round rather than flat. Without that stacking, cells settle individually and slowly, producing a deceptively low number.
Very high white blood cell counts (leukocytosis) can interfere with settling as well. When the blood contains a dramatically elevated number of white cells, the extra cells in the tube slow everything down. This kind of elevation usually points to a serious infection, a blood cancer, or an extreme stress response, all of which would be obvious from other symptoms or tests.
Heart failure and certain kidney or liver conditions can also produce a low sed rate, though the mechanism is less straightforward and these diagnoses are rarely made from an ESR result alone.
What a Low Sed Rate Does Not Tell You
The sed rate is a blunt instrument. It tells you something about the general level of inflammation in your body, but it can’t pinpoint where the inflammation is or what’s causing it. A high result prompts further investigation. A low result, on its own, doesn’t carry the same diagnostic weight.
It’s also worth knowing that the test has limits even when the result is elevated. The same proteins that make cells clump can rise with pregnancy, aging, obesity, or anemia, none of which are diseases. That’s why the sed rate is almost never used as a standalone test. It’s one piece of a larger picture that typically includes a complete blood count (CBC) and other markers of inflammation like C-reactive protein.
If Your Result Seems Unusually Low
If your sed rate came back at 0 or 1 mm/hr and you feel fine, there’s likely nothing to worry about. The test is most useful when it’s elevated. A low number in someone without symptoms is almost always a good sign, or at least a neutral one.
If you do have other unexplained symptoms, like persistent fatigue, unusual bleeding, or skin color changes, your provider will likely look at the full blood count rather than the sed rate to determine next steps. The CBC reveals the actual number and shape of your red blood cells, white blood cells, and platelets, providing far more specific information than the sed rate alone. A blood smear, where a technician examines your cells under a microscope, can identify abnormal shapes like sickle cells or spherocytes if those conditions are suspected.
For most people reading their lab results and wondering why the number is so low, the short answer is reassuring: a low sed rate typically means your body isn’t fighting significant inflammation, and that’s a good thing.