Measurement of Renal Functional Reserve

Inclusion Criteria: 1. Age ≥18 2. Estimated GFR \>30 mL/min/1.73m2 Exclusion Criteria: 1. Allergy to iothalamate, shellfish or iodine 2. Use of metformin or amiodarone 3. Inability to maintain a stable regimen of medications which affect GFR for \> one week prior to participation (e.g. non-steroidal anti-inflammatory drugs, angiotensin converting enzyme inhibitors, angiotensin receptor blockers) 4. Use of medications which directly affect elimination of creatinine (e.g. cimetidine and trimethoprim) 5. Acute exacerbation of asthma or chronic obstructive pulmonary disease within 3 months requiring hospitalization or oral steroid therapy 6. Inadequate intravenous access 7. Severe anemia (Hct \<21%) 8. Acute kidney injury (rise in creatinine to ≥1.5 times the previous baseline or by ≥ 0.3 mg/dL on most recent labs prior to enrollment) 9. History of contrast-induced nephropathy 10. Hyperthyroidism 11. Pheochromocytoma 12. Sickle cell disease 13. Urinary retention or incontinence 14. Status post organ transplant 15. Pregnancy or active breast feeding 16. Cognitive impairment with inability to give consent 17. Institutionalized status