CRP, ESR and procalcitonin

Interpretation

These blood tests show how much inflammation there is in the body; procalcitonin also helps tell bacterial infection from other causes.

Updated Nov 13, 2026

What it is

C-reactive protein (CRP), the erythrocyte sedimentation rate (ESR) and procalcitonin (PCT) measure the body's inflammatory response, not a diagnosis. CRP is an acute-phase protein made by the liver under interleukin-6; it starts rising within hours of a stimulus, peaks after about two days and falls quickly once the cause settles, so it is good for following an acute process. The ESR measures how far red cells settle in an hour; it rises when fibrinogen and immunoglobulins make red cells stack together, so it changes slowly, rises with age, female sex, anaemia, pregnancy and kidney disease, and is useful mainly in giant cell arteritis, polymyalgia rheumatica, myeloma and some chronic infections such as osteomyelitis. Procalcitonin, the precursor of calcitonin, is released by many tissues in bacterial infection and is partly suppressed by viral infection through interferon; it is used to support the diagnosis of bacterial infection and sepsis and, above all, to decide when antibiotics can be stopped. None of the three is specific: a normal value does not exclude localised or early infection, and a high value must be matched to the patient. Their value is in the trend and in discordance between them.

Reference values and concepts

CRP: usual adult value below about 10 mg/L (many laboratories report below 5 mg/L; some report in mg/dL, below about 1 mg/dL).
High-sensitivity CRP for cardiovascular risk: below 1 mg/L low, 1–3 mg/L intermediate, above 3 mg/L higher risk.
ESR (Westergren): a rough upper limit is age ÷ 2 mm/h for men and (age + 10) ÷ 2 mm/h for women.
Procalcitonin: usually below 0.1 ng/mL; below 0.25 ng/mL bacterial infection unlikely, above 0.5 ng/mL likely, above 2 ng/mL high risk of sepsis.
Procalcitonin rises within a few hours of bacterial infection and falls by about half each day once it is controlled.
ESR is lowered by polycythaemia, sickle cell and spherocytes, low fibrinogen and very high white cell counts.

Reading it step by step

Ask what question the test answers: infection, autoimmune activity or follow-up — the marker should fit the question.
Look at the clinical picture first — fever, focus, vital signs; a marker never replaces examination or cultures.
Read CRP against the range and the timing of symptoms — very early illness can still give a low CRP.
Read the ESR adjusted for age and sex and look for anaemia, pregnancy or kidney disease — they raise it without inflammation.
Compare CRP and ESR — discordance points to SLE, a paraprotein or a process that is resolving.
Use procalcitonin in suspected bacterial infection with the cut-offs of your local pathway — it supports, not makes, the diagnosis.
Follow the trend on repeat samples — a falling CRP or PCT means the treatment is working.
Recheck when a result does not fit the patient — surgery, trauma and some drugs change the values.

Marker patterns and what they suggest

Pattern	Likely meaning	Think of
CRP very high (above 100 mg/L), PCT high	Bacterial infection likely	Pneumonia, pyelonephritis, abscess, sepsis, septic arthritis
CRP mildly raised, PCT low	Viral infection or non-bacterial inflammation	Viral respiratory infection, inflammatory arthritis, recent surgery
ESR very high (above 100 mm/h)	Marked inflammation or paraprotein	Giant cell arteritis, polymyalgia rheumatica, myeloma, osteomyelitis, endocarditis, malignancy
ESR high, CRP normal	Discordant markers	SLE flare, paraproteinaemia, anaemia, pregnancy, kidney disease, resolving illness
CRP high, ESR normal	Very early acute inflammation	Early bacterial infection, acute injury; ESR has not caught up yet
PCT high without infection	Non-infectious rise	Major surgery or trauma, burns, cardiogenic shock, medullary thyroid carcinoma, first days of life

Patterns and common mistakes

Using a normal CRP to exclude infection in early presentation, in localised infection or in a patient on immunosuppressants.
Treating a raised ESR in an older anaemic woman as disease when age, sex and anaemia explain it.
Delaying antibiotics in a septic patient while waiting for a procalcitonin result.
Reading a high procalcitonin after major surgery, trauma or burns as proof of infection.
Ordering ESR and CRP routinely as screening tests in well people; abnormal values rarely change management.
Missing giant cell arteritis because only CRP was sent: in suspected GCA, send both and treat urgently on clinical suspicion.

Next step

Suspected sepsis: take cultures and start treatment by the sepsis pathway at once; do not wait for CRP or procalcitonin.
Antibiotics started: repeat CRP or procalcitonin after 2–3 days; a clear fall supports stopping or shortening treatment.
Suspected giant cell arteritis (new headache, jaw pain, visual symptoms over 50 years): call a senior today, as sight is at risk.
Unexplained very high ESR: look for myeloma (serum and urine protein electrophoresis), infection and malignancy.
Results that do not fit: recheck the patient and repeat the test rather than treat the number.

Test yourself

Think of the answer, then tap the question to see it.

A 72-year-old woman has new temporal headache, jaw pain on chewing and ESR 95 mm/h. What is the next step?

Suspect giant cell arteritis: call a senior the same day and start treatment by the local protocol without waiting for a biopsy, because sight is at risk. CRP is usually raised as well.

A woman with SLE has ESR 70 mm/h and CRP 4 mg/L; another has CRP 120 mg/L. How do the two results differ?

High ESR with normal CRP fits a lupus flare. A high CRP in SLE suggests infection or serositis, so the second patient needs a search for infection.

On day 3 of antibiotics for pneumonia, procalcitonin has fallen from 4 ng/mL to 0.4 ng/mL and the patient is better. What does this support?

A fall of about 90% and a value below 0.5 ng/mL in a patient who is improving supports stopping antibiotics early, as in procalcitonin-guided pathways.

Why can a normal CRP not exclude infection in a patient seen 3 hours after symptoms began?

CRP needs several hours to rise and peaks after about two days, so early in an infection it can still be normal. Repeat it and rely on the clinical picture.

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: C-Reactive Protein: Clinical Relevance and Interpretation (StatPearls (NCBI Bookshelf), 2025); Erythrocyte Sedimentation Rate (StatPearls (NCBI Bookshelf), 2025); Procalcitonin (StatPearls (NCBI Bookshelf), 2023).

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