Iron Deficiency : Part 6

by CDC

(Page 6 of 11)

Long-term residency at high altitude (greater than or equal to 3,000 ft) and cigarette smoking cause a generalized upward shift in Hb concentration and Hct. The effectiveness of screening for anemia is lowered if the cutoff values are not adjusted for these factors. Adjustment allows the positive predictive value of anemia screening to be comparable between those who reside near sea-level and those who live at high altitude and between smokers and nonsmokers.

In the United States, the distribution of Hb concentration values is similar among whites and Asian Americans, and the distribution of Hct values is similar among whites and American Indians. The distributions are lower among blacks than whites, however, even after adjustment for income. These different distributions are not caused by a difference in iron status indicators (e.g., iron intake, serum ferritin concentration, or transferrin saturation); thus, applying the same criteria for anemia to all races results in a higher rate of false-positive cases of iron deficiency for blacks. For example, in the United States during 1976-1980, 28% of nonpregnant black women but only 5% of nonpregnant white women had a Hb concentration of less than 12 g/dL and, according to the anemia criteria, would be classified as iron deficient, even though other tests for iron status suggested these women were not iron deficient. For this reason, the Institute of Medicine recommends lowering Hb concentration and Hct cutoff values for black children aged less than 5 years by 0.4 g/dL and 1%, respectively, and for black adults by 0.8 g/dL and 2%, respectively. Because the reason for this disparity in distributions by race has not been determined, the recommendations in this report do not provide race-specific cutoff values for anemia. Regardless, health-care providers should be aware of the possible difference in the positive predictive value of anemia screening for iron deficiency among blacks and whites and consider using other iron status tests (e.g., serum ferritin concentration and transferrin saturation) for their black patients.

Accurate, low-cost, clinic-based instruments have been developed for measuring Hb concentration and Hct by using capillary or venous blood. Small diurnal variations are seen in Hb concentration and Hct measurements, but these variations are neither biologically nor statistically significant. A potential source of error of using capillary blood to estimate Hb concentration and Hct in screening is improper sampling technique. For example, excessive squeezing (i.e., "milking") of the finger contaminates the blood with tissue fluid, leading to false low readings. Confirmation of a low reading is recommended by obtaining a second capillary blood sample from the finger or by venipuncture.

Although measures of Hb concentration and Hct cannot be used to determine the cause of anemia, a diagnosis of iron-deficiency anemia can be made if Hb concentration or Hct increases after a course of therapeutic iron supplementation. Alternatively, other laboratory tests (e.g., mean cell volume, red blood cell distribution width, and serum ferritin concentration) can be used to differentiate iron-deficiency anemia from anemia due to other causes.

In the United States in recent years, the usefulness of anemia screening as an indicator of iron deficiency has become more limited, particularly for children. Studies using transferrin saturation (a more sensitive test for iron deficiency) have documented that iron deficiency in most subpopulations of children has declined such that screening by Hb concentration no longer efficiently predicts iron deficiency. Data from NHANES II, which was conducted during 1976-1980, indicated that less than 50% of children aged 1-5 years and women in their childbearing years who had anemia (as defined by Hb concentration less than 5th percentile) were iron deficient (i.e., had at least two of the following: low mean cell volume, high erythrocyte protoporphyrin concentration, or low transferrin saturation). Causes of anemia other than iron deficiency include other nutritional deficiencies (e.g., folate or vitamin B12 deficiency), hereditary defects in red blood cell production (e.g., thalassemia major and sickle cell disease), recent or current infection, and chronic inflammation. The current pattern of iron-deficiency anemia in the United States indicates that selective anemia screening of children at known risk for iron deficiency or additional measurement of indicators of iron deficiency (e.g., erythrocyte protoporphyrin concentration and serum ferritin concentration) to increase the positive predictive value of screening are now suitable approaches to assessing iron deficiency among most U.S. children. The costs and feasibility of screening using additional indicators of iron deficiency may preclude the routine use of these indicators.

Mean Cell Volume

Mean cell volume (MCV), the average volume of red blood cells, is measured in femtoliters (10-15 liters). This value can be calculated as the ratio of Hct to red blood cell count or measured directly using an electronic counter. MCV is highest at birth, decreases during the first 6 months of life, then gradually increases during childhood to adult levels. A low MCV corresponds with the 5th percentile for age for the reference population in NHANES III.

Some anemias, including iron-deficiency anemia, result in microcytic red blood cells; a low MCV thus indicates microcytic anemia. If cases of lead poisoning and the anemias of infection, chronic inflammatory disease, and thalassemia minor can be excluded, a low MCV serves as a specific index for iron-deficiency anemia.

Red Blood Cell Distribution Width

Red blood cell distribution width (RDW) is calculated by dividing the SD of red blood cell volume by MCV and multiplying by 100 to express the result as a percentage:

RDW (%) = {SD of red blood cell volume (fL)/MCV (fL)} x 100

A high RDW is generally set at greater than 14.0%, which corresponds to the 95th percentile of RDW for the reference population in NHANES III. The RDW value obtained depends on the instrument used.

An RDW measurement often follows an MCV test to help determine the cause of a low MCV. For example, iron-deficiency anemia usually causes greater variation in red blood cell size than does thalassemia minor. Thus, a low MCV and an RDW of greater than 14.0% indicates iron-deficiency anemia, whereas a low MCV and an RDW less than or equal to 14.0% indicates thalassemia minor.

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