Background: The low anterior resection syndrome (LARS) score is a patient-reported outcome measure to evaluate the severity of bowel dysfunction after rectal cancer surgery by scoring the major symptoms of LARS. The aim of this study was to translate the English version of the LARS score into Japanese and to investigate the validity and reliability of the LARS score. Methods: The LARS score was translated in Japanese following current international recommendations. A total of 149 rectal cancer patients completed the LARS score questionnaire and were also asked a single question assessing the impact of bowel function on quality of life (QoL). A total of 136 patients answered the LARS score questionnaire twice. Results: The Japanese LARS score showed high convergent validity, based on its good correlation between the LARS score and QoL (p < 0.001). The LARS score was able to discriminate between patients according to the tumor distance to anal verge (p < 0.001), type of surgery (p < 0.001), and time since surgery (p = 0.001). Patients after ultra-low anterior resection and intersphincteric resection showed especially high scores. The score also had high test–retest reliability (intraclass correlation coefficient: 0.87). Conclusion: The Japanese LARS score is a valid and reliable tool for measuring LARS. The LARS score is appropriate for assessments in postoperative bowel function and international comparison. Using this score, patient-reported outcome measures of LARS in Japanese patients can be shared internationally. Additional validation reports from non-English speaking countries can support the LARS score as a worldwide assessment tool for postoperative bowel dysfunction.
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