The measurement of human pain

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Abstract
The assessment of human pain is complicated not only because pain, like other sensory experiences, is subjective, but also because pain, unlike other sensory experiences, may be modified by psychological, social, and situational factors. These factors can modify the neuronal response evoked by a relatively constant noxious stimulus (such as an injection) so that the resulting pain sensation may be enhanced or reduced. Consequently, in order to measure pain accurately, investigators must attempt to assess the subjective experience of pain.Although there are observational, self‐report, behavioral, and physiological methods available for measuring pain, direct scaling techniques may constitute a comprehensive measure by providing qualitative and quantitative estimates of pain expressed in terms of a patient's perception. A patient may rate both the strength and the affective dimensions of their clinical pain using simple measurement techniques that have been validated in well‐controlled studies on experimental pain. Direct scaling throughout a range of stimulus intensities (experimental pain) or throughout a range of time intervals (clinical pain) also provides a framework for standardization of individual responses. This standardization facilitates rigorous evaluation of analgesic efficacy for different pharmacological methods and for different pain conditions.The requirements for an objective measure of pain are similar to the requirements for any measuring instrument. These are: 1) reliability – that the procedure yields consistent results over time; 2) validity – that the procedure measures unequivocally a specific dimension of pain; 3) minimum inherent bias – that the procedure is relatively independent of method bias or patient/investigator response bias; and 4) versatility – that the procedure is applicable for both laboratory and clinical use and is practical for a wide variety of different medical settings.Visual analog scales, a direct scaling method in which a patient adjusts the length of a line so that the length is proportional to their pain, have been validated in a variety of experimental and clinical pain studies. Although these scales are deceptively simple, they have the reliability, validity, and versatility necessary for an objective pain measure.

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