Erythrocyte Sedimentation Rate (ESR)

The Westergren method draws anticoagulated blood into a vertical 200 millimetre tube and reports how many millimetres of clear plasma appear above the settling red cells after one hour. Red cells fall faster when they stack into rouleaux, and they stack when large asymmetric plasma proteins, chiefly fibrinogen and immunoglobulins, reduce the electrostatic repulsion between them. The ESR is therefore not a measurement of inflammation at all; it is a measurement of plasma protein composition and of the red cells themselves.

Fibrinogen has a half-life of days, so the ESR climbs over several days and takes weeks to return to baseline, while C-reactive protein moves in hours. Normal limits rise with age, and a widely used rule of thumb sets the upper bound at age divided by two in men and age plus ten divided by two in women. Values above 100 millimetres per hour narrow the differential sharply towards serious infection, malignancy or vasculitis, and an ESR at or above 50 is among the classification criteria for giant cell arteritis.

The two markers answer different questions rather than substituting for each other: CRP reports what is happening now, ESR the preceding weeks. Discordance is informative rather than a laboratory error, and a high ESR with a normal CRP points towards a paraprotein, anaemia or chronically raised immunoglobulins rather than active inflammation.

Its non-inflammatory determinants are the trap. Anaemia raises the ESR while polycythaemia and sickle cells lower it, pregnancy and advancing age raise it, and a monoclonal paraprotein can push it into three figures with no inflammatory process at all. A falling ESR during treatment may therefore reflect nothing more than a recovering haematocrit, and measuring it before and after starting a compound generates a difference that is mostly noise.