Table 2. Associations Between Age and Sex With Fifth Percentile Hb in a Multinational Sample of Apparently Healthy Individuals^a.

Variable	Factors associated with fifth percentile Hbb
	Preschool children 6-59 mo (n = 13 445)		Nonpregnant women 15-49 y (n = 25 880)
	β (SE)	P value	β (SE)	P value
Fixed effects
Intercept, Hb, g/dL	10.26 (0.32)	<.001	10.76 (0.05)	<.001
Age, yc	0.19 (0.02)	<.001	–0.03 (0.4)	.41
Age squared, yd	–0.00 (0.0)e	.62	0.00 (0.0)e	.10
Sex of child	0.28 (0.76)	.66	NA	NA
Variance decomposition (for random effects), %
ICC between surveys	3.6	<.001	3.5	<.001
ICC between participants across all surveys	96.4		96.5

Abbreviations: Hb, hemoglobin; ICC, intraclass correlation; NA, not applicable.

SI conversion factor: To convert Hb to grams per liter, multiply by 10.

^aHealthy is defined as no inflammation (C-reactive protein ≤0.5 mg/dL [to convert to milligrams per liter, multiply by 10] or α-1-acid glycoprotein ≤1 g/L), no iron deficiency (ferritin <12 ng/mL for children and <15 ng/mL for women [to convert to micrograms per liter, multiply by 1.0]), no vitamin A deficiency (retinol-binding protein or retinol <20.1 μg/dL [to convert to micromoles per liter, multiply by 0.0349], when available), and no known malaria.

^bLinear quantile mixed model results are shown.

^cAge was mean centered across surveys.

^dThe age squared term was added to assess curvilinear relations with Hb. Hb values were adjusted for altitude, when available (Afghanistan, Azerbaijan, Colombia, Ecuador, Great Britain, Laos, Malawi, Mexico 2006 and 2012, and Rwanda); otherwise, no adjustment was applied or, if altitude was less than 1000 m, no adjustment was needed. Hb values were further adjusted for smoking among women (Colombia, Ecuador, Mexico 2006 and 2012, Great Britain, and US).

^eValues were nonzero but round to 0.00 at 2 decimal points. Coefficient is −0.001 for children and 0.0013 for women.