You don't mention what your diagnosis is, your symptoms, or any other pertinent data, so it's hard to answer your question without more information.
Most people with Chronic Fatigue Syndrome (CFS) have extremely LOW sed rates - often between 1 and 3. A sed rate of 13 is actually fairly normal for a healthy person. The high sed rates you've had in the past are an indication of a lot of inflammation. You should be tested for illnesses that cause high levels of inflammation.
My understanding is that it is a blood test that shows inflammation. It's a fairly crude test - the lab lets your blood sit and measures how its sedimentation rate - how fast it divides out into layers.
Doctors usually use this test to look for evidence of various conditions that can cause severe inflammation - like rheumatopid arthritis and others. There is no lower limit, so doctors will often say sed rate is "normal" as long as it isn't high. But, most people with CFS have extremely LOW sed rates, often between 1 and 3, though the doctor and/or lab often don't notice because there's no lower "alarm" limit. It's one of those tests that can help to indicate a diagnosis of CFS, even though there's no single lab test for it (yet!).
Dr. David Bell discusses low sed rate in his excellent (though now a bit out of date) book "The Doctor's G7uide to Chronic Fatigue Syndrome."
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