TABLE 4.

Central 95% reference intervals for RBC folate concentrations by demographic variable in participants aged ≥4 y (NHANES 1988–2010)¹

	Prefortification (1988–1994)		Early post-fortification (1999–2004)		Late post-fortification (2005–2010)
Demographic variable	n	RBC folate	n	RBC folate	n	RBC folate
		nmol/L		nmol/L		nmol/L
MA
Male
4–11 y	810	473–1660	598	667–2180	603	673–1820
12–19 y	482	305–1430	1129	555–1840	628	500–1700
20–59 y	1603	314–1410	948	514–1930	1041	427–1810
≥60 y	504	311–1770	485	527–2730	3332	534–2320
Female
4–11 y	823	407–1370	587	678–2000	593	667–1800
12–19 y	500	318–1480	1130	575–2100	613	529–1730
20–59 y	1644	318–1670	1100	560–2310	1125	501–2250
≥60 y	454	319–2070	492	578–3180	3962	520–2680
NHB
Male
4–11 y	756	382–1300	577	642–1550	4432	572–1610
12–19 y	467	234–1140	1035	471–1380	621	449–1540
20–59 y	1421	246–1160	824	431–1600	968	429–1770
≥60 y	470	238–1640	3402	450–2240	480	449–2660
Female
4–11 y	723	313–1220	576	609–1620	453	594–1490
12–19 y	544	218–1120	963	438–1440	566	443–1420
20–59 y	1827	223–1480	930	448–1950	1089	431–1990
≥60 y	487	248–1870	3742	480–2500	464	442–2400
NHW
Male
4–11 y	593	475–1850	471	800–2230	588	688–1930
12–19 y	3272	336–1620	805	577–1920	644	488–1890
20–59 y	1616	326–1710	1902	600–2320	2224	540–2190
≥60 y	1381	370–2210	1343	650–3250	1535	614–3110
Female
4–11 y	577	462–1620	457	747–2170	534	685–1790
12–19 y	4182	318–1360	802	595–2070	573	540–1910
20–59 y	1930	314–1906	2142	558–2660	2361	514–2530
≥60 y	1502	370–2618	1336	661–3270	1454	627–3405

¹Values are reference intervals (2.5–97.5th percentile). RBC folate concentrations were measured by the Bio-Rad radioassay from 1988 to 1994 and from 1999 to 2006 and assay-adjusted to be comparable to the microbiologic assay, which was used from 2007 to 2010 (13, 16, 17). MA, Mexican American; NHB, non-Hispanic black; NHW, non-Hispanic white.

²Cell size is smaller than required ( = 448) to estimate the 2.5th and 97.5th percentiles with sufficient precision at an assumed mean design effect of 1.4.