A sed rate (or erythrocyte sedimentation rate or ESR) is a common blood test to rule out disease. A sed rate is "the distance that red blood cells settle in a tube of blood in one hour". It is non-specific, in other words it does not diagnose a specific disease. A sed rate is still quite valuable.
Inflammation in the body is indicated by an increase in the sed rate. Inflammation occurs with many diseases, infections and cancer. As inflammation increases so does the sed rate, as inflammation decreases (or the patient gets better) the sed rate goes back to normal.
(This doesn't always occur with cancer.)
A decrease in sed rate below normal can also occur. Decreased sed rates are associated with diseases that "change the size or shape or red blood cells" and/or "diseases that cause the body to make less protein or extra red blood cells".