Pain is the most common cause for patients seeking medical attendance [1]. It is estimated that approximately 19% of adults in Europe suffer from chronic pain, which has major impact on quality of life for the patients and economical consequences for the society. However, pain treatment is a major challenge in the clinic, and nearly half of the patients are influenced by inadequate treatment [2]. The challenge to treat patients is among other factors caused by lack of indebt knowledge of pain and analgesic mechanisms in individual patients. To gain such knowledge, improved methods to identify biomarkers for the mechanisms on a single subject basis are warranted.