Sedimentation measurement principle

The automatic reading of the Westergren sedimentation pipettes is performed by moving an optical sensor along the pipettes. While the sensor is moving, it is taking a reading every 0.25 mm. The sensor is reading the absorption of infrared light through the Westergren pipette filled with blood. From these readings, values at a number of absorption levels are determined. All absorption figures are relative to the darkest and lightest reading (darkest = 100 % and the lightest = 0 % absorption respectively).

By definition the levels are:

Graphic showing typical absorption values of a sample

Although the result of the ESR measurement is expressed in a quantifiable unit ("mm"), it is not a measurement that involves measurable quantities of any analytes or substances. What is measured is the result of a physiological process that is influenced by the properties of the blood sample and external factors. The ESR measurement must therefore be regarded as a qualitative process.