Application of item response theory to quantify substance use disorder severity

ObjectiveThe present investigation had two main goals: (1) Determine whether binary substance use disorder (SUD) diagnoses are indicators of a unidimensional trait indexing severity of disorder; and, (2) demonstrate the predictive, concurrent and construct validity of the SUD severity scale.MethodsBoys and their biological parents were administered structured diagnostic interviews to diagnose SUD. Item response theory (IRT) was applied to determine whether the diagnoses are indicators of a unidimensional trait. The score on this scale was correlated with substance use behavior, violence, treatment history, risky sex, and social adjustment.ResultsSUD diagnoses are indicators of a unidimensional latent trait. Maternal and paternal SUD severity predicted son's SUD severity at age 19. The score on the SUD severity scale correlated with drug use frequency, number of different drugs used in lifetime, treatment seeking, illegal behavior, social maladjustment, and risky sex.ConclusionSUD can be quantified on an interval scale indexing severity of disorder. The advantages of measuring SUD severity as a continuous trait are discussed.