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Arguments have been made for dimensional models over categorical for the classification of personality disorder, and for the five-factor model (FFM) in particular. A criticism of the FFM of personality disorder is the absence of measures designed to assess pathological personality. Several measures have been developed based on the FFM to assess the maladaptive personality traits included within existing personality disorders. One such example is the Five-Factor Obsessive-Compulsive Inventory (FFOCI). The current study applied item response theory analyses (IRT) to test whether scales of the FFOCI are extreme variants of respective FFM facet scales. It was predicted that both the height and slope of the item-response curves would differ for the conscientiousness-based scales, due to the bias towards assessing high conscientiousness as adaptive in general personality inventories (such as Goldberg’s International Personality Item Pool; IPIP). Alternatively, the remaining FFOCI scales and their IPIP counterparts were predicted to demonstrate no significant differences in IRCs across theta. Nine hundred and seventy-two adults each completed the FFOCI and the IPIP, including 377 undergraduate students and 595 participants recruited online. A portion of the results supported the hypotheses, with select exceptions. Fastidiousness and Workaholism demonstrated the expected trends, with the FFOCI providing higher levels of fidelity at the higher end of theta, and the IPIP demonstrating superior coverage at the lower end of theta. Other conscientiousness scales failed to demonstrate the expected differences at a statistically significant level. In this context, the suitability of IRT in the analysis of rationally-derived, polytomous scales is explored.