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f C 6
TPDN 1989, Vol. 49, No. 28
trimester, reach their nadir near the end of
second trimester, then gradually rise during the
third trimester. Because of the change of Hb and
Hct during pregnancy, anemia must be charac-
terized according to the specific stage of preg-
nancy. The normal range of Hb and Hct during
pregnancy is based on data aggregated from four
European studies of healthy iron-supplemented
pregnant women. These studies provide similar
findings at each specific month of pregnancy.
The month-specific fifth percentile values for
Hb of the pooled data have been adopted for use
in the CDC Pregnancy Nutrition Surveillance
System (Table 2). In addition, trimester-specific
cutoffs also have been developed for use in the
clinical setting (Table 2). These trimester-spe-
cific cutoffs are based on the mid-trimester
values; cutoffs for the first trimester, the time at
which most women arc initially seen for prenatal
care, arc based on a late-trimester value.
Table 2.
Pregnancy month-specific and trimester-
specific hemoglobin (Hb) cutoffs*
Gestation (wks) 12 16 20 24 28 32 36 40
Trimester 1' 2 2' 2 3 3' 3 term
Mean Hb (g/dL) 12.2 11.8 11.6 11.6 11.8 12.1 12.5 12.9
5th percentile
Hb values (g/dL) 11.0 10.6 10.5 10.5 10.7 11.0 11.4 11.9
Equivalent 5th
percentile
Hct' values (%) 33.0 32.0 32.0 32.0 32.0 33.0 34.0 36.0
"Based on pooled data from four European surveys of healthy women taking iron supplements
(7-10).
'Hb values adopted for the trimester-specific cutoffs.
'Hematocrit.
Adjustment of Hb and Het Cutoffs for Altitude
and Smoking
Persons residing at higher altitudes (>1,000 meters
[3,300 feet]) have higher Hb and Het levels than
those residing at sea level. This variation is due
to the lower oxygen partial pressure at higher al-
titudes, a reduction in oxygen saturation of
blood, and a compensatory increase in red cell
production to ensure adequate oxygen supply to
the tissues. Thus, higher altitude causes a gener-
alized upward shift of the Hb and Het distribu-
tions. This shift may be associated with the
underdiagnosis of anemia for residents of higher
altitudes when sea-level cutoffs are applied
(CDC, unpublished data). Therefore, the proper
diagnosis of anemia for those residing at higher
altitudes requires an upward adjustment of Hb
and Het cutoffs. The values for altitude-specific
adjustment of Hb and Het are derived from
data collected by the CDC Pediatric Nutrition
Surveillance System on children residing at
various altitudes in the mountain states (Table 3).
Altitude affects Hb and Het levels throughout
pregnancy in a similar way (JN Chatfield, un-
published data).
Table 3.
Altitude adjustments for hemoglobin (HB) and
hematocrit (Hct) cutoffs
Altitude (ft) Hb (g/dL) Hct (%)
<3000 0.0 0.0
3000-3999" +0.2 +0.5
4000-4999" + 0.3 +1.0
5000-5999" +0.5 +1.5
6000-6999 +0.7 +2.0
7000-7999' +1.0 +3.0
8000-8999' +1.3 +4.0
9000-9999' +1.6 +5.0
>10,000' +2.0 +6.0
'Based on data from CDC Pediatric Nutrition Surveillance System and reference 11.
'Based on reference 11 only.
The influence of cigarette smoking is similar to
that of altitude, in that smoking increases Hb and
Het levels substantially. The higher Hb and Het
of smokers is a consequence of an increased
carboxyhemoglobin from inhaling carbon mon-
oxide during smoking. Because Carboxyhemo-
globin has no oxygen carrying capacity, its
presence causes a generalized upward shift of
the Hb and Het distribution curves (CDC, unpub-
lished data). Therefore, a smoking-spccific ad-
justment to the anemia cutoff is necessary for the
proper diagnosis of anemia in smokers. The
smoking-specific Hb and Het adjustments are
derived from the NHANES I data (Table 4).
Table 4.
Smoking adjustments for hemoglobin (Hb) and
hematocrit (Hct)
Characteristic Hb (gm/dL) Hct (%)
Nonsmoker 0.0 0.0
Smoker (all) +0.3 +1.0
1/2-1 pack/day +0.3 +1.0
1-2 packs/day +0.5 +1.5
>2 packs/day +0.7 +2.0
The altitude and smoking adjustments are addi-
tive. For example, a woman living at 6,000 feet
and smoking two or more packs of cigarettes per
day would have her cutoff for anemia adjusted
upward by a total of 1.4 grams of Hb or 4% Hct.
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