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Illnesses, or diseases, cause patients to have different symptoms and signs and are often associated with abnormal test (investigation) results.

Not all patients have exactly the same pattern of complaints. The precise constellation of these problems allows doctors and researchers to classify the various types of arthritis they treat. This is useful because it allows them to predict (to some extent) which patients will do well, and which patients may need to be monitored closely for complications of their disease.
The disease classification can be useful to recruit patients to a trial, and be sure that the patients in the trial all have a similar type or severity of arthritis.
Where a patient has a pre-defined set of features, which relate to a particular disease, he or she may be said to fulfil the Diagnostic Criteria for the disease.
In the ACR 2010 Criteria for the Classification of rheumatoid arthritis (, the presence of synovitis (inflammation of the membrane of a joint), the number and site of involved joints, factors in blood and symptom duration should all be taken into account in classifying a patient’s disease. These are the Classification Criteria. If the criteria are fulfilled, then the patient is deemed to have rheumatoid arthritis and is said to have reached the diagnostic criteria.
Patients are often further sub classified according to their exact disease pattern.
Many patients’ symptoms and signs do not fit into a specific set of diagnostic criteria. In these cases, doctors and patients must work together to treat the illness, but both patient and doctor may need to live with some degree of uncertainty regarding the diagnosis, or label.

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