Your friendly neighborhood board certified lab tech here lol
esr and crp are nonspecific markers for acute phase reactants of inflammation. They do not have to be grossly elevated in order to receive a PsA diagnosis. I learned this from working in the lab, reading research data, working closely with providers, as well as my rheumatologist at Mayo and my local rheumatologist.
You can still have very active disease and have near normal markers.
I've had pristine markers before but still very obviously active, especially in the beginning stages when everything was ramping up. That's why so many people are misdiagnosed with just fibromyalgia and treatment is delayed due to the whole one size fits all mentality. I suffered over ten years due to this exact reason.

PsA is being studied even more closely than before, as they're even finding that patients with PsA can have the same organ involvement as RA patients. That came from NIH and Pubmed studies. The evidence is emerging, and the numbers of patients diagnosed with PsA is rising.

That said, family history, number of joints affected, erosion, symmetry, nail and or skin changes, any eye inflammation, RF factor result, ANA, and anti-ccp also dsdna, complement C3/C4, rad imaging/MRI all help to differentiate PsA from RA as well as lupus.
You also can have completely normal X-rays and still have PsA. The goal is to prevent erosion. The enthesitis plays a role in joint erosion as well. That's my issue, is the enthesitis and tenosynovitis. My rad baseline was without erosion, but that didn't start changing until this last 3 years. Hence another reason I went undiagnosed for so many years, because doctors were of the mindset that you had to have erosion for diagnosis. Nope! Again, Fake news.
In closing, it can be a process of elimination.

It is hard to diagnose. CRP and ESR are not good indicators because PsA is seronegative. meaning it may not elevate those markers. It takes a good rheumatologist and a little process of elimination.

It doesn't always show in blood tests, sero negative etc. But there are lots of symptoms , feet pain, tendonitis, lower back pain, fatigue . Google the symptoms and ask your doctor to refer you to a rheumatologist for diagnosis . X

You have to go a reumatologist, and tell your illness problems. He knows what the blood tests you have to do it.

we have found an awesome combo treatment for me that is working. skyrizzi , vital proteins collagen from Sams club, and medical CBD tincture (has only a trace of thc). my pain level is gone from excruciating to a 2 , my mobility has increased by 90%. I really believe I'm finally on the pathway to having my life back after 5 1/2 years of searching for an answer.