Editing Rheumatoid arthritis (section)

===Differential diagnoses===
{{Synovial fluid analysis}}
Several other medical conditions can resemble RA, and need to be distinguished from it at the time of diagnosis:<ref name=Merckmanual>{{cite book | editor=Berkow R | title=The Merck Manual | edition=16th | publisher=Merck Publishing Group | year=1992 | pages=1307–1308 | isbn=978-0-911910-16-2 | url=https://archive.org/details/merckmanualofdia16berk }}{{subscription required}}</ref>
* Crystal induced arthritis ([[gout]], and [[pseudogout]]) – usually involves particular joints (knee, MTP1, heels) and can be distinguished with an aspiration of joint fluid if in doubt. Redness, asymmetric distribution of affected joints, pain occurs at night and the starting pain is less than an hour with gout.
* [[Osteoarthritis]] – distinguished with [[X-ray]]s of the affected joints and blood tests, older age, starting pain less than an hour, asymmetric distribution of affected joints and pain worsens when using joint for longer periods.
* [[Systemic lupus erythematosus]] (SLE) – distinguished by specific clinical symptoms and blood tests (antibodies against double-stranded DNA)
* One of the several types of [[psoriatic arthritis]] resembles RA – nail changes and skin symptoms distinguish between them
* [[Lyme disease]] causes erosive arthritis and may closely resemble RA – it may be distinguished by blood test in endemic areas
* [[Reactive arthritis]] – asymmetrically involves heel, [[sacroiliac]] joints and large joints of the leg. It is usually associated with [[urethritis]], [[conjunctivitis]], [[iritis]], painless buccal ulcers, and [[keratoderma blennorrhagica]].
* [[Axial spondyloarthritis]] (including [[ankylosing spondylitis]]) – this involves the spine, although an RA-like symmetrical small-joint polyarthritis may occur in the context of this condition.
* [[Hepatitis C]] – RA-like symmetrical small-joint polyarthritis may occur in the context of this condition. Hepatitis C may also induce rheumatoid factor auto-antibodies.

Rarer causes which usually behave differently but may cause joint pains:<ref name=Merckmanual/>
* [[Sarcoidosis]], [[amyloidosis]], and [[Whipple's disease]] can also resemble RA.
* [[Hemochromatosis]] may cause hand joint arthritis.
* Acute rheumatic fever can be differentiated by a migratory pattern of joint involvement and evidence of antecedent [[streptococcal]] infection.
* Bacterial arthritis (such as by ''[[Streptococcus]]'') is usually asymmetric, while RA usually involves both sides of the body symmetrically.
* [[Gonococcal]] arthritis (a bacterial arthritis) is also initially migratory and can involve [[tendon]]s around the wrists and ankles.

Sometimes arthritis is in an undifferentiated stage (i.e. none of the above criteria is positive), even if synovitis is witnessed and assessed with ultrasound imaging.