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Background: The then-test, also known as the retrospective pre- and post-test design method, is a measurement used to evaluate response shift. The method requires patients to assess their previous health-related quality of life (HRQoL) and provide a retrospective judgement based on their current perspectives. The then-test, however, has been criticized for its reliability and validity. The objective of this systematic review is to summarize the current literature for the use of the then-test for evaluating HRQoL in cancer patients and account for potential response shift effects., Methods: A literature search was conducted in May 2022 using MEDLINE, PubMed, and PsychINFO dated from 2005 to the time of the search. Studies were included if they (I) used the then-test and (II) involved a population of cancer patients (any cancer site)., Results: The literature search resulted in 16 studies, published between 2005 and 2020. All studies used the then-test to detect response shifts. The EORTC QLQ-C30 and EORTC QLQ-BR23 questionnaires were the most common assessments used to evaluate HRQoL. Of the 16 articles, 5 exclusively reported on breast cancer, 5 reported on prostate cancer, and the remaining included all cancer sites. Most studies looked at patients undergoing a combination of chemotherapy, radiotherapy, and hormonal therapy. The mean differences between the retrospective assessment at both 3 and 6 months were significant for various quality of life (QoL) dimensions. Patients in some studies recalled their pretreatment HRQoL (then-score) as better than the pretreatment baseline scores and others reported them as worse, both confirming the existence of a response shift., Conclusions: Our review demonstrates that the then-test measurement tool has been commonly used for the detection of response shift. Newer measures for response shift have become more accepted; the then-test, if used, should be used with caution considering its limitations and the emergence of more advanced methods.