There is no single test that can fully diagnose fibromyalgia and there is debate over what should be considered essential diagnostic criteria and whether an objective diagnosis is possible. In most cases, people with fibromyalgia symptoms may also have laboratory test results that appear normal and many of their symptoms may mimic those of other rheumatic conditions such as arthritis or osteoporosis. The most widely accepted set of classification criteria for research purposes was elaborated in 1990 by the Multicenter Criteria Committee of the American College of Rheumatology. These criteria, which are known informally as "the ACR 1990", define fibromyalgia according to the presence of the following criteria:
• A history of widespread pain lasting more than three months – affecting all four quadrants of the body, i.e., both sides, and above and below the waist.
• Tender points – there are 18 designated possible tender points (although a person with the disorder may feel pain in other areas as well). Diagnosis is no longer based on the number of tender points.
In 2010, the American College of Rheumatology approved provisional revised diagnostic criteria for fibromyalgia that eliminated the 1990 criteria's reliance on tender point testing.The revised criteria use a widespread pain index (WPI) and symptom severity scale (SS) in place of tender point testing under the 1990 criteria. The WPI counts up to 19 general body areas in which the person has experienced pain in the preceding two weeks. The SS rates the severity of the person's fatigue, unrefreshed waking, cognitive symptoms, and general somatic symptoms, each on a scale from 0 to 3, for a composite score ranging from 0 to 12. The revised criteria for diagnosis are:
• WPI ≥ 7 and SS ≥ 5 OR WPI 3–6 and SS ≥ 9,
• Symptoms have been present at a similar level for at least three months, and
• No other diagnosable disorder otherwise explains the pain.