<p>Low back pain is highly prevalent and disabling worldwide. Several investigations focus on detecting improvement in patient-reported outcomes that reflect minimal important differences following care. However, determining minimal important differences for worsening is just as important. This study determined the estimated probability of people with low back pain reporting being worse as a function of their change in pain and disability scores following a chiropractic encounter. This secondary analysis of The Danish Chiropractic Low Back Pain Cohort (ChiCo) included 2,848 adults (mean age = 45 [SD=14] years; 59% male) with low back pain. Pain intensity (numeric rating scale) and disability (Roland-Morris Disability Questionnaire) were measured at baseline and at two weeks after a chiropractic encounter. Self-report of being “worse” at two weeks was measured on the global perceived change score (seven-point scale: 1 = much better; 7 = much worse). The association of absolute and relative changes in pain and disability scores with being “worse” was evaluated using logistic regression. Only 3.1% of participants reported worsening. Participants with increased pain or disability scores showed an increased probability of reporting worsening. However, the probability of reporting being worse with increased pain and disability rarely reached over 50%. Minor improvements or no changes in pain and disability were also perceived as worse by some. Detecting negative minimal important differences is crucial for patient safety and to fully inform people of the risks of interventions.</p>