Sedimentation Rate Test Health Article

Definition

The sedimentation rate test, also called the erythrocyte sedimentation rate (ESR) or sed rate test, measures the speed at which the red blood cells (erythrocytes or RBCs) separate from the liquid part of the blood (plasma) and settle to the bottom in a tube of anticoagulated blood.

Purpose

The sedimentation rate is a non-specific indicator of the presence of inflammation or infection. Although newer methods for diagnosing specific diseases have decreased the test's usefulness, it is still an important tool for the diagnosis and monitoring of two diseases: polymyalgia rheumatica and temporal arteritis. Sedimentation rate testing at regular intervals can also be helpful in predicting relapse in patients with chronic diseases such as Hodgkin's disease and other cancers.

Precautions

Patients who have bleeding disorders or are taking blood thinners might have trouble with bleeding following a venipuncture. Before having a blood sample drawn, such patients should tell the phlebotomist about their condition.

Description

The sedimentation rate test dates back to the early 1900s. In 1921, Westergren introduced a new method for performing the test that has become the time-honored classic. It is the recommended method of the International Committee for Standardization in Haematology and the National Committee for Clinical Laboratory Science. While automated methods can perform the sed rate in less time and with smaller samples of blood, many labs continue to use the Westergren method because it is simple and inexpensive.

For accurate results, the blood sample should be fresh (within two hours of collection is best). The sample might be rejected because it is too old or because the collection tube is underfilled, not labeled correctly, or contains blood that is hemolyzed or clotted.

The standard Westergren method includes the following steps:

• Diluting whole blood, or blood anticoagulated with EDTA, in citrate (1 volume of citrate to 4 volumes of blood).
• Aspirating the diluted blood to the 200 mm mark of a Westergren tube (30 cm in length and 2.55 mm in diameter with a uniform bore diameter) by means of a mechanical device or teat.
• Placing the tube in a vertical position in a Westergren rack in a location that is free of vibration and that is not exposed to direct sunlight.
• After exactly one hour, reading the distance the erythrocytes have fallen, and recording this value as the sedimentation rate.

Preparation

This test requires no special preparation.