Assessment of Hyperkalemia's Illness and Treatment Burden in Chronic Kidney Disease and Heart Failure Patients: HK Registry Pilot Study

Inclusion Criteria: * Outpatients aged ≥20 years * Hyperkalemia patients defined as meeting either of the following criteria: 1. Having a history of S-K ≥5.1 mmol/L ≥2 times within 6 months before enrolment 2. Having a history of S-K ≥5.5 mmol/L once within 6 months before enrolment 3. Currently treated by potassium binders for the treatment of hyperkalemia at enrolment * Having been diagnosed as CKD (≥stage 3b) or HFrEF by investigators as defined below: CKD is diagnosed based on the guidelines of CKD issued by the Japanese Society of Nephrology (JSN, 2018) as being either or both of condition 1 and 2 for ≥3 months 1. Clear sign of kidney impairment based on urinalysis, imaging, blood test, or biopsy. Especially, existence of ≥0.15 g/gCr of proteinuria (≥30 mg/gCr of albuminuria) is important. 2. GFR \<45 mL/min/1.73m2 Within the routine clinical practice, GFR is estimated by serum creatinine, gender, and age using following the formulation. eGFR creat (mL/min/1.73m2) = 194 x serum creatinine (mg/dL)-1.094 x age (years)-0.287 (for female patients, x 0.739) ≥Stage 3b CKD is diagnosed based on the following eGFR categories: * Stage 3b: 30 mL/min/1.73m2 ≤ eGFR \<45 mL/min/1.73m2 * Stage 4: 15 mL/min/1.73m2 ≤ eGFR \<30 mL/min/1.73m2 * Stage 5: eGFR \<15 mL/min/1.73m2 Patients with HFrEF is enrolled if patients meet following criteria within 6 months: 1. EF ≤40% 2. NYHA class II-IV * Provision of signed, written, and detailed informed consent * Signed written informed consent by themselves Exclusion Criteria: * Currently on any chronic RRT (including hemodialysis or peritoneal dialysis \>30 days, or kidney transplant) within 6 months before enrolment * Patients with acute kidney injury at enrolment •Patients who took blood transfusion or potassium supplements within 6 months before enrolment * Active malignancy or life expectancy of less than 6 months. * Patients who have GI disturbance/chronic diarrhoea/stoma, and investigators determine those affect significantly serum K level * Patients who have autoimmune disorders, and investigators determine those affect significantly serum K level * Patients whose lab data have suspicion for pseudohyperkalemia * Patients who are pregnant, lactating, or planning to become pregnant * Current participation in interventional studies and/or clinical trials * Patients who, in the opinion of the investigators, would be unlikely to comply with self-assessments