Plasma Kynurenic Acid/Tryptophan Ratio: A Sensitive and Reliable Biomarker for the Assessment of Renal Function

Figures & data

Table 1. Renal function and plasma TRP, KYN, and KYNA (mean ± SD).

Parameter	Control	CKD	p-Value	CR (%)^a
N (male/female)	60 (45/15)	20 (5/5)	>0.05	NA
Age (years)	53 ± 13	58 ± 15	0.180	NA
BUN (mmol/L)	4.76 ± 1.04	6.85 ± 2.07	<0.001	44
UmAlb (mg/24-h)^b	18.9 (14.7–23.1)	75.5 (19.9–289)	<0.001	NA
SCr (µmol/L)	76.2 ± 15.8	118.9 ± 48.8	<0.001	56
eGFR (mL/min/1.73 m^2)	96.3 ± 18.3	62.9 ± 22.8	<0.001	–35
TRP (µmol/L)	62.2 ± 9.8	56.6 ± 10.7	0.017	–9
KYN (µmol/L)	2.05 ± 0.43	2.60 ± 0.78	<0.001	27
KYNA (nmol/L)	24.2 ± 7.8	43.3 ± 18.2	<0.001	79
KYN/TRP (%)	3.34 ± 0.68	4.80 ± 1.66	<0.001	44
KYNA/KYN (%)	1.19 ± 0.30	1.65 ± 0.39	<0.001	39
KYNA/TRP (‰)	0.396 ± 0.133	0.802 ± 0.378	<0.001	103

Notes: CKD, chronic kidney disease; BUN, blood urea nitrogen; SCr, serum creatinine; UmAlb, urine micro-albumin; eGFR, estimated glomerular filtration rate; TRP, tryptophan; KYN, kynurenine; KYNA, kynurenic acid; NA, not applicable; CR, change rate.

^aCR (%) = (CKD – Control)/Control × 100.

^bData were shown as median (25–75 percentile) and were compared using logarithmic concentration.

Table 2. Correlations between parameters of renal function and plasma TRP, KYN, and KYNA (n = 80).

	Correlation coefficient (r)
Parameter	TRP	KYN	KYNA	KYN/TRP	KYNA/KYN	KYNA/TRP
BUN	–0.232	0.533***	0.620***	0.597***	0.423***	0.622***
UmAlb/24-h^a	–0.297**	0.316**	0.445***	0.487***	0.367***	0.518***
SCr	–0.121	0.704***	0.787***	0.741***	0.467***	0.797***
eGFR	0.157	–0.598***	–0.652***	–0.643***	–0.428***	–0.662***
Mean^b	0.202	0.538	0.626	0.617	0.421	0.650

Notes: TRP, tryptophan; KYN, kynurenine; KYNA, kynurenic acid; BUN, blood urea nitrogen; SCr, serum creatinine; UmAlb, urine micro-albumin; eGFR, estimated glomerular filtration rate.

* denotes p < 0.05; ** denotes p < 0.01 and *** denotes p < 0.001.

^aUsing logarithm for the evaluation.

^bCalculated by absolute value.

Figure 1. Relationship between kynurenine (KYN) and tryptophan (TRP) or kynurenic acid (KYNA). (A), Control (n = 60); (B), Chronic kidney disease (n = 20).

Figure 2. A schematic diagram of the changes of blood TRP, KYN, KYNA, KYN/TRP, and KYNA/TRP in CKD patients. CKD, chronic kidney disease; IDO, indoleamine 2,3-dioxygenase; TDO, tryptophan 2,3-dioxygenase; KAT, kynurenine aminotransferase; TRP, tryptophan; KYN, kynurenine; KYNA, kynurenic acid.