OBJECTIVES The Pain Assessment for Lower Back Symptoms (PAL-S) is a Patient Reported Outcome (PRO) instrument being developed to assess the key symptoms of chronic low back pain (cLBP).  Qualitative development included both concept elicitation and cognitive interviews.  As part of the ongoing development of the instrument, we further evaluated and refined the PAL-S using a mixed methods approach. METHODS Adults self-reporting a clinical diagnosis of cLBP were recruited from an existing US-based commercial survey panel to participate in a pilot quantitative study.  Qualifying participants completed a web-based survey consisting of the 14-item PAL-S and items assessing clinical, treatment, and demographic characteristics. Study data was analyzed to assess item- and scale-level performance of the PAL-S using Rasch Measurement Theory analyses.  Following analysis and modification, two waves of cognitive interviews were conducted to evaluate respondent understanding of the revised PAL-S. RESULTS The dataset included 598 respondents (mean age: 55.5±12.6; 67.9% female; 88.0% white; and 54.0% married) who had cLBP for mean of 15.2±11.5 years.  The Rasch analyses item threshold maps showed only two items having ordered thresholds, suggesting that respondents experienced increased difficulty distinguishing between options at the lower levels of the 0-10 scale.  Simulations collapsing the responses to a best-fit 4-point response scale resulted in improved ordering of thresholds, suggesting a more optimal response option structure.  Based on these findings, the numeric response scale of the PAL-S items was replaced with a 4-point verbal rating scale incorporating response choices such as not at all, slight, moderate, and severe