Table 2.

Significantly altered metabolites in serum of patients with mild or severe OA compared to that of healthy controls.

	Metabolite	(t R/min)	(m/z)	Ion	Comparison		Related pathway
					OA versus HC	m-OA versus s-OA
1	4-Oxoproline	0.73	130.051	[M + H]+	∗↓	NS	Amino acid metabolism
2	L-Glycine	0.78	120.030	[M + FA − H]−	∗∗↓	∗↓	Amino acid metabolism
3	L-Histidine	0.88	156.077	[M + H]+	∗∗↓	∗↓	Amino acid metabolism
4	Hypoxanthine	0.91	135.031	[M − H]−	∗∗↑	NS	Purine metabolism
5	Pyridoxine	0.96	170.082	[M + H]+	∗↓	NS	Energy metabolism
6	Pyruvate	1.02	133.014	[M + FA−H]−	∗↑	NS	Glycolysis
7	Homocysteine	1.09	136.043	[M + H]+	∗∗↑	NS	Energy metabolism
8	Urate	1.16	169.035	[M + H]+	∗∗↑	NS	Purine metabolism
9	3-Carboxy-4-methyl-2-oxopentanoate	2.85	173.045	[M − H]−	∗↑	NS	Amino acid metabolism
10	Methyl-hippuric acid	3.11	194.082	[M + H]+	∗↑	NS	Amino acid metabolism
11	L-Tryptophan	3.59	205.097	[M + H]+	∗∗↑	NS	Amino acid metabolism
12	Sphingomyelin (d18:1/16:0)	6.09	703.575	[M + H]+	∗↓	NS	Sphingolipid metabolism
13	Propionyl-L-carnitine	6.98	218.137	[M + H]+	∗↓	NS	Fatty acid metabolism
14	LPC (18:0)	13.11	524.372	[M + H]+	∗∗↓	NS	Lipid metabolism

↑: increased; ↓: decreased; ^* P < 0.05; ^** P < 0.01; NS, not significant; m-OA, mild OA; s-OA, severe OA.