Table 4.  . Percent stability (mean ± SD) of tryptophan metabolites after 1–5 freeze–thaw cycles.

Compound	Baseline mean ± SD	% change
		Thawing 2	Thawing 3	Thawing 4	Thawing 5
TRP	1.5 ± 0.5 μM	104 ± 4.9	100 ± 4.9	99 ± 4.9	101 ± 4.9
L-KYN	60.2 ± 30.6 nM	98 ± 4.1	94 ± 4.1	94 ± 4.07	95 ± 4.1
KYNA	2.4 ± 0.8 nM	100 ± 4.9	96 ± 4.9	107 ± 4.9	101 ± 4.9
QUIN	23.6 ± 19.7 nM	105 ± 1.5	101 ± 1.5	102 ± 5.9	100 ± 5.9
PIC	14.5 ± 3.0 nM	101 ± 4.7	97 ± 4.7	99 ± 4.6	101 ± 6.3
PIC/QUIN ratio	0.8	101.8 ± 7.1	105.7 ± 10.7	105.9 ± 9.2	108.1 ± 8.4
NAA	22.8 ± 20.9 nM	101 ± 2.3	98 ± 2.0	98 ± 7.6	105 ± 6.6
3-HK	3.1 ± 1.2 nM	103 ± 2.9	97 ± 1.7	97 ± 1.7	96 ± 1.7

Compared with 100% at t = 0, mean ± SD from 4 to 6 individual CSF samples. For baseline and all freeze–thaw cycles, n = 6.

3-HK: 3-Hydroxykynurenine; CSF: Cerebrospinal fluid; KYNA: Kynurenic acid; L-KYN: L-Kynurenine; NAA: Nicotine amide; PIC: Picolinic acid; QUIN: Quinolinic acid; SD: Standard deviation; TRP: Tryptophan.