Erythrocyte sedimentation rate: old test, new machine

One of the oldest and still most frequently requested blood tests is the erythrocyte sedimentation rate (ESR). It has been in use for nearly 100 years and remains one of the most popular tests in the pathology laboratory repertoire. It hasn’t changed fundamentally in the way it is carried out since first described by Westergren in 1921.1 In fact, as recently as 2011, the International Council for Standardization in Haematology and the Clinical and Laboratory Standards Institute in the USA confirmed and defined the reference method for ESR to be the Westergren method.2,3

Blood viscosity: indirect versus direct assessment
Essentially, ESR is a measure of blood viscosity and works the same way as a falling ball viscometer, using the patient’s red cells as millions of falling balls. The Westergren ESR result is not really a rate at all but a measure of how far the red cells have fallen in one hour when placed in a glass tube, 20 cm long and 2.55 mm internal diameter. The measurement is taken from the meniscus of the sample to the top of the red cell column below. The addition of one part of isotonic trisodium citrate to four parts of whole blood helps to standardise the sedimentation in samples with higher haematocrit values.

Red blood cells have a tendency to stack together, rather like a pile of plates, in formations called rouleaux, and when this happens the stack falls faster than do individual cells. Certain proteins, called acute-phase reactants, (eg C-reactive protein [CRP] and fibrinogen), alter the electrical properties of the red cell membrane, increasing the tendency of the RBCs to form rouleaux, thereby speeding up their rate of fall.