Screening Characteristics of Hemoglobin and Mean Corpuscular Volume for Detection of Iron Deficiency in Pregnancy

Iron deficiency in pregnancy remains underdiagnosed despite professional society recommendations for first-trimester complete blood count (CBC) screening. To determine the effectiveness of the CBC hemoglobin and mean corpuscular volume (MCV) to identify iron deficiency in pregnancy, we conducted a retrospective analysis of 20,550 pregnancies from 2009 to 2022 at the University of California, San Francisco, obstetrics clinics. A total of 16,547 (80.5%) pregnant individuals had first-trimester screening CBC; 345 (2.1%) had a coincident ferritin test. Hemoglobin level less than 11 g/dL and MCV level less than 80 fL each had sensitivity of only 30% (95% CI, 20-41%) to detect first-trimester iron deficiency (ferritin level less than 30), corresponding to a negative likelihood ratio of 0.90 (95% CI, 0.77-1.05) and 0.85 (95% CI, 0.73-0.99), respectively. More than 50% of the 1,749 women with documented iron deficiency anytime during pregnancy were neither anemic nor microcytic at the time of diagnosis.