Clinical dimensions of fibromyalgia symptoms and development of a combined index of severity: The CODI index

Although a large body of work indicates that fibromyalgia (FM) is not a discrete entity, few studies have attempted to classify
the heterogeneity of FM symptoms. The objectives of the present study were to confirm the existence of two latent dimensions
underlying FM symptoms (Core-FM symptoms and Distress) by means of factor analysis techniques, and to develop and validate
a new combined index of symptom severity (the CODI).

We analyzed and combined the baseline scores on six visual analog scales of the FIQ (pain, general fatigue, morning fatigue,
stiffness, anxiety, and depression) and on the STAI-T (trait anxiety) of 216 Spanish patients diagnosed with FM (97.7% women)
who were participating in a randomized, controlled trial.

The principal component analysis indicated the presence of two correlated factors (labeled as Core-FM symptoms and Distress)
that explained 64% of total variance. The subsequent confirmatory factor analysis yielded more empirical support for the two-factor
model than the one-factor model (all items loading on one latent dimension). The two factors possessed adequate internal consistency
and construct validity given the pattern of significant correlations with the Euroqol items. The Core-FM dimension had a stronger
relationship with mobility, self-care, usual activities, and pain/discomfort than with anxiety/depression, whereas the Distress
dimension showed the opposite pattern. Finally, summing the standardized scores of the two dimensions, a new combined index
of symptom severity (the CODI) was developed.

The clinical implications and utility of the CODI are discussed in relation to previous research on FM.