Single arm physiological study to measure renal functional reserve as a biomarker of kidney function and health

The investigators will measure glomerular filtration rate using urinary clearance of cold iothalamate before and after an oral protein load in patients with chronic kidney disease and healthy volunteers.

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