RT Book, Section A1 McKinney, Christopher A1 Nuss, Rachelle A1 Wang, Michael A2 Bunik, Maya A2 Levin, Myron J. A2 Abzug, Mark J. A2 Schreiner, Teri L. SR Print(0) ID 1211984374 T1 Hematologic Disorders T2 Current Diagnosis & Treatment: Pediatrics, 27th Edition YR 2025 FD 2025 PB McGraw Hill PP New York, NY SN 9781265739898 LK accessmedicine.mhmedical.com/content.aspx?aid=1211984374 RD 2025/03/15 AB The normal ranges for peripheral blood counts vary with age. Neonates have a hematocrit of 45%–65% accompanied by an increased reticulocyte count of 2%–8% in the first 24 hours. Within the first few days of life, erythrocyte production decreases as serum erythropoietin levels fall in response to increased tissue oxygenation, and the hemoglobin and hematocrit fall to a physiologic nadir around 10 g/dL and 30%, respectively, at 6–8 weeks of life. Thereafter, the normal values for hemoglobin and hematocrit gradually increase until adult values are reached after puberty with post-pubertal males having an average hemoglobin concentration about 1 g/dL higher than postpubertal females. Premature infants may have a more exaggerated and delayed hemoglobin nadir of 7–8 g/dL at 8–10 weeks. Anemia is defined as a hemoglobin concentration less than the 2.5th percentile for a normal population of the same gender and age.