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.