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Diagnostic Accuracy of Predicting Somatization From Patients’ ICD-9 Diagnoses

From the Departments of Medicine and Psychiatry (R.C.S.) and Epidemiology (J.C.G., Z.L.), Michigan State University, East Lansing, Michigan; and the Department of Medical Humanities and Social Sciences (K.R.), Florida State University College of Medicine, Tallahassee, Florida.

Objectives: To hypothesize in a new and different population that administrative database (ADB) screening would identify somatizing patients by increasing numbers of visits, female gender, and greater percent of International Classification of Diseases, 9^th Edition (ICD-9) primary diagnosis codes in musculoskeletal, nervous, gastrointestinal (GI), and ill-defined body systems. We labeled these codes as having "somatization potential." Our earlier study demonstrated that ICD-9 codes and other data from the ADB effectively identified somatization.

Methods: Using a prospective observational design in a staff model health maintenance organization, we evaluated 1364 patients aged 18 to 65 years who had 8 visits yearly in the 2 years before study. Clinician raters applied a reliable method of medical chart review to identify patients meeting the criteria for somatization. We randomly selected 2/3 for the derivation set (n = 901) for logistic regression to evaluate the contribution of potential ADB correlates (age, gender, all encounters, primary diagnosis codes (ICD-9), revenue codes, and charges) of a diagnosis of somatization. This prediction rule was then applied to the remaining 1/3 of subjects, the validation set (n = 463).

Results: Patients averaged 47.1 years, 12.8 visits per year, and 71.6% were female; 319 had somatization. Age, visits, and somatization potential were associated with clinician-rated somatization, with a c-statistic 0.72 in the derivation set and 0.68 in the validation set.

Conclusions: These data support our earlier findings that selected ICD-9 diagnoses in the ADB predict somatization, suggesting their potential in identifying a common, costly, and usually unrecognized problem.

Key Words: administrative database screening • somatization • medically unexplained symptoms • sensitivity and specificity • diagnostic accuracy

Abbreviations: ADB = administrative database; ICD-9 = International Classification of Diseases, 9th Edition; HMO = health maintenance organization; RCT = randomized controlled trial; DSM-IV = Diagnostic and Statistical Manual of Mental Disorders, 4th Edition; PHQ-15 = Patient Health Questionnaire, 15 items; PHQ-15-R = PHQ-15, Revised.