Original Research

Using Ferritin Levels To Determine Iron-Deficiency Anemia in Pregnancy

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BACKGROUND: Standard obstetrical practice has included iron therapy for patients with anemia without requiring the determination of iron deficiency. However, the proportion of pregnant women with anemia who have such a deficiency may be relatively modest. We instituted a practice protocol using serum ferritin levels to determine the proportion of women undergoing prenatal care who had both anemia and iron deficiency.

METHODS: We evaluated consecutive women entering prenatal care. Those with anemia (hemoglobin level <11 mg/dL) underwent testing for serum ferritin level and other hematologic variables.

RESULTS: A total of 182 patients entered prenatal care. Hemoglobin data were available for 173 (95%). Thirty-eight (22%) had anemia (hemoglobin level <11 mg/dL); 1 of those women was excluded from the study. Using a ferritin level of 12 mg per dL as the cutoff for iron deficiency, 54% (20) of the 37 remaining patients with anemia had an iron deficiency, and 46% (17) had anemia not related to such a deficiency. Use of hematologic indices provided on complete blood count were not useful in predicting iron deficiency based on serum ferritin levels.

CONCLUSIONS: In our population of prenatal patients with anemia, only approximately half had an iron deficiency. Diagnostic and therapeutic approaches to screening for anemia in pregnancy should be reconsidered and further evaluated.

Standard obstetrical practice has included screening for anemia and the provision of iron supplements to anemic patients. This approach has been based on assumptions about anemia and iron deficiency that are not supported by the literature.

Anemia in pregnancy has been reported to be associated with preterm delivery.1,2 However, this may not take into consideration the lower hemoglobin values normally present during the second trimester. The potentially spurious association between anemia and preterm delivery could be explained by the lower hemoglobin values that are expected during the second trimester. When measured at the time of preterm delivery, these lower hemoglobin values, which are normal in the second trimester (but not in the third trimester or in a nonpregnant woman), are often believed to have led to the preterm delivery.

Normal hemogloblin values from nonpregnant women cannot be assumed to apply to those who are pregnant. Average hemoglobin levels decrease to 11.6 g per dL at 20 to 24 weeks’ gestation, with the fifth percentile at 10.5 g per dL (hematocrit=32%). Anemia in pregnancy has been defined by criteria from the Centers for Disease Control and Prevention (CDC) as a hemoglobin level of less than 11 g per dL during the first and third trimesters and less than 10.5 g per dL during the second trimester.3

When anemia is present in pregnancy it cannot be assumed to be the result of iron deficiency, even though this type of anemia has been previously reported as the most common cause.4 The Camden study5 of 826 pregnant women showed preterm delivery and low birth weight associated with iron-deficiency anemia. Only 27.9% of the pregnant women had anemia, however, and only 12.5% of the patients with anemia had an iron deficiency. Thus, only 3.5% of the entire cohort had iron-deficiency anemia.6

Iron deficiency in pregnancy has been defined by the National Academy of Sciences panel on nutrition and pregnancy7 as ferritin levels lower than 12 ng per mL. A systematic overview8 of 55 studies relevant to laboratory tests for diagnosis of iron-deficiency anemia in variable patient populations found serum ferritin radioimmunoassay to be the most powerful test. Ferritin levels are considered the gold standard for the diagnosis of iron-deficiency anemia in pregnancy.9

We report a descriptive study of the use of ferritin levels to determine the need for iron supplementation among pregnant women with anemia.

Methods

Lebanon Family Health Services is a nonprofit federally subsidized community agency providing prenatal care and women’s health services to a diverse population without restriction on the basis of financial status. The prenatal patients cared for in this practice included women aged 15 to 40 years (23.6% were younger than 19 years; 70.8%, 19 to 30 years; and 5.6%, older than 30 years) of whom 65.3% were white, 29.2% Hispanic, 4.2% African American, and 1.4% Asian.

We evaluated all patients entering into prenatal care at Lebanon Family Health Services from April 1, 1997, through December 31, 1998, using prospective data collection and retrospective record review. Prenatal vitamins (including elemental iron 30 to 60 mg/day) were prescribed to all patients. Complete blood count was tested at the initial evaluation as part of a comprehensive screening. For patients who entered prenatal care at earlier than 28 weeks’ gestation complete blood count was checked again when they had reached that point.

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