Table 5.

Stability of PEth 16:0/18:1 in whole blood stored at 4°C and comparison of calculated concentrations obtained for different laboratories

Sample ID	Stability of PEth 16:0/18:1 in whole blood stored at 4°C				Ethanol (g/kg)b	Comparison of PEth 16:0/18:1 concentrations obtained with concentrations obtained by reference laboratories
	Calculated PEth 16:0/18:1 concentration (stored 0 days)		Stability in whole blood (%)			Sample ID	PEth 16:0/18:1 concentration(ng/mL)	Reference laboratory 1c		Reference laboratory 2
	nmol/L	ng/mL	36 daysa	71 daysa				ng/mL	%‐diff.d	ng/mL	%‐diff.d
1	1.5	1.1	93	128	0.0	a	47	55	18	56	21
2	3.3	2.3	97	92	0.0	b	175	210	20	232	33
3	5.0	3.5	106	109	0.0	c	283	360	27	302	7
4	6.2	4.4	105	106	0.0	d	1090	1430	31	1118	3
5	11	7.5	123	116	0.0	e	3395	4260	25	3866	14
6	17	12	98	109	0.0
7	22	16	104	90	0.0
8	36	25	102	103	0.0
9	42	30	108	113	0.0
10	52	36	104	96	0.0
11	73	51	98	96	0.0
12	110	77	104	114	0.0
13	124	87	95	92	0.0
14	230	162	106	112	0.0
15	279	196	130	128	0.7
16	430	302	98	96	0.0
17	598	420	99	100	0.0
18	641	451	96	91	0.0
19	765	538	96	94	0.2
20	915	643	88	87	0.9
21	1267	891	104	97	0.0

Diff., difference.

^aStability was calculated as: Calculated concentration of blood sample stored (36 or 71 days)/calculated concentration of blood sample analyzed the day of arrival at our laboratory. In this test calibrators were prepared in Type 1 water (for all three analyses) and not in whole blood.

^bEthanol concentrations were determined by immunological screening as described by Kristoffersen et al.45

^cPEth 16:0/18:1 concentrations determined by UHPLC‐MS/MS on a Luna Omega 1.6 μm Polar C₁₈ column (30 × 2.1 mm ID) with a sample preparation with PPT using 2‐propanol.

^d%‐difference for Peth 16:0/18:1 concentrations between reference laboratory and the concentrations obtained by the developed UHPLC‐MS/MS method.