A digital sedimentator for measuring erythrocyte sedimentation rate using a linear image sensor

A digital apparatus was fabricated to determine accurately the erythrocyte sedimentation rate (ESR) using a linear image sensor. Currently, ESR is utilized for clinical diagnosis, and in the laboratory as one of the many rheological properties of blood through the settling of red blood cells (RBCs). In this work, we aimed to measure ESR automatically using a small amount of a sample and without moving parts. The linear image sensor was placed behind a microhematocrit tube containing 36 μ l of RBC suspension on a holder plate; the holder plate was fixed on an optical bench together with a tungsten lamp and an opal glass placed in front. RBC suspensions were prepared in autologous plasma with hematocrit H from 25% to 44%. The intensity profiles of transmitted light in 36 μ l of RBC suspension were detected using the linear image sensor and sent to a personal computer every minute. ESR was observed at the settling interface between the plasma and RBC suspension in the profile in 1024 pixels (25 μ m / pixel ) along a microhematocrit tube of 25.6 mm total length for 1 h at a temperature of 37.0±0.1 ° C . First, we determined the initial pixel position of the sample at the boundary with air. The boundary and the interface were defined by inflection points in the profile with 25 μ m resolution. We obtained sedimentation curves that were determined by the RBC settling distance l(t) at the time t from the difference between pixel locations at the boundary and the interface. The sedimentation curves were well fitted to an empirical equation [Puccini et al., Biorheol. 14, 43 (1977)] from which we calculated the maximum sedimentation velocity s max at the time t max . We reached t max within 30 min at any H , and s max linearly related to the settling distance l(60) at 60 min after the start of sedimentation from 30% to 44% H with the correlation coefficient r=0.993 . Thus, we may estimate conventional ESR at 1 h from s max more quickly and accurately with less effort.