Caveats with the criteria
Despite these well-defined criteria, the diagnosis is not as clear-cut as it may appear. In 1990, the ACR convened a panel of 24 experts to define and standardize the diagnosis of fibromyalgia. The basis for this consensus was a group of 293 patients with fibromyalgia, each of whom had been assessed by one of the expert investigators according to “his or her usual method of diagnosis.” 1
The investigators determined the unique characteristics of fibromyalgia by comparing the 293 cases to 265 controls who had other chronic pain conditions (eg, low back pain syndromes, neck pain syndromes, regional tendonitis, possible systemic lupus erythematosus, rheumatoid arthritis). The investigators considered a multitude of symptoms and signs including sleep disturbance, morning stiffness, paresthesias, irritable bowel syndrome, fatigue, and anxiety. Their conclusion was that widespread pain and tender points were the most sensitive (88.4%) and specific (81.1%) distinguishing criteria for fibromyalgia.1
No reference standard. However, these calculations of sensitivity and specificity are less meaningful than in studies where an independent reference standard or gold standard is available. The ACR expert panel derived the criteria in a circular way using a nonindependent reference standard—ie, patients thought to have fibromyalgia compared with control patients thought not to have fibromyalgia. The expert panel essentially set the specificity of the criteria at 100%, since the specificity is based on the rate of false positives.
Furthermore, because there was no objective gold standard for determining who truly had fibromyalgia (and we do not yet have an independent biologic “test” for this condition), this panel could not determine whether additional symptoms or signs that should be considered in the diagnosis of fibromyalgia.
Biases, dubious representation? Unknown elements in this analysis are 1) how closely the reference population used to develop these criteria represents the true population of persons with fibromyalgia, and 2) the biases of the ACR experts. Finally, the positive and negative predictive values of these criteria will depend on the prevalence of fibromyalgia and other similar conditions.
Morbidity not predicted by criteria. In addition, the 1990 ACR criteria assume the number of tender points and degree of pain are directly proportional to overall morbidity; however, a person with fewer than 11 tender points may experience significant morbidity, indicating that the sensitivity of the criteria may be low.40-42 As suggested by Wolfe in 1997, “the tender point count functions as a sedimentation rate for distress” in persons with chronic pain.42 Thus, the authors of the 1990 ACR study stated that ACR criteria should not be applied rigidly in diagnosing and treating fibromyalgia,42 leaving a large role for clinical judgment.
Subjective factors. A final difficulty with the diagnosis of fibromyalgia is its dependence on patient report and examiner technique.1 In the 1990 ACR criteria, tender points were defined as a complaint of pain (or any more dramatic response) when an examiner applied 4 kg of pressure with the pulp of the thumb or first two or three fingers, calibrated with a dolorimeter (a device that can measure the amount and rate of pressure applied over a specified surface area).1 It has been shown that practitioners require training to apply 4 kg of force with regularity.43
However, applying exactly 4 kg of pressure may not be clinically important. Other studies have shown that finger palpation or dolorimetry identifies tender points with equal accuracy (LOE: 3, case-control design with non-independent reference standard).44,45
Manual palpation
A controlled study of manual palpation was conducted to standardize the tender point survey described in the Figure. 46 This method compares well with the ACR 1990 method, with a sensitivity of 88.6% and a specificity of 71.4%.46
To speed up the examination, a particular sequence of palpating survey points was established, with the patient positioned as outlined in the Figure. Using the thumb pad of his/her dominant hand, each examiner applied 4 kg of pressure, at a rate of 1 kg per second, just once at each survey point. Examiners learned to apply the proper amount of pressure by standing a patient on a scale and watching the scale while pressing down perpendicularly on the trapezius survey point.
The examinee was seated throughout the exam, except when lying on the side for palpation of the trochanter and lying supine for palpation of the knee. A tender point was identified when the patient rated the pain resulting from palpation at least 2 out of 10 (0, no pain; 10 worst pain) (LOE: 3, case-control design, nonindependent reference standard).46
Until a firm biologic basis for fibromyalgia is discovered and a true gold standard for testing is developed, the diagnosis of fibromyalgia will remain a matter of clinical judgment and convention (SOR: C).