Definitions

This is not intended to be an exhaustive list of medical and scientific terms and occasionally I will probably use a term that you don't understand in a post. This list is also a work in progress! If you want to understand more, please let me know! I am assuming some knowledge of biology for readers of this blog, but I am happy to explain anything you find confusing.
Please let me know if you see errors in the definitions below as well. I'm still learning every day.

Disease definitions and diagnosis

2010 ACR-EULAR classification criteria for rheumatoid arthritis

This table shows the criteria developed by the American College of Rheumatology and The European League Against Rheumatism for diagnosing Rheumatoid Arthritis.

Target population (Who should be tested?): Patients who have at least 1 joint with definite clinical synovitis (swelling) with the synovitis not better explained by another disease
Classification criteria for RA (score-based algorithm: add score of categories A–D; a score of ≥6/10 is needed for classification of a patient as having definite RA)	Score
A. Joint involvement
1 large joint	0
2-10 large joints	1
1-3 small joints (with or without involvement of large joints)	2
4-10 small joints (with or without involvement of large joints)	3
>10 joints (at least 1 small joint)	5
B. Serology (at least 1 test result is needed for classification)
Negative RF and negative ACPA	0
Low-positive RF or low-positive ACPA	2
High-positive RF or high-positive ACPA	3
C. Acute-phase reactants (at least 1 test result is needed for classification)
Normal CRP and normal ESR	0
Abnormal CRP or abnormal ESR	1
D. Duration of symptoms
<6 weeks	0
≥6 weeks	1

Please note, this chart is not intended to be used to diagnose yourself without the aid of a physician. If you think you might have rheumatoid arthritis, go see your doctor. Early diagnosis and treatment is key!

DAS-28 Disease Activity Score

This is a score that quantifies the activity of disease in patients based on the number of painful and swollen joints (28 are examined, and the list excludes some common sites like feet and ankles), inflammatory markers (C-reactive protein or Erythrocyte Sedimentation Rate), and the patient's own assessment of their health (based on a scale of 0-10, basically).

This page allows you to enter in the parameters and calculate the score.

The DAS-28 (using either ESR or CRP) is frequently used in clinical trials to measure the success of the candidate drug or intervention for reducing the activity of RA in enrolled patients.

It is somewhat flawed, given the number of joints excluded and the subjective nature of most of the parameters.

Remission

There are a couple definitions of remission, but the clearest is that set by the American College of Rheumatology, which requires that no more than one joint can be counted as tender or swollen, than CRP must be less than 1 mg/dL, and the patient should rank their overall arthritis symptoms as a 0 or 1 on a scale of 1-10. There is also the option to add all these parameters together; if a score of 3.3 or less is determined, then it's also remission. 

For the latter, that would allow you to have a joint that is painful and swollen but still be in remission.

Medical Tests

C-reactive protein (CRP)

This is a protein that is found in your blood that serves as an inflammatory marker. For a healthy adult, CRP is usually less than 4 mg/dL--and then generally on the lower end of the scale. It is a non-specific marker, and can mean you have inflammation anywhere in your body. For example, if you are sick or have cancer or a heart condition, this marker can be increased. IL-6, a pro-inflammatory cytokine, is thought to induce the release of CRP.

Erythrocyte Sedimentation Rate (ESR)

This is an older test to assess inflammation. Erythrocytes are red blood cells, the primary cells found in your blood. Fibrinogen is a protein that is also found in blood and is involved in blood clotting. During the body's inflammatory response, fibrinogen levels increase in the blood. This causes the red blood cells to clump together in a test tube and fall to the bottom. The ESR is a measurement of how fast the red blood cells fall in millimeters per hour. Just like CRP, this is a non-specific marker of inflammation and can also be increased in cases of infection or other inflammation. A high ESR does not necessarily mean you have RA or another autoimmune disease, but the cause should be investigated.