This study investigates the influence of haematocrit, fibrinogen concentration and fibrinogen availability (amount of fibrinogen per red blood cell) on erythrocyte sedimentation. The Westergren technique was applied to blood samples from 36 subjects and to their blood manipulated to haematocrits of 10, 20, 30 and 40%. Readings were taken every 10 minutes for 300 minutes. Previous studies indicate that erythrocyte sedimentation occurs in three phases. In this study, we show that haematocrit has little influence on either the rate of fall of particles in the first phase (
$m_{1}$
) or the duration of the first phase. This is also true for fibrinogen availability and for fibrinogen concentration at low haematocrits. At high haematocrits
$m_{1}$
increases with fibrinogen concentration. The rate of fall of rouleaux during phase 2 (
$m_{2}$
) and ESR
$_{60}$
both decrease exponentially with haematocrit and increase linearly with fibrinogen concentration. While
$m_{2}$
is more closely correlated to fibrinogen availability than to fibrinogen concentration or to haematocrit, this is not the case for ESR
$_{60}$
. Thus haematocrit, fibrinogen concentration and fibrinogen availability are more important to the velocity of sedimentation in the second phase than to the sedimenting velocity during phase 1 or to the duration of phase 1.
