OBJECTIVE: The objective of this study was to establish thresholds for clinical importance (TCIs) for the five functioning and nine symptom scales of the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 (EORTC QLQ-C30).

STUDY DESIGN AND SETTING: In this diagnostic study, cancer patients with mixed diagnoses and treatments completed the EORTC QLQ-C30 and anchored the questions in each domain in terms of their clinical importance. The anchor questions, concerned limitations in daily life, need for help/care, and the worries of the patient and his/her partner/family. These questions allowed categorizing patients into whether they exhibited a clinically important symptom/functional impairment for each scale and performing a receiver operating characteristic curve analysis to establish TCIs.

RESULTS: Data from 498 patients from six European countries (mean age 60.4 years, 55.2% women) were analyzed. For the TCIs generated using the patient questionnaire data, the EORTC QLQ-C30 scales showed sensitivity values between 0.71 and 0.97 and specificity values between 0.62 and 0.92 (area under the curve above 0.80 for all scales).

CONCLUSION: This EORTC Quality of Life Group study provides TCIs for the functioning and symptom scales of the EORTC QLQ-C30. These TCIs can increase the interpretability of the questionnaire results and foster its use in daily clinical practice and in clinical research.