There have been several discussions lately about inflammatory markers, their presence in psoriatic disease, and their use as a diagnostic tool. Well, as they say, it’s complicated.
Inflammatory markers – CRP and ESR – are pretty sensitive lab tests which measure local or systemic inflammation. They will be ordered when you are being diagnosed and periodically during your treatment by a rheumatologist.
One of the quirky and difficult things about PsA is that a lot of the time it doesn’t cause a rise in CRP or ESR. An inflammatory disease that doesn’t show up in the blood tests for inflammation! Go figure. But anyway, read this quote –
Laboratory tests are usually done at diagnosis and periodically thereafter. However, there is to date no diagnostic test. The acute phase reactants (erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP)) are often normal. These are raised in less than 50% of people with psoriatic arthritis. Rheumatoid factor is usually negative.
Gladman, Dafna, and Chandran Vinod, Psoriatic Arthritis: the Facts, Chapter 5, Loc. 665 of 3009.
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So, a normal CRP or ESR doesn’t necessarily mean that you don’t have PsA. Then there’s the whole issue about “what is raised?”. Labs and doctors rely on something called a “reference range” which is kind of a statistical middle-of-the-road number for what is considered “normal” or average for a healthy human. So at the start, when you are just being diagnosed, your CRP and your ESR are compared with the reference range. As Gladman and Chandran say, around 50% of people with PsA don’t go above the reference range for ESR/CRP. So looking at inflammatory markers to diagnose PsA doesn’t get them very far.
So let’s say the rheumatologist has given you the physical joint examination, and knows that you have joint inflammation. To narrow down which of the 100 or so types of arthritis it is, the rheumatologist will use this generally-accepted checklist if PsA might be one of the possibilities: evidence of psoriasis, psoriatic nail dystrophy, a negative test for Rheumatoid Factor, dactylitis, and Radiological evidence of juxta-articular new bone formation. (http://www.rheumtutor.com/caspar-criteria/) Nothing about elevated CRP or ESR, because they’re not reliable indicators for diagnosing PsA.
But wait … there’s more. Once the rheumatologist has diagnosed you with PsA, your ESR and CRP numbers (even if they are normal) are still important. They can be really useful for monitoring how well your treatment is working. The doctor will still be interested in whatever your inflammatory markers are (even if they aren’t out of “normal” range). That’s because the doc can compare them before and after treatment, to get an idea of how well MTX or your biologic (or whatever) is working for you.
And then, just to add to this intriguing puzzle, there are the many other factors which the rheumatologist should be taking into consideration: your level of functioning, your fatigue, and the other symptoms which you report. Rheumatology is as much an art as it is a science. Going “by the numbers” doesn’t make for a good diagnosis, and if you get the feeling that your rheumatologist is working only by numbers, you should probably, at the least, be looking for a second opinion. Just my humble opinion!
Given all of that, a thoughtful diagnosis and good treatment, you might still be in pain, unfortunately. Well, maybe in that case the pain isn’t caused by PsA at all. Maybe it’s a joint that has been damaged somehow (osteoarthritis). And that will never show on a lab test for inflammation, because it isn’t!
So the bottom line here: just because your inflammatory markers are normal, doesn’t mean that you don’t have PsA. But your markers, whatever they are, can still be really useful over time to your rheumatologist in monitoring your treatment.
I have a friend who says that “rheumatologists are the Sherlocks of the medical world”. I agree!