Table 1.

Median ammonia concentrations among different subgroups of neonates.

	Number of neonates (%)	2.5th and 97.5th reference limits (90% CI) µmol/L	Confidence ratioa	Z maxb	SD Ratiob	Median ammonia concentration in µmol/L (minimum, maximum)	P value
All neonates	127 (100%)	12 (10-15)	0.14			32 (<10, 86)	-
		82 (72-86)
Sex				1.6	1.4		0.0079
Male	73 (57%)	11 (9-13)	0.22			29 (<10, 86)
		77 (66-91)
Female	54 (43%)	20 (17-22)	0.22			35 (18, 82)
		65 (57-73)
Gestational age at birth				1.7	1.0		0.0119
Term (≥37 weeks)	43 (34%)	13 (11-15)	0.27			28 (10, 86)
		66 (55-79)
Preterm (<37 weeks)	84 (66%)	14 (12-16)	0.19			35 (<10, 82)
		79 (69-90)
Collection method				n/a	n/a		0.2610
Venipuncture	80 (63%)	n/a				33 (<10, 86)
Umbilical venous  catheter	47 (37%)	n/a				31 (12, 74)
Collection time				n/a	n/a		0.2820
<24 hours of life	7 (6%)	n/a				31 (12, 57)
24-48 hours of life	67 (53%)					33 (<10, 82)
48-72 hours of life	38 (30%)					28 (14, 86)
>72 hours of life	15 (12%)	n/a				38 (14, 61)
Feeding at the time of ammonia draw				n/a	n/a		0.5789
Total parenteral  nutrition only	12 (9%)	n/a				35 (10, 72)
Enteral nutrition onlyc	80 (63%)	n/a				32 (14, 86)
Both parenteral and  enteral nutrition	35 (28%)	n/a				32 (<10, 55)
Hyperbilirubinemia requiring phototherapy				n/a	n/a		0.3798
Yes	62 (49%)	n/a				33 (<10, 86)
No	65 (51%)	n/a				31 (10, 82)

^aConfidence ratio <0.20 is desirable.

^bEither Z max >2.2 or SD Ratio >1.5 suggests a separate reference interval (RI) may be warranted.

^cIncludes formula and breastmilk by oral intake and tube feedings. Some infants in this group were receiving dextrose-containing intravenous fluids, but not parenteral amino acid or lipid formulations.