Understanding Inflammation Blood Tests

Key inflammation markers compared

CRP vs ESR: which is better?

What does a very high CRP mean?

Procalcitonin: the bacterial infection marker

Procalcitonin (PCT) is produced by the body specifically in response to bacterial infection: not viral infections. This makes it useful for distinguishing bacterial pneumonia (high PCT) from viral pneumonia (low/normal PCT), and for deciding whether antibiotics are needed. Rising PCT indicates worsening infection; falling PCT with treatment indicates improvement. PCT is also used to guide antibiotic duration, antibiotics can often be safely stopped when PCT falls below 0.25 ng/mL.

Questions to ask your doctor

• Is my CRP raised: does this suggest active infection or inflammation?
• Is my ESR elevated: could this be temporal arteritis or myeloma?
• Has my procalcitonin been checked to guide antibiotic use?
• Is my inflammation responding to treatment (CRP trending down)?

High-sensitivity CRP and cardiovascular risk

Standard CRP testing detects acute inflammation and infection. High-sensitivity CRP (hsCRP) measures the same protein at much lower concentrations and serves a different purpose: estimating the long-term risk of heart attack and stroke in people with no active illness.

A landmark study of 27,939 women in the Women's Health Study followed participants for eight years and found that hsCRP was a stronger predictor of future cardiovascular events than LDL cholesterol.¹ Women with hsCRP above 3.0 mg/L had twice the risk of a first cardiovascular event compared with those in the lowest quartile, even when LDL was within the normal range. This evidence helped establish hsCRP as a recognised cardiovascular risk marker alongside lipids in clinical decision-making.

Interpreting hsCRP for cardiovascular risk: below 1.0 mg/L is low risk; 1.0–3.0 mg/L is intermediate; above 3.0 mg/L is high risk. The test is only meaningful when there is no concurrent infection or inflammatory condition, both of which raise CRP independently of vascular risk. A single high result during an illness tells you about the infection, not your heart.

How quickly do inflammation markers respond to treatment?

CRP falls rapidly once the trigger is removed, making it useful for real-time monitoring. In bacterial pneumonia responding to antibiotics, CRP should fall by roughly 50% within three to four days. A CRP that does not fall – or rises – after starting treatment is a warning sign of treatment failure, a resistant organism, or an undrained focus of infection (abscess, empyema). ESR falls much more slowly; it can take weeks to normalise even after complete recovery, making it unsuitable for monitoring acute treatment response but useful for tracking chronic inflammatory conditions over months.

In polymyalgia rheumatica and giant cell arteritis, both CRP and ESR typically fall dramatically within 48–72 hours of starting prednisolone – so promptly that a failure to respond should prompt a reassessment of the diagnosis. During steroid dose reduction, rising CRP or ESR precedes clinical relapse and provides the earliest laboratory signal to adjust therapy.

References

Sources cited on this page. PubMed links open the original abstract.

1. Ridker PM, Hennekens CH, Buring JE, Rifai N. C-reactive protein and other markers of inflammation in the prediction of cardiovascular disease in women. N Engl J Med. 2000;342(12):836–843. PMID 10733371 · doi:10.1056/NEJM200003233421202