Validation of a new instrument for measuring disease-specific quality of life: A pilot study among patients with chronic kidney disease and hyperkalemia

BACKGROUND: To prepare for a longitudinal study of the effects of potassium-lowering treatment on quality of life (QOL), we quantified the validity of a new disease-specific instrument for measuring QOL, using data from patients who had hyperkalemia (HK) due to chronic kidney disease (CKD) or chronic heart failure, and were also being treated with potassium binders (PBs). METHODS: In this cross-sectional study, the participants were 98 patients at five outpatient clinics in Japan. The outcome measures were the Medical Outcomes Study 36-item short-form (SF-36), a widely used generic measure of QOL, and the Quality of Life Disease-specific Impact Scale (QDIS-7), a recently-developed disease-specific measure of QOL. Internal-consistency reliability was quantified, and factor analysis was done to confirm hypothesized QOL dimensions. Validation tests used two external criteria: CKD stage, and PB formulation. PB formulation was used because different formulations are associated with different degrees of patients' burden. Using a previously-described method, we computed the relative validity (RV) of the two measures. RESULTS: Two factor scoring of the SF-36 and one factor scoring of the QDIS-7, as standardized from previous studies, were confirmed. The RVs showed that the QDIS-7 was much more valid than the SF-36, for discriminating between groups defined clinically (by CKD stage), and also between groups defined by PB formulation. Reliability was satisfactory: 0.73-0.95 for the SF-36 and 0.86 for the QDIS-7. CONCLUSIONS: The QDIS-7 with CKD or PB attributions was more valid than the SF-36 for measuring the effects of CKD and of PB formulation on QOL.