The dimensionality of pain: Palliative care and chronic pain patients differ in their reports of pain intensity and pain interference

The accurate, precise, and consistent assessment of pain is of particular importance in palliative care. The European Palliative Care Research Collaborative is developing a computer-based pain assessment instrument and has been evaluating the content and dimensionality of existing pain questionnaires. The most important dimensions of pain are intensity and interference. However, since pain interference is a consequence of and largely reflects pain intensity, we postulated that it might either provide information to enhance the evaluation of intensity, or that an overall summary measure of pain severity could be constructed by combining the 2 dimensions. Cancer patients in palliative care (n=395) and chronic pain patients (n=168) completed questionnaires that included 23 pain items culled from existing questionnaires. Psychometric analyses confirmed the existence of 2 main dimensions, intensity and interference, and also guided identification of items that contributed most strongly to these dimensions. However, there was strong evidence that the relationship between the intensity and the interference items differs markedly in palliative care patients compared to chronic pain patients. As hypothesized, there was strong correlation between intensity and interference, lending support to the possibility that, for some purposes, these dimensions may be combined to provide a higher-level summary measure of patients’ pain experience. We conclude that these dimensions should be kept distinct when assessing patients in general, although for a single type of patient (such as palliative care patients), it may be possible to regard intensity and interference as contributing to an overall measure of pain severity. Pain intensity and interference are highly correlated but represent clinically distinct and important dimensions. For clinical trials, a single “severity” summary score is proposed.