Sex differences in heat pain thresholds as a function of assessment method and rate of rise

Studies of sex differences in the responses to experimentally induced pain demonstrate greater pain sensitivity among females than males. However, studies investigating heat pain responses have produced inconsistent results. Differences in stimulus characteristics and assessment methods probably account for this variability. This study examined sex differences in the heat pain threshold as a function of two different assessment methods and varying rates of rise. Nineteen female and 18 male healthy volunteers underwent heat pain threshold assessment via the method of levels and the method of limits. In addition, both fast (4.0 degrees C/s) and slow (0.5 degrees C/s) rates of rise were used for the method of levels assessments. In order to examine the reliability of threshold values, each subject participated in two sessions, separated by approximately 8 days. Females evinced lower thresholds than males for the method of levels assessments with both slow and fast rates of rise (ps < 0.05), while no sex differences emerged for the threshold assessed via the method of limits. Test-retest reliability coefficients were relatively high. However, thresholds generally increased significantly from session 1 to session 2. Between method correlations were generally low to moderate. These findings indicate that the method of levels may be more sensitive to sex differences than the more commonly used method of limits. Also, thresholds appear to increase from session 1 to session 2, and thresholds assessed via different methods are not strongly correlated. Potential implications of these results for experimental pain assessment are discussed.