CRPvsESR

CRP and ESR both tell you that something inflammatory is going on, but they measure it in different ways and on different clocks. CRP is a direct count of one protein that rises within hours and falls within days. ESR is an indirect, slower test that reads how fast your red cells sink, and it is moved by things that have nothing to do with inflammation.

What is the difference between CRP and ESR?

CRP is a protein the liver releases under the control of the signalling molecule IL-6. After a single trigger, CRP rises above 5 mg/L by about 6 hours and peaks around 48 hours. Its half-life in the blood is about 19 hours and stays the same in health and disease, so the CRP level depends only on how fast the liver is making it, which tracks how active the process driving it is. ¹ The CRP marker page covers the ranges in detail.

ESR is measured by standing a tube of blood upright and recording how many millimetres the red cells fall in one hour. Inflammation raises proteins such as fibrinogen and immunoglobulins, which make red cells stack together and sink faster. Because the result depends on those proteins and on the red cells themselves, anything that changes the number, size or shape of red cells, or the protein mix of the plasma, changes the ESR too. ³ The ESR marker page covers its ranges.

C-reactive protein is a better indicator of inflammation than the erythrocyte sedimentation rate. It is more sensitive and responds more quickly to changes in the clinical situation.Harrison M. Erythrocyte sedimentation rate and C-reactive protein. Australian Prescriber, 2015 ²

CRP and ESR side by side

Why CRP moves fast and ESR moves slowly

CRP is cleared from the blood at a steady rate, so when the liver stops making extra, the level falls by about half every 19 hours. ¹ ESR depends mostly on fibrinogen and antibodies, which stay in the blood far longer, so the ESR keeps reading high well after the inflammation itself has eased. ²

In practice, on day 5 of a chest infection that is clearly improving, a falling CRP with a still-high ESR is the expected pattern, and a clinician tracking recovery will usually follow CRP for that reason.

What pushes ESR up or down besides inflammation

ESR has more non-inflammatory influences than CRP, and they explain most surprising results. Kidney disease, female sex and older age all raise ESR. ² Other common influences: ³

• Anemia raises ESR, because fewer red cells settle faster. See hemoglobin.
• Pregnancy raises ESR as fibrinogen climbs.
• High immunoglobulin levels, as in some blood cancers such as myeloma, can raise ESR a long way. See IgG.
• Low albumin is linked to ESR and CRP reading in different directions. ⁴ See albumin.
• Polycythemia (too many red cells) and abnormally shaped red cells, as in sickle cell disease, slow settling and lower ESR.
• A sample left standing too long before testing can change the result.

What affects CRP

In healthy young adults the median CRP is 0.8 mg/L and the 99th centile is 10 mg/L; after a strong trigger it can climb more than 10,000-fold within hours. ¹ Bacterial infection, injury, surgery and active inflammatory disease cause the large rises. Excess body fat and smoking hold CRP slightly higher over the long term, which is the low range the separate hs-CRP test reads for heart risk. Anemia and red cell shape do not change CRP.

Lupus is the well-known exception. CRP often stays fairly low even when lupus is active, so ESR keeps a monitoring role there, and a high CRP in someone with lupus raises the question of infection. ^{2 3}

When CRP and ESR disagree

Disagreement is common enough that it has been studied directly. In 2,069 hospital patients who had both tests on the same day, 2.6% had a high ESR with a low CRP and 1.5% had a high CRP with a low ESR. Infection, kidney disease and low albumin were linked to the high-ESR, low-CRP pattern. ⁴

High ESR, normal CRP

This is the more common split. The usual explanations are an inflammation that has already settled (CRP has fallen, ESR has not caught up), anemia, kidney disease, low albumin, older age, pregnancy, or raised antibody proteins. ^{3 4} A very high ESR with a normal CRP and no clear reason is one of the patterns that leads clinicians to check blood proteins more closely.

High CRP, normal ESR

This usually means something started recently. In the first day or two of an infection or injury, CRP is already up and ESR has not moved. ² It can also happen when something holds ESR down, such as polycythemia or abnormally shaped red cells. ³

Both high

Agreement makes active inflammation more likely. In 764 people having a temporal artery biopsy for suspected giant cell arteritis, a raised ESR and CRP together roughly tripled the odds of a positive biopsy (odds ratio 3.06). ⁵

Both normal

Two normal results make significant inflammation less likely, but not impossible. In the same biopsy study, CRP picked up 86.9% of confirmed giant cell arteritis and ESR 84.1%, and 4% of people with confirmed disease had both tests normal at diagnosis. ⁵ Symptoms still count when the numbers look fine.

Which one should you ask about?

• For a suspected infection, recovery after surgery, or checking whether treatment is working: CRP, because it moves with the illness. ²
• For suspected polymyalgia rheumatica or giant cell arteritis: usually both, since each catches cases the other misses. ⁵
• For lupus monitoring and some bone infections: ESR still adds information. ²
• For heart and blood vessel risk: neither standard test. That question uses hs-CRP, a more sensitive version of the CRP assay.
• For joint symptoms: both are often ordered with rheumatoid factor and ANA; see joint pain.

Whichever test you have, compare it with your own earlier results from the same lab rather than reading one number alone. A CRP falling from 80 to 20 mg/L over three days says more than either value. The reasons are set out in why a trend beats a single reading.

CRP and ESR usually sit beside a white blood cell count and sometimes fibrinogen. How they fit together is covered in how to read inflammation markers.

CRP tells you about the last day or two, ESR about the last few weeks, and ESR is also moved by age, sex, anemia, kidney function and blood proteins. A mismatch between them is usually explained by one of those, and a clinician reads both against your symptoms.

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