Using a recently developed and validated screening instrument for ICDs in PD, we found that (1) the QUIP is a sensitive instrument for the detection of ICDs whether completed by a patient or informant, (2) subsyndromal ICD symptoms are common in PD, and (3) patient-informant agreement of ICD symptoms is high for patients with an ICD, but in a sample that includes both ICD and non-ICD patients the agreement is moderate.
In our sample, the sensitivity of the QUIP for the detection of an ICD was 100%, regardless if a patient or informant completed the instrument. This suggests that if the purpose of administering the QUIP is to make sure that an ICD is not overlooked, then it does not matter if it is completed by the patient or an informant who knows the patient well. Phrased slightly differently, a negative screen from either the patient or an informant essentially rules out the presence of an ICD.
In addition, we found that approximately 40% of patients without an ICD had a positive QUIP based on either patient- or informant-reporting. As the QUIP is worded to identify core ICD symptoms (e.g., thoughts, drives and behaviors), this suggests that a substantial percentage of PD patients without an ICD experience subsyndromal ICD symptoms. Whether such patients are at increased risk of developing an ICD over time is an important area for future research.
There was moderate agreement between patient- and informant-reporting of any ICD symptoms in this population. The varying degrees of agreement for each individual ICD suggest that it is easiest for patients and informants to agree on the clinical significance of gambling behaviors. Regarding the other ICDs (buying, sex and eating), either behaviors may be more likely to be concealed (only patient endorsement on the QUIP) or some patients may be unaware of the clinical significance of their ICD behaviors (only informant endorsement on the QUIP).
The moderate agreement suggests that patient- and informant-reporting of ICD symptoms in PD should not be used interchangeably, especially for patients with subsyndromal symptoms. One possible reason for a mismatch between patient- and informant-reporting include a lack of awareness or minimization of symptoms by patients. In addition, patients may be motivated to hide undesirable behaviors. Some ICD behaviors may be easy to conceal [9
], and high agreement between patients and informants depends upon behavior observability [7
]. Finally, ICD signs and symptoms include not only behaviors, but thoughts and drives, and the latter may not be observable to others.
Limitations of this study included the relatively small number of patients with an ICD and a gender imbalance (80.3% of participants and 21.1% of informants were male). Future research examining assessment instruments for ICDs in PD should involve larger samples, includes patients from different countries, and be diverse in terms of clinical and demographic characteristics. In addition, it is possible that both the QUIP and the diagnostic interview were inaccurate (e.g., if a patient with an ICD did not endorse symptoms on both the QUIP and the diagnostic interview), so clinicians must be open to the possibility that patients or informants may be reluctant to disclose ICD symptoms in a clinical setting.
Both patient- and informant-rated QUIPs perform well as a screening instrument for ICDs in PD. For patients with subsyndromal ICD symptoms, which appear to be common in PD, the data from patient- and informant-rated QUIPs may not be interchangeable. A positive QUIP requires a follow-up clinical interview to confirm the presence of clinically-significant ICD symptoms, and patients found to have subsyndromal symptoms should be closely monitored. Additional research is needed to determine if this relatively large percentage of PD patients are at an increased risk of developing one or more ICDs in the future.