We found that the QUIP, a brief, self-completed screening questionnaire for ICDs and other compulsive behaviors in PD, has good discriminant validity using formal diagnostic criteria as the “gold standard” diagnosis.
To our knowledge, this is the first instrument to be developed and have score performance supporting validity as a screening instrument for the range of ICDs and other compulsive behaviors reported in PD. The Minnesota Impulsive Disorders Interview (MIDI)(22
), which includes sections for compulsive gambling, sexual behavior, and buying, has been used in PD(4
). However, the MIDI does not cover compulsive eating or other compulsive behaviors, and thresholding of scores to identify cases has varied across studies. Different rating scales (e.g., the South Oaks Gambling Screen(25
)), questionnaires (Punding Questionnaire(20
)), and diagnostic criteria (DSM-IV diagnostic criteria for pathological gambling, McElroy criteria for compulsive buying(26
), Voon criteria for compulsive sexual behavior)(6
), and descriptions of other compulsive behaviors(19
) have either been used(3
) or created for use(6
) in PD, but no existing single instrument fulfills the criteria of being comprehensive, self-rated, and validated for use in this population.
The ICD section of the QUIP had at least 80% sensitivity and specificity for each of the 4 ICDs at the recommended cut-off points. As ICDs were frequently co-morbid but not always co-endorsed, combining the four ICDs increased the sensitivity for identifying an individual with any ICD to 97%. Thus, in many cases, ICD patients who failed to screen positive for one ICD were still identified with a positive screen for another ICD.
The sensitivity and specificity for the hobbyism subsection were both >90%. The instrument was limited in sensitivity for the punding and walkabout subsections (60–65%). However, interpretation of the walkabout results was limited by the low number of cases. Furthermore, the full meaning and range of behaviors associated with punding were difficult to convey and capture in a brief questionnaire. Combining the ICD section and the other compulsive behaviors sections increased the sensitivity for identifying an individual with any disorder/behavior to 96%.
On post hoc analysis we found that the psychometric properties for each ICD in the QUIP-S were similar to their counterparts in the QUIP. Additionally, the QUIP-S ICD section overall had a sensitivity of 100% for identifying a patient with at least one ICD, and the total QUIP-S had a sensitivity of 94% for identifying a patient with at least one ICD or other compulsive behavior.
The median completion time for the QUIP (30 questions total) was 5 minutes. We estimate that the median completion time for the QUIP-S (13 questions total) is 3 minutes. Although the shortened version was not formally tested, we do recommend it for routine use as the specific questions and overall structure of the instrument were not modified in any way. Clinicians or clinical researchers who want the additional information provided by the full questionnaire may choose to administer this version.
The QUIP was designed and validated as a screening instrument, not as a diagnostic or rating instrument. The negative predictive values (NPVs) for each ICD were very high, so a negative screen appears to signal with a great degree of certainty that an ICD is not present. For a screening instrument, a high NPV is crucial, while a low PPV can be counterbalanced by conducting a follow-up clinical interview.
Positive predictive values (PPVs) were low overall, indicating that a positive screen needs to be followed by a clinical interview to verify if the patient truly has clinically significant ICD or other compulsive behaviors. Since ICDs and other compulsive behaviors are commonly co-morbid, patients in particular who screen positive for a single ICD should be queried about the range of ICDs and other compulsive behaviors reported in PD.
The low PPVs for most of the ICDs and other compulsive behaviors are in part a reflection of the relatively low frequency of each disorder in our study population (the most common behavior, hobbyism, was diagnosed in <15% of patients). In addition, there are other reasons a patient might endorse symptoms on the QUIP but not meet diagnostic criteria for an ICD or other compulsive behavior when interviewed. First, ICDs or other compulsive behaviors may be present at subsyndromal levels, in which case follow-up and monitoring is appropriate as such patients may be at higher risk of developing a disorder (i.e., having symptoms that lead to distress or some form of psychosocial impairment). Similarly, a clinician might consider the presence of subsyndromal symptoms in therapeutic decisions for a given patient, such as whether to utilize a DA or levodopa. Second, a patient may acknowledge symptoms leading to a positive screen with the QUIP, but minimize symptom severity on formal interview leading to a negative diagnostic interview. An example from clinical experience are married male patients who acknowledge compulsive sexual behaviors, but report them as not being clinically significant, whereas the spouse will report that such symptoms are causing distress or impairment in the marital relationship. This situation highlights the value of including an informed other, if available, when clinically evaluating the significance of ICD and other compulsive behaviors. Third, the diagnostic criteria used for the different disorders for the validation process may have imposed a limitation. Pathological gambling has arguably the most well-established diagnostic criteria(1
), and the QUIP had the highest PPV for this disorder. The lowest PPVs were for compulsive buying and eating, disorders for which it may be difficult to differentiate pathological behaviors from excessive behaviors that do not rise to the level of a disorder. Finally, a patient may be more willing to endorse symptoms on a self-administered questionnaire than to an interviewer.
There are several limitations to note. First, the diagnostic criteria for two of the disorders and behaviors (i.e., binge-eating disorder and walkabout) were slightly modified to be consistent with clinical experience. Second, only one patient met diagnostic criteria for compulsive medication use thus limiting validation of this section of the questionnaire, and less than 10 subjects each had compulsive eating or walkabout. Third, as we used a time frame of “anytime during PD” to assess ICD and other compulsive behaviors, recall bias may have influenced the accuracy of the information provided. Fourth, as our study population was a convenience sample of PD patients, we cannot say if the instrument would have performed differently in a random sample of patients. Finally, as a result of providing many examples and using terms from existing instruments, the QUIP is rated at a 12th grade reading level, and it is unknown how this would affect the screening of PD patients with less than 12 years of formal education.
Both the QUIP and QUIP-S are appropriate for use in clinical care and clinical research as screening instruments for the range of ICDs and other compulsive behaviors reported in PD. Future studies should include a larger number of patients with a range of ICD histories to validate our preliminary findings. In addition, the QUIP should be tested in other populations who may be at risk for ICD development (e.g., restless leg syndrome [RLS] patients receiving DA therapy(31