How do doctors diagnose rheumatoid arthritis?
BMJ Group Medical Reference
Doctors sometimes find it difficult to diagnose rheumatoid arthritis. This is because the symptoms can start very slowly, and there isn't a test that shows whether or not you have it. Your doctor will need to collect lots of types of information to make a diagnosis.
Your doctor may do the following things to see if you have rheumatoid arthritis.
Ask you questions about your joints: Your doctor will want to know if your joints are stiff or if they get swollen, and how long you have had problems.
Examine your joints: Your doctor will look to see if your joints are swollen and how easily they move.
Do a blood test: Some people with rheumatoid arthritis have a protein in their blood called rheumatoid factor. Your doctor may test for this. But if you don't have this protein, you could still have rheumatoid arthritis.
Do x-rays of your joints: X-rays can show whether your bones are getting thin or changing shape.
After collecting this information, your doctor may use the following list to make a diagnosis. If you have four or more of the signs of rheumatoid arthritis that are on the list below, it is very likely that you have the condition: 
Morning stiffness in your joints that lasts for more than an hour
Swelling and inflammation in three or more joints, that lasts more than six weeks
Swelling and inflammation in your hand joints or wrists, that lasts more than six weeks
Symptoms in the same joints on both sides of your body, that lasts more than six weeks
Rheumatoid nodules (these are firm lumps under your skin)
Rheumatoid factor in the blood (many people with rheumatoid arthritis have this protein in their blood)
X-rays showing changes in your joints (x-rays can show if your bones are getting thin and changing shape).
Inflammation is when your skin or some other part of your body becomes red, swollen, hot, and sore. Inflammation happens because your body is trying to protect you from germs, from something that's in your body and could harm you (like a splinter) or from things that cause allergies (these things are called allergens). Inflammation is one of the ways in which your body heals an infection or an injury.
X-rays are pictures taken of the inside of your body. They are made by passing small amounts of radiation through your body and then onto film.
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