Laboratory tests - inflammatory markers

CRP and ESR

CRP and ESR are non-specific markers of systemic inflammation, useful for tracking disease activity and treatment response but unable to distinguish infection from other inflammatory causes (autoimmune disease, malignancy, tissue injury) without clinical correlation
Amongst the most frequently ordered tests in acute and chronic illness assessment given their availability and utility in monitoring trends over time

CRP rises and falls rapidly (within hours to 1-2 days), making it useful for tracking acute response to treatment; ESR rises and falls more slowly (days to weeks) and is influenced by factors beyond inflammation alone (anaemia, pregnancy, age) - this differing kinetics explains why CRP is generally preferred for monitoring acute treatment response, while ESR retains value in some chronic inflammatory conditions (e.g. polymyalgia rheumatica, temporal arteritis)

Neither test is diagnostic in isolation - a normal CRP/ESR does not exclude infection (particularly early in the course, or with certain organisms/sites less associated with a systemic inflammatory response), and an elevated result does not confirm infection over another inflammatory cause
Serial trending (rather than a single value) is most useful for tracking response to antimicrobial treatment or disease activity over time

A falling CRP with clinical improvement supports continuing current treatment; a static or rising CRP despite treatment should prompt reassessment (inadequate source control, resistant organism, wrong diagnosis) rather than simply extending the same antimicrobial course
Extreme elevation can support (but not confirm) more severe/invasive infection and prompt more urgent source-identification imaging

Sepsis monitoring, polymyalgia rheumatica/giant cell arteritis (ESR's continued diagnostic role), treatment response tracking, malignancy/autoimmune disease as non-infective causes of elevation

A normal CRP/ESR does not exclude infection, particularly early in the disease course - these are trending tools for monitoring known or suspected inflammatory processes, not sensitive screening tests capable of ruling infection in or out on a single value

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