Sed rate and CRP tests are generally not accurate for AS patients. You may have active symptoms and no deviation from normal in these tests. The only accurate test for inflammation is an MRI where it can be seen. Unfortunately, by the time an MRI is scheduled and performed your inflammation may have resolved itself. Your rheumatologist should perform a physical exam at every office visit, paying close attention to areas that you report as painful. It's important to find a rheumatologist who treats the symptoms you report rather than one who waits for evidence.

Sed rate is just one consideration.

Probably the best way to answer your question is look at what the Mayo Clinic Staff says"

Definition

By Mayo Clinic staff
Sed rate, or erythrocyte sedimentation rate (ESR), is a blood test that
can reveal inflammatory activity in your body. A sed rate test isn't a
stand-alone diagnostic tool, but it may help your doctor diagnose or
monitor the progress of an inflammatory disease.

When your blood is placed in a tall, thin tube, red blood cells
(erythrocytes) gradually settle to the bottom. Inflammation can cause
the cells to clump together. Because these clumps of cells are denser
than individual cells, they settle to the bottom more quickly.

The sed rate test measures the distance red blood cells fall in a test
tube in one hour. The farther the red blood cells have descended, the
greater the inflammatory response of your immune system.

I have fused with a normal sed rate.

An elevated SED rate indicates inflammation, but inflammation doesn't indicate an elevated SED rate.  :[  It's crazy frustrating because you can have visible joint inflammation and still get a test with normal results.  Don't let it drive you over the cliff.  It can be maddening.  :[

Even during my worst flares my sed rate was always normal. It was so frustrating in the beginning because I thought it meant that nothing was really wrong with me!  Now I know that that can be a normal thing with an AS patient.

According to my rheumatologist it is not accurate for AS, often giving low numbers during periods of high inflammation (high numbers still indicate inflammation.) The problem is that it is the only form of arthritis for which it isn't accurate. So the tendency exists to assume it is correct when it is still off base.

Same here. Sed Rate very rarely elevated. I have hip, knee, neck, elbows, wrist, rib, heel and plantar pain...did I leave any joint out?  Normal sed rate with obvious swelling.  Took until I hit 50 to get a diagnosis. Most Doctors see a normal sed rate and move on. Finally after 20 years of pain I found a Rheumatologist who asked the right questions, ran a HLA B27 and finally a diagnosis. I have a daughter,  older sister and nephew who also suffer and are now waiting for a diagnosis. I just started Humira a week ago and waiting to see if it works.

When I was younger, like 19 to 24 years old, my sed rate was always elevated.  Then it went to normal range and they do not check it anymore.  Then they started testing the CRP and it was really high.  That is when the Humira I take has gotten it back down.  So stuff is still going on even though those indicators are in normal range.  My lumbar vertebrea are in the process of fusing, and SI joints totally fused.

My ESR has been normal every time it is tested, and every time my doctor dismisses my self reported symptoms because I have no inflammation markers in my body. The last time I mentioned less than 70% of people with AS have a raised ESR level  (per www.spondylitis.org/about/as_diag.aspx ) and asked if that was possible that I was that 30%, but he dismissed it and made me feel horrible saying that I did not have the active disease…I have started going to the doctor praying that my blood tests will be bad enough that he will take me seriously.