Parkinson’s Disease Questionnaire-39

Abstract

The Parkinson’s Disease Questionnaire-39 (PDQ-39) is a widely recognized, disease-specific psychometric instrument designed to assess the health status and Health-Related Quality of Life (HRQoL) in individuals diagnosed with Parkinson’s disease. This self-administered questionnaire comprises 39 items distributed across eight distinct domains, reflecting the multidimensional impact of the neurological disorder on the patient’s life. The PDQ-39 is essential for both clinical trials and routine outcome measurement.

The instrument is structured into eight subcategories that cover the most relevant aspects of living with Parkinson’s, including physical function, emotional state, and social interactions. Scoring is achieved using a five-point Likert-type scale, ranging from “never” to “always” or “completely unable,” allowing for nuanced capture of symptom frequency and severity.

Keywords

Parkinson’s disease, PDQ-39, Quality of Life, HRQoL, Mobility, Activities of Daily Living, Stigmatization, Health Status, Neurological disorder, Outcome measure.

Authors

Peto V, Fitzpatrick R, Jenkinson C, Verjee-Lorenz A (Dutch translation).

Purpose

The primary purpose of the PDQ-39 is to provide a comprehensive, patient-centered measure of the subjective impact of Parkinson’s disease on daily life and well-being. Unlike purely clinical assessments that focus on motor symptoms (e.g., the UPDRS), the PDQ-39 captures the patient’s perspective on how the disease affects their functional capacity and overall quality of life.

The scale serves as a critical outcome measure in clinical research, evaluating the effectiveness of new treatments, therapeutic interventions, and pharmacological agents. Furthermore, it aids clinicians in identifying specific areas of distress—such as emotional well-being or social support deficits—that require targeted intervention for improved patient care.

Construct

The PDQ-39 measures the construct of Health-Related Quality of Life (HRQoL) specific to individuals with Parkinson’s disease. This construct is operationalized through 39 items grouped into eight distinct dimensions, designed to reflect the diverse physical, psychological, and social consequences of the illness. High scores on the PDQ-39 indicate poorer HRQoL.

The eight domains assessed by the PDQ-39 are:

• Mobility: Assessing difficulties related to movement, walking, and falls.
• Activities of Daily Living (ADL): Covering essential daily tasks like dressing, hygiene, and feeding.
• Emotional Well-being: Measuring feelings such as anxiety, depression, and mood changes.
• Stigmatization: Assessing feelings of shame, embarrassment, or being judged by others due to the disease.
• Social Support: Evaluating perceived lack of support or difficulty in relationships.
• Cognition: Addressing issues related to memory, concentration, and thought processes.
• Communication: Assessing difficulties in speech, writing, or being understood.
• Bodily Discomfort: Measuring pain, cramps, and other physical discomforts.

Validity

The PDQ-39 demonstrates robust validity across numerous international studies. Construct validity is supported by significant correlations between the PDQ-39 summary index and other established measures of disease severity, such as the Unified Parkinson’s Disease Rating Scale (UPDRS), particularly the ADL and motor sections. It also shows strong correlations with general quality of life scales (e.g., SF-36).

Known-groups validity confirms that the PDQ-39 can successfully differentiate between patient groups known to differ in disease severity or stage. Furthermore, the instrument exhibits responsiveness to change, meaning it is sensitive enough to detect clinically meaningful improvements or deteriorations in HRQoL following therapeutic interventions or disease progression.

Reliability

The reliability of the PDQ-39 is generally high, confirming its suitability for clinical and research use. Internal consistency, measured using Cronbach’s alpha, is excellent for the overall summary index (typically > 0.90) and satisfactory to good for the majority of the eight subscales (generally ranging from 0.70 to 0.90). This indicates that the items within each domain are highly interrelated and measure the same underlying construct.

Test-retest reliability has also been established in stable patient populations, demonstrating the scale’s stability over time when no clinical change has occurred. Studies typically report high intraclass correlation coefficients (ICCs), confirming the consistency of scores upon repeat administration.

Factor Analysis

Initial factor analysis conducted by Peto et al. (1995) supported the conceptual framework of the PDQ-39, confirming the existence of the eight distinct, yet correlated, factors (subscales). This eight-factor structure accounts for a substantial proportion of the variance in the items, establishing the scale’s multidimensional nature.

While the 39-item version remains the standard, subsequent research utilizing exploratory and confirmatory factor analysis has led to the development of shorter versions, such as the PDQ-8. These shorter versions often collapse the eight factors into a four-factor model or retain only the items with the highest factor loadings, while still maintaining high reliability and validity for screening purposes.

Instrument

Test Type: Psychometric Questionnaire / Self-Report Scale

Format: 39 items scored on a 5-point Likert scale.

Language Available: English (Original), Dutch, and numerous other validated international translations.

Population Group: Adults, Elderly

Age Group: Typically 40 years and older, corresponding to the primary incidence range of Parkinson’s disease.

Population Details: Individuals diagnosed with Parkinson’s disease, regardless of disease stage or treatment regime.

Test Methodology: Self-administered paper-and-pencil or computerized questionnaire. Respondents rate the frequency of difficulties experienced over the preceding month. The scoring options are: 0=Never, 1=Rarely, 2=Sometimes, 3=Often, 4=Always or Cannot do at all. Scores are typically transformed to a range of 0 (best HRQoL) to 100 (worst HRQoL) for each domain and for the summary index.

Keywords

Psychometrics, Neurological disorder, Outcome measure, Assessment scale, Self-report, Internal consistency, Reliability, Validity, PDQ-8.

Authors

Author ORCID Identifier: N/A (Not provided in source)

Affiliation Email addresses: N/A (Not provided in source)

Correspondence Address: N/A (Contact details typically associated with the Nuffield Department of Population Health, University of Oxford, UK, for original authors)

Permissions & Fee and Test Year

The original version of the PDQ-39 was published in 1995. The Dutch version was published in 1999. Use of the PDQ-39 often requires permission from the copyright holders, usually Oxford University Innovation, particularly for commercial or large-scale academic use, and may involve licensing fees.

Reference’s

Peto V, Jenkinson C, Fitzpatrick R. The development and validation of a Parkinson’s disease questionnaire (PDQ-39): summary results. J Neurol. 1995 Dec;242(9):594-8.

Verjee-Lorenz A, van der Meulen P, Kleyweg RP, van Hilten JJ, Roos RA. De Nederlandse versie van de Parkinson’s Disease Questionnaire (PDQ-39). [The Dutch version of the Parkinson’s Disease Questionnaire (PDQ-39)]. Ned Tijdschr Geneeskd. 1999 Jul 31;143(31):1576-80.

The explanatory form (Toelichtingsformulier) for the Dutch version can be downloaded here: Toelichtingsformulier (PDF).

The measurement instrument (Meetinstrument) for the Dutch version can be downloaded here: Meetinstrument (PDF).

Items of the Parkinson’s Disease Questionnaire-39

IMPORTANT: The following scale items must be preserved in their original language and must not be changed in any way.

The PDQ-39 consists of 39 items, divided into 8 subcategories. Per item, the score is determined by a five-point scale (nooit, zelden, soms, vaak, altijd of kan helemaal niet).

The eight subcategories measured are:

• mobiliteit (Mobility)
• ADL (Activities of Daily Living)
• emotioneel welzijn (Emotional Well-being)
• stigmatisering (Stigmatization)
• sociale steun (Social Support)
• cognitie (Cognition)
• communicatie (Communication)
• lichamelijk ongemak (Bodily Discomfort)