What is psoriatic arthritis

[Caroline]

How do they tell if it is PsA and not other types of arthritis? What if you have osteoarthritis and PsA? Can they tell the difference?

That is going to be very difficult as there are no markers in the blood that can reveal Psoriatic Arthritis.
The only thing they can do is to do a puncture and get some inflammation fluid from the swollen joints. If you have Psoriatic Arthritis this contains pyruvate crystals, which can be seen under the microscope.
However hardly any rheumatologist knows this.