There is no single, specific diagnostic laboratory test for diagnosing fibromyalgia. Before receiving a diagnosis of fibromyalgia, you may go through several medical tests, such as blood tests and X-rays, only to have the results come back normal (although these tests may rule out other conditions, such as rheumatoid arthritis, lupus and multiple sclerosis, they cannot confirm fibromyalgia).

The American College of Rheumatology has established general classification guidelines to help in the assessment and study of fibromyalgia. These criteria, known informally as the "ACR 1990, " define fibromyalgia according to the presence of the following criteria:

• Widespread pain. A history of widespread pain lasting more than three months - affecting all four quadrants of the body (left side, right side, above the waist and below the waist).
• Tender points. There are 18 designated possible tender or trigger points (although a person with the syndrome may feel pain in other areas as well). Tender points are located on either side of the neck, chest, upper back, lower back, spine, buttocks, knee caps, or the inside of either arm where it bends at the elbow. The patient must feel pain at 11 or more of these points for fibromyalgia to be considered. Four kilograms of force is about the amount of pressure required to blanch the thumbnail when applying pressure.

This set of criteria was developed by the American College of Rheumatology as a means of diagnosing fibromyalgia for both clinical and research purposes. Although these guidelines were originally established for research purposes and were not intended for clinical diagnosis, they have become the de facto diagnostic criteria in the clinical setting.

Not all doctors agree with these guidelines. Some believe that they are too rigid and that you can have fibromyalgia even if you don't meet the required number of tender points. Others question how reliable and valid tender points are as a diagnostic tool.