Table 1.

Kidney clearances of secretory solutes and correlations with eGFR

Solute	Kidney Clearance, ml/min (IQR)a	Correlation with eGFRb,c	Protein Binding in CKD, %d
Hippurate	465 (274, 807)	0.33	51±13
Pyridoxic acid	456 (284, 716)	0.56	87±1
Dimethyluric acid	452 (258, 804)	0.35	68±7
Trimethyluric acid	260 (134, 499)	0.38	80±11
Isovalerylglycine	225 (142, 348)	0.50	4±7
Tiglylglycine	182 (111, 286)	0.56	24±15
Kynurenic acid	92 (63, 136)	0.60	96±2
Xanthosine	77 (46, 121)	0.44	15±13
Cinnamoylglycine	57 (33, 101)	0.39	95±3
Indoxyl sulfate	34 (22, 51)	0.55	93±2
p-Cresol sulfate	10 (6, 15)	0.52	96±1

^aMedian (IQR) of kidney clearances (not standardized for body surface area).

^bPearson correlations between log-transformed secretory-solute clearances and log-transformed eGFR, each standardized to 1.73 m² body surface area.

^cP values for all correlations <0.001.

^dShown as mean±SD. To evaluate the extent of protein binding, we performed ultrafiltration experiments in plasma obtained from 14 patients with advanced CKD not receiving dialysis. Briefly, plasma was filtered using a centrifugal filter (Amicon Ultra, 3 kD MWCO) at 11,200×g for 30 min at room temperature. The concentration of solutes in the filtrate was then determined using the same method as for plasma and compared with the concentration of solutes in unfiltered plasma. The proportion of the solutes in the filtrate was assumed to be unbound in plasma. Preliminary experiments were performed to confirm that observed differences between the filtrate and unfiltered plasma could not be attributed to adsorption to the Amicon Ultra filtration units (data not shown).