DefinitionAgglutinins are antibodies that cause the red blood cells to clump together.Cold agglutinins are active at cold temperatures.Febrile (warm) agglutinins are active at normal body temperatures.This article discusses the blood test used to measure the level of these antibodies in the blood.Alternative NamesCold agglutinins; Weil-Felix reaction; Widal's test; Warm agglutininsHow the test is performedBlood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight v