🔬 CRP and ESR

CRP and ESR are non-specific markers of systemic inflammation. CRP is an acute-phase protein produced by the liver in response to IL-6, rising quickly in acute injury or infection. ESR measures how fast red cells settle; it is influenced by fibrinogen and immunoglobulins, making it a better marker for chronic inflammation and certain haematological protein disorders.

CRP is indicated for suspected acute infection, sepsis, or post-operative monitoring for complications. It is also used to assess flares in inflammatory bowel disease. ESR is particularly indicated when investigating suspected GCA, PMR, multiple myeloma, or chronic bone infections (osteomyelitis). They are often ordered together as part of a 'vasculitis screen' or for 'fever of unknown origin'.

CRP (C-Reactive Protein) is measured from a serum sample (yellow top) using an immunoassay. ESR (Erythrocyte Sedimentation Rate) is measured using an EDTA sample (purple top); it measures the rate at which red blood cells sink in a standardized tube over one hour. Modern labs often produce CRP results much faster than ESR.

Normal CRP is typically <5-10 mg/L. A normal ESR is age and sex-dependent (roughly Age/2 for men and (Age+10)/2 for women), generally staying below 15-20 mm/hr in younger adults. Normal levels suggest a lack of significant systemic inflammation but do not rule out localized or very early disease.

CRP rises and falls rapidly (half-life of 19 hours), making it a 'real-time' marker of acute inflammation. ESR is a 'slow' marker; it takes days to rise and weeks to fall, reflecting the previous weeks' inflammatory status. Interpretation must account for the clinical picture—CRP can be high in trauma or malignancy without infection. In GCA, a very high ESR (>50 or >100 mm/hr) is a classic finding.

Elevated CRP (>10 mg/L) suggests acute inflammation; levels >100 mg/L are highly suggestive of bacterial infection, while very high levels (>200-300 mg/L) suggest severe sepsis or abscess. Elevated ESR is non-specific but seen in chronic infections (e.g., TB, endocarditis), autoimmune diseases (SLE, RA), and plasma cell dyscrasias (myeloma). A 'discrepant' result (high ESR, normal CRP) can occur in SLE or low-grade chronic conditions.

CRP is the primary marker for monitoring the acute phase response to infection and treatment response (e.g., assessing if antibiotics are effective). ESR is useful for specific chronic conditions like Giant Cell Arteritis (GCA) and Polymyalgia Rheumatica (PMR), where it often forms part of the diagnostic criteria. Both are part of the 'inflammatory markers' panel used widely across all medical and surgical specialties.

Using CRP to 'rule out' infection in the first few hours of symptoms is a mistake (it takes ~6-12 hours to rise). Attributing a high ESR entirely to inflammation without checking the full blood count is a pitfall, as anaemia itself will increase the ESR. Over-reliance on these markers in the absence of clinical symptoms (the 'treating the numbers' trap) should be avoided.

Both tests are highly sensitive but very non-specific. They cannot distinguish between infection, autoimmune inflammation, or malignancy. ESR is affected by red blood cell shape and number (e.g., anaemia falsely raises ESR). CRP can be misleadingly low in certain conditions like SLE (where ESR is usually high instead) or in the very early stages of a serious infection.

MLA focus: Remember that CRP is preferred for acute infection/monitoring. Know the classic 'high ESR' conditions: GCA, PMR, and Myeloma. Understand that CRP can be normal in SLE unless there is a co-existing infection. Be aware of the 'lag' in ESR—it doesn't change quickly enough for acute sepsis management.