Unfortunately, there are no objective, biological markers or blood tests to document the presence or the severity of pain. The most accurate and reliable method of determining the intensity of pain is to simply ask the person how bad the pain is. Consequently, experts recommend that people’s compaints of pain be taken seriously. This is particularly true for chronic pain that is often exhausting and associated with a multiplicity of physical, mental, and social consequences.
(Source: Chronic Pain: Biomedical and Spiritual Approaches, by Harold G. Koenig, M.D.)