sedimentation rate
Meaning of sedimentation rate: The falling red cells are only a stand-in; what the test really tracks is a shift in the proteins dissolved around them.
Definition of sedimentation rate
A sedimentation rate is a blood test measuring how far red cells sink down a vertical tube in one hour. Inflammation raises certain plasma proteins, chiefly fibrinogen, which cancel the charge normally keeping red cells apart. The cells stack together, become heavy for their surface area, and fall faster. A raised result signals inflammation somewhere without saying where or why.
How does it compare with C-reactive protein?
| Feature | Sedimentation rate | C-reactive protein |
|---|---|---|
| What is measured | How fast red cells settle | The concentration of one protein |
| Speed of rise | Slow, over days | Fast, within hours |
| Speed of fall | Slow, over weeks | Fast, once the stimulus ends |
| Affected by anemia | Yes | No |
The two are often ordered together because they answer slightly different questions. C-reactive protein tracks what is happening now, while the sedimentation rate lags behind and therefore describes the recent past. A protein level already falling alongside a still-raised sedimentation rate usually indicates inflammation that has settled rather than one continuing, and reading them the other way round is a common error.
What else changes the result?
Anything that alters the number, the shape or the surroundings of red cells. Anemia raises the rate simply because fewer cells fall with less crowding, while a high red cell count lowers it. Cells that cannot stack, as in sickle cell disease, give a low result even during a severe crisis. High levels of antibody protein raise it, and so do pregnancy, increasing age and being female, so a single number means little without knowing whose blood it came from. A grossly raised value in someone who appears well can itself point toward an abnormal protein produced by a bone marrow cancer. Technique matters too: a tube not held upright, or blood left standing too long before testing, shifts the reading.
When is a very high result meaningful?
A markedly raised rate narrows the field considerably. Giant cell arteritis and polymyalgia rheumatica, inflammation of the arteries around the scalp and of the muscles around the shoulders and hips, characteristically drive it very high, and the test forms part of how both are assessed. Deep-seated infection such as bone or heart valve infection, myeloma, and some cancers do the same. The reverse does not hold: a normal result excludes none of these, and the test is read alongside the examination and the rest of the picture rather than treated as an answer in itself.
Also known as
Used in a sentence
A sedimentation rate was ordered as part of the workup for suspected inflammatory disease.
Good to know
An elevated sedimentation rate reflects the general presence of inflammation without indicating its cause or location, and it also rises and falls slowly, so the result is interpreted together with other tests and clinical findings.
Word origin
Where “sedimentation rate” comes from
The related term erythrocyte, referring to the red blood cells being measured, comes from Greek erythros, meaning red, and kytos, meaning cell.