CRP and ESR are the two most commonly requested markers of inflammation, but they measure different things and respond at different rates. CRP is a protein produced by the liver; its level rises within a few hours of inflammation starting and falls just as quickly, making it more useful for acute infections and monitoring treatment. ESR (erythrocyte sedimentation rate, known in Poland as OB — odczyn Biernackiego) changes more slowly and is affected by age, sex, pregnancy and anaemia, so it is more often used to monitor chronic conditions, such as rheumatic diseases.
What are CRP and ESR?
C-reactive protein (CRP) is an acute-phase protein produced in the liver, mainly in response to interleukin 6, one of the chemical messengers involved in inflammation. It was discovered in 1930 by Tillett and Francis, and its name comes from its reaction with the C polysaccharide in the capsule of the pneumococcus bacterium. The test measures the concentration of a specific substance in the blood, usually in mg/l.
CRP levels change rapidly. Following a strong inflammatory stimulus, the concentration exceeds 5 mg/l after about 6 hours and peaks after about 48 hours. The half-life of CRP in the blood is about 19 hours and is constant in both healthy people and those who are ill. This means that the CRP level depends solely on how much of the protein the liver is producing at that moment. As inflammation subsides, the result falls within a few days. The result report is issued in Polish.
Erythrocyte sedimentation rate (ESR) is an indirect test. The laboratory measures how many millimetres red blood cells settle in one hour in a vertical tube containing blood that has been prevented from clotting. During inflammation, the amount of certain plasma proteins in the blood increases (particularly fibrinogen), making red blood cells more likely to stick together in stacks and settle faster. The result is reported in mm/h.
The Polish name commemorates the physician Edmund Faustyn Biernacki (1866–1911). He described the practical diagnostic use of measuring red blood cell sedimentation in 1894 in “Pamiętnik Towarzystwa Lekarskiego Warszawskiego”, and in 1897 published the paper “Spontaneous blood sedimentation as a scientific and practical clinical method of investigation” in “Gazeta Lekarska”, as well as a German version. From 1902, he worked in Lviv. Internationally, the test was long attributed to the Swedes Fåhræus and Westergren, who studied it in 1918–1921. The Westergren method is now the reference method, but in Poland the test is still called odczyn Biernackiego.
CRP or ESR: how do they differ in practice?
Both tests indicate that inflammation is taking place in the body, but neither shows where it is or what is causing it. They differ in how quickly they respond and how susceptible they are to factors unrelated to inflammation.
| Feature | CRP | ESR |
|---|---|---|
| What it measures | Concentration of an acute-phase protein | Rate at which red blood cells settle |
| Unit | mg/l | mm/h |
| Rise after inflammation starts | After a few hours, peaking after about 48 h | More slowly, usually after 24–48 h |
| Return to normal | Within a few days after the cause resolves | Gradually, sometimes over weeks |
| Effect of age, sex, pregnancy and anaemia | Minor | Marked |
| Typical use | Acute infections, monitoring treatment, assessment after surgery | Rheumatic diseases and chronic inflammation, monitoring over time |
The results of the two tests do not always move together. One may be raised while the other is normal, which is why doctors often request both and assess them in the context of your symptoms.
When should you have a CRP test, and when an ESR test?
A C-reactive protein (CRP) test is usually chosen when timing matters: if you have a fever or a suspected infection, after surgery, or to assess whether treatment is working. A falling CRP level suggests that treatment is having an effect. CRP also helps diagnose and monitor inflammatory bowel disease and autoimmune diseases.
An ESR test is requested for symptoms suggesting chronic inflammation: persistent joint pain and stiffness, neck and shoulder girdle pain, unexplained fever or weight loss, headaches in older people, and anaemia. ESR has a particular role in diagnosing giant cell arteritis, polymyalgia rheumatica and rheumatoid arthritis, and is also included in some rheumatic disease activity scores used by EULAR.
In practice, if your symptoms started yesterday, CRP will tell you more. If your symptoms have lasted for weeks, both tests may be useful.
Normal CRP and ESR ranges: how to read your result
Always use the reference ranges on your laboratory report, as they depend on the method used. The values below are a guide only.
| Test | Result | General interpretation |
|---|---|---|
| CRP | below about 3 mg/l | Typical value for a healthy person (median in healthy young adults about 0.8 mg/l) |
| CRP | 3–10 mg/l | Slight rise; can occur in healthy people, more often in women and older people, and with obesity or smoking |
| CRP | 10–100 mg/l | Moderate rise; usually ongoing inflammation or infection (bacterial or viral) |
| CRP | above 100 mg/l | Marked rise; acute infections (more often bacterial), injuries, major surgery |
| ESR, men up to 50 years old | up to 15 mm/h | Normal range (Westergren method) |
| ESR, women up to 50 years old | up to 20 mm/h | Normal range |
| ESR, men over 50 years old | up to 20 mm/h | Normal range |
| ESR, women over 50 years old | up to 30 mm/h | Normal range |
| ESR, children | up to 10 mm/h | Normal range |
Because ESR naturally rises with age, a simple rule published in the “British Medical Journal” in 1983 (Miller et al.) is used in the literature to estimate the upper limit of normal in adults: age divided by 2 for men, and (age + 10) divided by 2 for women. For a 70-year-old woman, this gives about 40 mm/h. This is an approximation and does not replace a doctor’s assessment.
A low ESR rarely has clinical significance. It may accompany polycythaemia (an excess of red blood cells), sickle cell anaemia, certain abnormalities of blood cell shape, a very high white blood cell count or heart failure.
Bacterial or viral infection: what does CRP tell you?
This is one of the most common questions, and the answer requires caution. High CRP occurs in both bacterial and viral infections. According to information on mp.pl, viral infections usually raise CRP to about 50 mg/l, rarely to 100 mg/l, while the highest values, above 500 mg/l, occur in severe bacterial infections (particularly with sepsis) and after major injuries and operations. The StatPearls review indicates that values above 500 mg/l are associated with bacterial infection in about 90% of cases.
In its pneumonia guidelines (CG191), the UK’s NICE recommends considering a rapid CRP test in adults with symptoms of a lower respiratory tract infection when pneumonia has not been diagnosed after examination and it is unclear whether an antibiotic should be prescribed. With a result below 20 mg/l, antibiotics are not routinely prescribed; at 20–100 mg/l, a delayed prescription is considered (to be dispensed if symptoms worsen); and above 100 mg/l, an antibiotic is recommended. This is a tool for doctors, not a threshold for deciding on treatment yourself. ESR is not suitable for distinguishing bacterial from viral infections.
CRP and hs-CRP: testing cardiovascular risk
hs-CRP (high-sensitivity CRP) measures the same protein, but uses a method capable of detecting very low concentrations. It is not used to look for infection, but to assess chronic, low-grade inflammation in blood vessels as one of the risk factors for heart disease. The joint statement issued by the American Heart Association and CDC in 2003 defines three categories:
- below 1 mg/l: low risk,
- 1–3 mg/l: average risk,
- above 3 mg/l: high risk.
The same guidelines recommend measuring hs-CRP twice, ideally about 2 weeks apart, while the person is healthy and in a stable condition, and averaging the results. A result above 10 mg/l should be excluded as unreliable for risk assessment; infection or another source of inflammation should be investigated, and the test repeated. hs-CRP does not replace a lipid profile, blood pressure measurement or assessment of other risk factors, and its usefulness in screening remains a matter of debate.
How should you prepare for CRP and ESR tests?
CRP and ESR tests alone do not require any special preparation. However, because both are usually performed alongside a full blood count, glucose test or lipid profile, in practice it is best to attend in the morning on an empty stomach (8–12 hours without food; water is allowed). It is also worth:
- avoiding strenuous exercise and alcohol the day before,
- telling your doctor about any medicines and supplements you take, particularly non-steroidal anti-inflammatory drugs (e.g. ibuprofen, aspirin), statins and menopausal hormone therapy,
- for hs-CRP, choosing a time when you do not have an infection and waiting at least 2 weeks after a cold,
- for women, taking account of the timing of your period and any possible pregnancy, as these affect ESR.
What can distort ESR results?
ESR is sensitive to many factors unrelated to inflammation, and this is its main limitation. The result is raised by:
- pregnancy (a phenomenon described as early as 1918 by Fåhræus),
- age: ESR rises with age, and women have slightly higher values than men,
- anaemia: when there are fewer red blood cells, they settle faster, so ESR may be raised without inflammation,
- menstruation, obesity, kidney disease and certain cancers.
Factors that lower the result include polycythaemia, sickle cell anaemia, spherocytosis, regular alcohol consumption, greater physical activity and certain medicines (valproic acid, statins, NSAIDs). This is why ESR is always interpreted alongside a full blood count.
CRP is less susceptible to these effects, but factors that can raise it slightly include obesity, smoking, insomnia, depression and menopausal hormone therapy.
What should you do with your result, and which tests should you have alongside it?
A single abnormal result is not a diagnosis. If your CRP or ESR is above the normal range, it is worth seeing your GP, who will assess it alongside your symptoms and decide on further investigations. A high fever, shortness of breath, severe pain, signs of dehydration, or a CRP level of several tens of mg/l or more together with feeling unwell require urgent medical assessment. People over 50 with a new headache, pain when chewing or visual disturbances and a raised ESR should see a doctor without delay, as this could be giant cell arteritis.
The test most commonly requested alongside CRP and ESR is a full blood count with differential: the number and types of white blood cells help assess the nature of the inflammation, while haemoglobin shows whether anaemia may be affecting ESR. You can find more about interpreting a full blood count in our articles on full blood counts. Depending on your symptoms, your doctor may add urinalysis, kidney and liver tests, and, for rheumatological investigations, rheumatoid factor and anti-CCP antibodies.
Frequently asked questions
Is it better to have a CRP or ESR test?
For sudden symptoms, such as fever or a suspected infection, CRP will tell you more because it responds within hours. For persistent joint symptoms or monitoring a rheumatic disease, doctors often request both tests because they complement each other.
Does a high CRP mean I need an antibiotic?
Not automatically. CRP also rises in viral infections. Very high values, above 100 mg/l, are more often associated with bacterial infection, but the doctor decides whether to prescribe an antibiotic based on an examination and your symptoms.
Why is my ESR raised when I feel well?
The reason may be age, being female, pregnancy, menstruation, anaemia or obesity. A slight rise in ESR without symptoms is often not significant, but it is worth discussing it with your doctor and checking your full blood count.
Are CRP and hs-CRP the same test?
Both measure the same protein, but hs-CRP detects very low concentrations and is used to assess cardiovascular risk. Standard CRP is used when infection or inflammation is suspected.
Sources
- MedlinePlus (U.S. National Library of Medicine), “C-Reactive Protein (CRP) Test”, online version. link
- MedlinePlus (U.S. National Library of Medicine), “Erythrocyte Sedimentation Rate (ESR)”, online version. link
- Pearson T.A. et al., American Heart Association, Centers for Disease Control and Prevention, “Markers of Inflammation and Cardiovascular Disease: Application to Clinical and Public Health Practice”, Circulation, 2003. link
- NICE, “Pneumonia in adults: diagnosis and management (CG191)”, 2014. link
- Pepys M.B., Hirschfield G.M., “C-reactive protein: a critical update”, Journal of Clinical Investigation, 2003. link
- Singh B., Goyal A., Patel B.C., “C-Reactive Protein: Clinical Relevance and Interpretation”, StatPearls (NCBI Bookshelf), 2025. link
- Tishkowski K., Zubair M., “Erythrocyte Sedimentation Rate”, StatPearls (NCBI Bookshelf), 2025. link
- Kucharz E.J., “Sto pięćdziesiąta rocznica urodzin Edmunda Faustyna Biernackiego, twórcy testu pomiaru szybkości opadania krwinek czerwonych” (The 150th anniversary of the birth of Edmund Faustyn Biernacki, creator of the erythrocyte sedimentation rate test), Forum Reumatologiczne, 2016. link
- Wiercińska M., “Badanie CRP. Jakie są normy CRP?” (CRP testing: what are the normal ranges?), mp.pl, 2021. link
- Miller A., Green M., Robinson D., “Simple rule for calculating normal erythrocyte sedimentation rate”, British Medical Journal, 1983. link
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