Brief Answer:
ACR/EULAR criteria

Detailed Answer:
HI, thanks for using healthcare magic

Rheumatoid arthritis is diagnosed based on clinical signs and symptoms, lab values and also x ray results.
A combination of these factors is used to make the diagnosis.

There are criteria called ACR/ EULAR that looks at : (a)type and number of joints involved, (b)blood tests showing inflammation, (c)serology results (looking for rheumatoid factor eg), (d)different symptoms present and length of time present.

Points are given for the different areas and then totaled.

(a)Clinical signs and symptoms:

(1)arthritis of multiple joints particularly the hands and feet. Involves both hands and/or feet
(2)deterioration of the joints
(3)there should be swelling or tenderness of joints on examination

Points are actually given based on whether it is a large joint (eg knee, elbow etc) or small joints
Points also depend on number of joints involved.

(b) there are some blood tests such as c reactive protein and esr which go up if there is any inflammation in the body.
They do not pin point the site inflammation but show that there is inflammation present somewhere in the body.

If one of these is high, a point is also given

(c)Serology tests would look for certain factors such as rheumatoid factor (whether high positive or negative) and other factors.
Points also awarded based on results

(d)The length of time of symptoms are also examined- less than 6 weeks- no points,
More than 6 weeks -one point

Xrays are also assessed.

Summary: diagnosis is based on - types and number of joints involved

presence of inflammatory blood factors

serology results

duration of symptoms

xrays also important

I hope this helps,feel free to ask any other questions

---