“…38 Studies comparing urinary and plasma clearance of either iothalamate or iohexol show high agreement, although some studies showed that plasma clearance slightly overestimates urinary clearance at lower levels of GFR (Table 5). 17,34 In studies of repeated measurements of plasma clearance of iothalamate or iohexol, the coefficient of variation ranged from 5.6 to 11.5%, 37,39,42,43 similar to the variation observed with urinary clearance reported above. More studies are necessary to determine the precision of these methods.…”