Cross-sectional validity and specificity of comprehensive measurement in lymphedema and lipedema of the lower extremity: a comparison of five outcome instruments

Background Literature on the validity of outcome measurement in lymphedema and lipedema is very sparse. This study aimed to examine the convergent, divergent and discriminant validity of a set of 5 instruments in both conditions. Methods Cross-sectional outcome was measured by the generic Short Form 36 (SF-36), the lymphedema-specific Freiburg Quality of Life Assessment for lymphatic disorders, Short Version (FLQA-lk), the knee-specific Knee Outcome Survey Activities of Daily Living Scale (KOS-ADL), the Symptom Checklist-90-revised (SCL-90R), and the Six-Minute Walk Test (6 MWT). Construct convergent/divergent validity was quantified by bivariate correlations and multivariate factor analysis, and discriminant validity by standardized mean differences (SMDs). Results Health was consistently better in lymphedema (n = 107) than in lipedema (n = 96). The highest construct convergence was found for physical health between the SF-36 and KOS-ADL (bivariate correlations up to 0.78, factor loads up to 0.85, explained variance up to 56.8%). The second most important factor was mental health (bivariate correlations up to 0.79, factor loads up to 0.86, explained variance up to 13.3%). Discriminant validity was greatest for the FLQA-lk Physical complaints (adjusted SMD = 0.93) followed by the SF-36 Bodily pain (adjusted SMD = 0.83), KOS-ADL Function (adjusted SMD = 0.47) and SF-36 Vitality (adjusted SMD = 0.39). Conclusions All five instruments have specific strengths and can be implemented according to the scope and aim of the outcome examination. A minimum measurement set should comprise: the SF-36 Bodily pain, SF-36 Vitality, FLQA-lk Physical complaints, FLQA-lk Social life, FLQA-lk Emotional well-being, FLQA-lk Health state, KOS-ADL Symptoms, KOS-ADL Function, and the SCL-90R Interpersonal sensitivity.