The ESR, or erythrocyte sedimentation rate, measures how fast your red blood cells sink to the bottom of a tube in one hour. That is all it does. A high value means your cells sank faster than expected, usually because more inflammatory proteins are floating in your plasma making the cells clump together.
So it is an indirect measurement. And that is where the trouble starts.
Because anything that affects your red cells also affects your ESR. Anaemia raises it without a shred of inflammation being involved. For an endurance athlete with diluted blood, that is not a theoretical detail.
What is a normal ESR value?
That depends on your age and your sex, which already tells you something about how crude this test is. A common rule of thumb: for men, your age divided by two; for women, your age plus ten, divided by two. For a man of 40 that works out to roughly 20 mm per hour as an upper limit.
Your own lab uses its own range, and it is printed on your result. Use that, not the rule of thumb.
What stands out most: the upper limit climbs with age. An ESR of 22 mm/hour means something different in a man of 25 than in a man of 70. Few other blood values depend so heavily on who you are.
What is the difference between ESR and CRP?
Speed and directness. CRP is a protein your liver makes during inflammation, and it rises within six to eight hours. The ESR measures a consequence of those proteins, reacts days later, and then lingers for weeks. CRP is the smoke alarm; the ESR is the smoke that reaches the hallway later.
Research placing the two side by side in acute inflammation found exactly that difference in kinetics (PMID 31598629). For fresh inflammation CRP is the more useful number, while for tracking something long-running the ESR can hold its value (PMID 25290132).
| ESR (sedimentation rate) | CRP | |
|---|---|---|
| What it measures | How fast red cells sink | An inflammatory protein, measured directly |
| Rises after | Days | Six to eight hours |
| Falls after | Weeks | Days |
| Sensitive to anaemia | Yes, a low haematocrit raises the ESR | No |
| Sensitive to age and sex | Yes, strongly | Barely |
| Useful for quiet, low-grade inflammation | Too crude | Only the high-sensitivity version, hs-CRP |
If you may pick only one of the two for an acute complaint, CRP is almost always the better number. That is not an opinion, that is kinetics.
Why can your ESR be high without inflammation?
Because the test depends on your red blood cells themselves. If fewer cells sink through the tube, they collide with each other less and sink faster. The result: in anaemia your ESR rises while there is no inflammation anywhere. The test cannot tell the difference.
That is exactly where it gets interesting for endurance athletes. Training makes your body produce extra blood plasma, so your haematocrit falls without your oxygen transport getting any worse (PMID 10688281). A diluted blood count with a mildly raised ESR looks worrying on paper while you are fitter than ever.
Imagine a cyclist mid-build with a haematocrit of 0.38 l/l and an ESR of 18 mm/hour. His hs-CRP is 0.6 mg/l, so low. The ESR is raised, the inflammation marker is not. That is not a contradiction; that is the ESR measuring his diluted blood.
I have seen this more often than I would like, and it is an unnecessary scare every time.
Which causes of a raised ESR are there?
A raised sedimentation rate is a broad signal and says little about the cause on its own. Infections, autoimmune disease, anaemia, pregnancy, kidney disease and even excess weight can raise it. That is the core of the criticism of this test: it is sensitive but not specific (PMID 29094869).
A mild rise without symptoms is therefore rarely a diagnosis. It is a reason to look further.
The NHG guidelines seldom use the ESR alone. It almost always sits alongside other values, and that is exactly how you should read it yourself.
When should you discuss a raised sedimentation rate with your GP?
When it is markedly raised, when it stays high on a repeat, or when you have symptoms alongside it. Persistent joint pain, fever, unexplained weight loss or night sweats belong in a conversation, whatever the number says. An ESR sitting just above the range with no symptoms is a different story.
Take your blood count to that conversation. Judging the ESR apart from your haematocrit is precisely the mistake this test invites.
My position: if your hs-CRP is low and your haematocrit sits on the low side, your ESR deserves an explanation from your red series before anyone starts thinking about inflammation.
Where do you start?
Never let your ESR be judged as a standalone number. It belongs next to your haematocrit, your haemoglobin and preferably a CRP. Our complete blood count test gives you that red series, with an assessment by a BIG-registered doctor included.
How to read all the values together is in complete blood count explained. Why your haematocrit falls with training is in low red blood cell count. And what quiet, low-grade inflammation does to your energy is in hs-CRP and fatigue.
References
- Kratz A, et al. Erythrocyte sedimentation rate and C-reactive protein in acute inflammation. American Journal of Clinical Pathology. 2020. PMID 31598629.
- Litao MK, Kamat D. Erythrocyte sedimentation rate and C-reactive protein: how best to use them in clinical practice. Pediatric Annals. 2014. PMID 25290132.
- Bray C, et al. Erythrocyte sedimentation rate and C-reactive protein measurements and their relevance in clinical medicine. WMJ. 2016. PMID 29094869.
- Fallon KE, Fallon SK, Boston T. Sports haematology. Sports Medicine. 2000. PMID 10688281.
- NHG. Dutch College of General Practitioners, inflammatory markers in primary care. Accessed 2026.
- RIVM. Blood testing and reference values. Accessed 2026.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. For treatment decisions, discuss your results with your GP.
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