Using Ferritin Levels To Determine Iron-Deficiency Anemia in Pregnancy

Journal of Family Practice, Sept, 2000 by Brian S. Alper, Roger Kimber, Anuradha Kudumala Reddy

The CDC criteria for defining anemia are hemoglobin levels less than 11 g per dL during the first and third trimesters, and less than 10.5 g per dL during the second trimester. A hemoglobin level of less than 11 g per dL at any time during the pregnancy was used as the cutoff point for anemia in this clinical practice, in consideration of patients with uncertain or inaccurate pregnancy dating.

Patients with anemia underwent blood testing for serum ferritin level, generally 1 week after a complete blood cell count. At the time, ferritin levels were determined using different test tubes than those used for other prenatal testing, thus resulting in a delay in obtaining blood for ferritin testing. If serum ferritin was 12 ng per dL or lower, iron-deficiency anemia was diagnosed and ferrous sulfate was prescribed for the remainder of the pregnancy and the postpartum period. If serum ferritin was greater than 12 ng per dL iron deficiency was excluded, anemia was generally considered to be pregnancy related, and further evaluation and treatment was at the discretion of the treating clinician.

Variables recorded for all patients with anemia were: estimated date of delivery, last menstrual period, date of any testing for complete blood count or ferritin levels, hemoglobin value, hematocrit value, red blood cell count (RBC), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), red cell distribution width (RDW), any notation regarding microcytosis, and serum ferritin levels.

RESULTS

A total of 182 patients were consecutively entered into prenatal care during the study period. Hemoglobin data were not obtained from 9 patients--4 transferred to another practice for care before initial blood testing, 2 did not carry the pregnancy until initial blood testing, and 3 were lost to follow-up before initial blood testing. Thus, hemoglobin data were available for 173 patients (95%).

Thirty-eight women (22%) had anemia defined as a hemoglobin level lower than 11 mg per dL at any time during the pregnancy. One patient was excluded from further evaluation because she entered prenatal care late, had an initial hemoglobin level of 9.6 mg per dL at 39 weeks' gestation, and was given iron without serum ferritin determination.

Of the 37 patients with anemia who had serum ferritin level measurements, the values ranged from 3 to 91 ng per dL. Twenty of these 37 patients (54%) had iron-deficiency anemia (ferritin levels ranging from 3-10 mg/dL), and 17 patients (46%) had anemia not related to iron deficiency (range=13-91 mg/dL).

Of the 38 patients with anemia by our definition, 20 had hemoglobin levels between 10.5 and 10.9 mg per dL, of whom 10 were between 14 and 28 weeks' gestation (second trimester). Thus only 28 patients (16%) had anemia according to the CDC criteria. Of the 27 patients with anemia according to the CDC criteria (one patient was excluded), 17 (63%) had iron-deficiency anemia, and 10 (37%) had anemia not related to iron deficiency.