Definition and Basic Principles

• Pathological Anemia: Hemoglobin (Hb) or hematocrit (Hct) levels below 2 standard deviations from the mean for gestation, postnatal age, and cardiorespiratory status.
• Physiologic Anemia of Infancy: Normal postnatal decline in Hb due to improved oxygenation at birth, leading to down-regulation of erythropoietin (EPO). Nadir of 9.5–11 g/dL is reached at 8–12 weeks in term infants.
• Anemia of Prematurity (AOP): Occurs earlier (4–6 weeks) and lower (7–8 g/dL) in preterm neonates due to shortened RBC lifespan, rapid growth-induced volume expansion, and blunted hepatic EPO response to hypoxia.
• Sampling Discrepancy: Capillary Hct is typically higher than venous Hct by 2.7% to 3.7%. Pre-heating the heel narrows this artifactual gap.

Etiological Classification

Anemia is dynamically classified into three mechanistic categories:

1. Anemia Due to Blood Loss (Normal Bilirubin)

• Obstetric Causes: Placental incision during cesarean section, vasa previa, cord rupture, velamentous insertion, or abruptio placentae.
• Occult / Fetal Causes: Massive Fetomaternal Hemorrhage (FMH), Twin Anemia Polycythemia Sequence (TAPS) in monochorionic twins, or fetoplacental pooling (e.g., tight nuchal cord).
• Neonatal / Iatrogenic Causes: Excessive phlebotomy (most common NICU cause), internal bleeding (subgaleal hemorrhage, cephalhematoma, intraventricular hemorrhage, hepatic/splenic/adrenal rupture), or gastrointestinal hemorrhage.

2. Anemia Due to Hemolysis (Elevated Bilirubin + Reticulocytosis)

• Immune-Mediated: Isoimmune hemolytic disease (Rh, ABO, minor blood group incompatibilities like Kell, Duffy, c, E) or maternal autoimmune diseases (e.g., systemic lupus erythematosus).
• Inherited RBC Defects: Membrane defects (hereditary spherocytosis, elliptocytosis), enzyme deficiencies (G6PD deficiency, pyruvate kinase deficiency), or hemoglobinopathies ($\alpha$-thalassemia syndromes).
• Acquired: Sepsis (bacterial/viral), TORCH infections, or Disseminated Intravascular Coagulation (DIC).

3. Anemia Due to Impaired RBC Production (Normal Bilirubin + Reticulocytopenia)

• Congenital: Diamond-Blackfan anemia, osteopetrosis, Congenital Dyserythropoietic Anemia (CDA).
• Acquired/Infectious: Congenital parvovirus B19 infection, rubella, or congenital leukemia.

Clinical Evaluation

History

• Maternal & Obstetric: Intrapartum bleeding events, acute fetal distress (indicative of FMH), chorionicity (TAPS), or maternal autoantibodies.
• Family History: Unexplained neonatal jaundice, early gallstones, anemia, or splenectomy in parents/siblings (inherited RBC defects).

Physical Examination

• Acute Blood Loss: Presentation with hypovolemic shock, tachycardia, poor peripheral perfusion, respiratory distress, and metabolic acidosis with minimal initial jaundice.
• Chronic Blood Loss: Marked pallor, but hemodynamically compensated; mild tachypnea, tachycardia, or irritability may be present.
• Hemolytic Anemia: Early-onset jaundice, progressive pallor, and hepatosplenomegaly.
• Hypoplastic / Infectious: Growth restriction, purpura/petechiae ("blueberry muffin" rash), cataracts, and microcephaly (TORCH features).

Diagnostic Approach and Lab Investigations

Step 1: Initial Hematological Screening

• Complete Blood Count (CBC) with RBC Indices: Establishes severity and baseline parameters.
• Reticulocyte Count: Differentiates regenerative (hemolytic/blood loss) from non-regenerative (production defect) etiologies.
• Peripheral Blood Film (PBF): Evaluates diagnostic RBC morphology (e.g., spherocytes, schistocytes, Heinz bodies).
• Total Serum Bilirubin (TSB): Identifies accelerated RBC destruction.

Step 2: Diagnostic Algorithm Based on Initial Matrix

Step 3: Specific Confirmative Testing

• Kleihauer-Betke (KB) Test or Flow Cytometry: Quantifies fetal cells in maternal circulation to confirm FMH.
• Apt Test (Alkali Denaturation): Differentiates swallowed maternal blood from neonatal gastrointestinal bleeding (fetal Hb remains pink; adult Hb turns yellow-brown).
• Neurosonogram and Abdominal Ultrasound: Identifies internal bleeding sites (IVH, subgaleal, subcapsular hepatic hematoma).
• Targeted Assays: G6PD enzyme levels (post-acute phase), osmotic fragility, parental CBCs, and TORCH PCR profiles.

Management and Transfusion Guidelines

1. Packed Red Blood Cell (PRBC) Transfusion Thresholds

The choice to transfuse depends on gestational age, postnatal age, and clinical respiratory support status rather than arbitrary numerical triggers.

Restrictive Transfusion Thresholds for Preterm Infants (<32 weeks)

Transfusion Thresholds for Term Infants

• < 10.0 g/dL: Severe pulmonary disease or major surgical intervention.
• < 8.0 g/dL: Moderate cardiopulmonary disease.
• < 7.0 g/dL: Symptomatic anemia (apnea, bradycardia, poor weight gain, tachycardia).

2. Blood Product Specifications

• Dose & Rate: Standard top-up transfusion is 10–15 mL/kg administered over 2–3 hours (extendable to 4 hours in VLBW infants).
• Leukoreduction: Leukocyte-depleted PRBCs are mandatory for infants <1200 g to minimize CMV transmission and febrile non-hemolytic reactions.
• Irradiation: Indicated for all preterm neonates to eliminate donor lymphocytes and prevent Transfusion-Associated Graft-Versus-Host Disease (TA-GVHD). Use within 24 hours of irradiation to avoid hyperkalemia.
• Age of Blood: Fresh blood (<5 days old) is vital for exchange or massive transfusions to preserve 2,3-DPG levels and prevent potassium toxicity; standard issue blood (<35 days old) is acceptable for routine top-up transfusions.

3. Partial Exchange Transfusion

• Indication: Severe anemia presenting with hypervolemia or congestive heart failure.
• Method: Corrects tissue hypoxia rapidly without augmenting the total blood volume. Uses donor PRBCs adjusted to a hematocrit of ~70%.
• Formula: $$\text{Volume to be Exchanged (mL)} = \frac{\text{Total Blood Volume (mL)} \times (\text{Desired Hct} - \text{Observed Hct})}{\text{Donor Hct} - \text{Observed Hct}}$$ (Total blood volume is estimated at 80–90 mL/kg).

4. Pharmacological and Preventive Interventions

• Delayed Cord Clamping (DCC): Delays clamping by 30–60 seconds in stable infants; increases placental transfusion, optimizing iron stores and reducing subsequent transfusion needs.
• Umbilical Cord Milking (UCM): Avoided in extreme preterm infants (23–27 weeks) due to an established association with severe intraventricular hemorrhage.
• NICU Blood Conservation: Implement micro-sampling techniques, utilize inline point-of-care monitors, and salvage placental cord blood for baseline admission testing.
• Enteral Iron Supplementation: Initiated at 2–4 weeks of age in all preterm infants at a dose of 2–4 mg/kg/day to maintain iron profiles for neurodevelopment; it does not alter the timing or depth of the physiological nadir.
• Erythropoiesis-Stimulating Agents (rh-EPO): Not routinely utilized due to its inconsistent efficacy and clinical correlation with increased risk of severe Retinopathy of Prematurity (ROP).

5. Complications of Neonatal Transfusions

• Transfusion-Associated Necrotizing Enterocolitis (TANEC): Ischemic bowel injury occurring within 48 hours of transfusion in vulnerable neonates. Evidence does not support routinely withholding feeds during top-up transfusions.
• Transfusion-Associated Circulatory Overload (TACO): Left ventricular strain due to overly rapid infusion or high volume.
• Infectious Transmission: Regulated via Nucleic Acid Testing (NAT) for HIV, HBV, and HCV.