Assessing residual renal function and efficiency of hemodialysis - an application for urographic contrast media

Background: In patients on hemodialysis with end-stage renal disease there is an increasing interest in measuring both residual renal function (RRF) a nd quantity and quality of dialysis because insufficient dialysis gives hig her mortality. For that purpose we have measured clearances of two urograph ic iodine (I) contrast media (CM) with different molecular masses (iohexol 821 u and iodixanol 1,550 u). These CM are filtered through glomeruli and d ialysis membranes and have higher molecular masses than urea and creatinine and might rep resent the dialyzability of the hypothetic uremic toxins wit h a molecular mass of 300-5,000 u. Methods: Thirteen patients (8 of them we re anuric) immediately after hemodialysis received 15 ml iohexol (300 mg l/ ml i.v.) and 2 weeks later in the same way 15 mi iodixanol (320 mg l/ml). N ine other patients (2 anuric) received CM after only one dialysis; 8 got io hexol and 1 got iodixanol. After the CM injections the iodine concentration s were measured with X-ray fluorescence in blood and, when available, urine during the following 2 days including both the start and end of the next d ialysis. Eighteen patients after two dialysis sessions, 2 weeks apart, rece ived 10 ml iohexol i.v., and a single blood sample was taken at the start o f the next dialysis 2 days later to determine RRF alone. Results: In the 10 anuric patients the extrarenal clearances (mean +/- SD) were 2.5 +/- 1.1 a nd 2.7 +/- 1.7 ml/min/1.73 m(2) for iohexol and iodixanol, respectively. in patients with RRF good correlations were demonstrated between body clearan ce, based on two blood samples, and renal clearance of CM. Good correlation s (r(2) = 0.853 for iohexol, r(2) = 0.933 for iodixanol) were noted between two-sample and single-sample body clearances. Repeated single sample iohex ol clearances gave a coefficient of variation of 15%. During dialysis the c learances of iohexol and iodixanol were, respectively, 69 +/- 16 and 58 +/- 11 ml/min/1.73 m(2) when calculated from a single-pool model (hemodialysis clearance of CM from plasma). A median increase (rebound) of CM concentrat ions in plasma 45 min dialysis was 8% for iodixanol and 18% for iohexol. Wh en the CM concentration 45 min after dialysis was used, the clearance value s were by 8-10% lower and represented the hemodialysis clearance of CM from the extracellular compartments. The dialysis eliminations of iohexol and i odixanol were similar to that of urea, measured as percentage reduction of serum levels during dialysis. Conclusions: A single injection of CM at the end of dia lysis followed by a single blood sam pie at the sta rt of the ne xt dialysis gives total body clearance, i.e., an estimation of the RRF. An additional blood sample at the end of the next dialysis gives dialysis effi ciency. Copyright (C) 2000 S. Karger AG, Basel.