DIALYSIS TECHNICAL NOTE How to Measure Residual Renal Function in Patients on Maintenance Hemodialysis Susie Q. Lew Many patients with end-stage renal disease who are on maintenance hemodialysis still have significant residual renal function. Exogenous markers such as inulin and radionuclides, and endogenous markers such as creatinine and urea are commonly used to quantify this residual function. These various methods are described in detail and compared with one another. Residual renal function needs to be considered when the dialysis prescription is written. This contribution of residual renal function to dialysis adequacy and its importance are discussed. © 1994 by the National Kidney Foundation, Inc. Index Words: Creatinine clearance; residual renal function; hemodialysis; glomerular filtration rate; inulin; iothalamate; OTPA; EOTA; urea; clearance. T he functional level of glomerular filtration rate (GFR) is measured for clinical or research purposes. The technique used in measuring GFR will depend on the degree of accuracy needed, convenience, and cost. In a research setting, exact measurements of GFR are usually needed to compare renal function before and after an intervention or observa- tion period. Inulin clearance is used because of its accuracy. In the clinical setting, multiple measurements of renal function over time are used to define any absolute change or rate of change of renal function as well as to permit calculation of the appropriate dosage of poten- tially toxic drugs excreted by the kidney. In clinical practice, creatinine clearance is the preferred technique of measuring GFR be- cause of its convenience and cost. Before a patient reaches end-stage renal disease (ESRO), frequent measurements of renal function alert the clinician to the need for intervention. Modification of the diet with low protein intake, and renal replacement therapy with dialysis or transplantation are examples of such interventions. Renal replace- ment therapy is usually instituted in symptom- atic patients when GFR is less than 10 mLi minute. Measurement of renal function does not cease when the patient is on renal replace- ment therapy. At the time of instituting renal replacement therapy and at periodic intervals thereafter, measurements of residual renal function are performed to assist in the prescrip- tion of an adequate dialysis regimen for pa- tients with ESRO. Periodic assessment of residual renal func- tion is needed to document changes over time. To provide adequate dialysis and to decrease morbidity and mortality, objective parameters such as Kt/V, protein catabolic rate (PCR), and urea reduction ratio (URR) have been introduced. Guidelines, recommen- dations, and standards have been formulated and used for comparative studies. In hemodi- alysis and peritoneal dialysis, fractional urea clearance factored for body urea space is used for calculating the contribution of dialysis to small solute removal (Kt/V). Urinary urea clearance is measured to derive total Kt/V for hemodialysis and peritoneal dialysis (usually expressed on a per treatment basis for HO, and on a weekly basis for PO). Urinary creati- nine clearance is used primarily in PO to calculate total weekly creatinine clearance. There are several methods that can be used to measure residual renal function in patients receiving renal replacement therapy. Each method is described and compared in a clini- cal setting with respect to accuracy, cost, and ease of application (Table 1). Clearance studies for patients on hemodialysis are performed during the interdialytic period. Oliguria poses a problem with clearance techniques that require timed urine collections. Water loading to initiate and maintain a diuresis may not be From The Division of Renal Diseases and Hypertension, Department of Medicine, The George Washington University Medical Center, Washington, DC. Address correspondences to Susie Q. Lew, MD, 2150 Pennsylvania Ave, NW, Rm 4-425, Washington, DC 20037. © 1994 by the National Kidney Foundation, Inc. 1073-4449/94/0102-0012$3.00/0 Advances in Renal Replacement Therapy, Vol 1, No 2 (July), 1994: pp 185-193 185