Having spent over a week contemplating various aspects of AS, from diagnosis to treatment options, from prognostication to tapering; it may come as somewhat of a surprise to you that the criteria for Diagnosis, Remission and Flare in AS are still works in progress.
The latest diagnostic (classification, to be precise) criteria proposed in 2012 are still awaiting validation and widespread adoption (usually under the joint auspices of the ACR and EULAR).
We are all agreed that the TARGET of therapy in AS is REMISSION, as in other rheumatic diseases for which we actually have effective therapies.
But what does remission in AS look like? Is it just pain relief and functional restoration, or is the prevention of structural progression of ultimate importance? Surely without defining it, we may be wasting lots of costly arrows (eg the biologics) without quite achieving our target.
We need to do better than answer, as Judge Stewart did for hardcore pornography, "I know it when I see it".
Like the natural history of AS, definitions, criteria and guidelines move much slower compared to RA. One major difficulty in defining remission criteria in AS is that disease activity may be different in the various domains of the disease (eg axial inflammation, enthesitis, uveitis).
Even if the diagnosis is not in doubt, and treatment has delivered deep clinical and radiological remission in all domains of AS, the task at hand is to detect disease flare early enough to allow for a brief therapeutic intensification to restore remission.
Flare criteria/definition is not simply the loss of remission criteria (which is still a work in earnest progress). If the experience with RA is anything to go by, a recent study suggests that the RA patient is better able to discern a flare than all the metrology and the physician's assessment. I call this the "How-ya-doin' " score. It actually involves talking to the patient; not just filling charts, checking bloods and scanning joints.