What Does The Sedimentation Rate Mean?

The sedimentation rate (“sed rate”) is not a perfect test, and using it can pose a challenge for physicians. An elevated sed rate is understood by physicians to be neither sensitive nor specific for RA. A highly sensitive test for RA would give a positive result for all people with RA; a highly specific test would give a positive result for only those people with RA and for no one else. A very sensitive and specific test would be a boon to patients and physicians alike. With a single blood test, the physician could make the diagnosis and start treatment. Unfortunately, such a test does not currently exist.

The sed rate is often elevated in RA, but not universally so. In fact, only 60% of people with RA have an elevated sed rate. Additionally, if a person has an elevated sed rate, it does not mean that individual has RA because an elevated sed rate is a nonspecific finding. In other words, it does not identify any particular disease, but merely indicates that inflammation is occurring in the body somewhere. Diseases other than RA that can produce an elevated sed rate include polymyalgia rheumatica, systemic lupus erythematosus, ulcerative colitis, heart attack, certain cancers, leukemia, tuberculosis, and other bacterial and viral infections. A sed rate can even be increased during a normal pregnancy.

Patients often wonder why they need a blood test that is so nonspecific and does not appear to help make the diagnosis of RA. Physicians understand that the sed rate sometimes helps to confirm an uncertain diagnosis or enables the physician to better monitor the disease activity. A new patient who presents with some joint or muscle pain but has a normal sed rate is less likely to have RA. A patient with established RA who complains of increased pain in his knee might be having a flare of his RA that will require an increase of medication or might be suffering from the effects of minor trauma. An elevated sed rate in this situation would suggest an increase in inflammation due to worsening RA. The physician could then treat the person with antirheumatic drugs with greater confidence and a better chance of improvement.

The sed rate test shouldn't be ordered as part of an annual physical examination, unless the doctor has a strong suspicion that the patient is sick. A sed rate in this setting is analogous to a low-grade fever. A low-grade fever isn't normal, but the list of its possible causes is almost endless and can result in needless worry and fruitless searches for a cause. In most situations, the C-reactive protein (CRP) test is a better option than the sedimentation rate for identifying RA; the CRP test is discussed in more detail in C-reactive protein.