## Standard Dosing of Anti-D Immunoglobulin **Key Point:** The standard anti-D dose for prevention of Rh isoimmunisation after delivery of an Rh-positive baby is **500 IU (100 μg) IM**, which corresponds to **500 IU per mL of fetal red cells** (or **1000 IU per mL of fetal whole blood/plasma**). This is Option A. ### Understanding the Dosing Ratio **High-Yield:** - **Standard prophylactic dose:** 500 IU (100 μg) IM, given within 72 hours of delivery - **Coverage:** One standard 500 IU dose neutralises up to ~1 mL of fetal red cells (or ~2 mL fetal whole blood) - **For larger fetomaternal haemorrhage (FMH):** Additional anti-D is required — calculated as 500 IU per mL of fetal RBCs detected beyond the standard volume ### Kleihauer–Betke Test and FMH Quantification | FMH Volume (fetal RBCs) | Anti-D Required | |---|---| | ≤1 mL | 500 IU (standard dose) | | 1–2 mL | 1000 IU | | 2–3 mL | 1500 IU | | >4 mL | Quantify by KB test; 500 IU per mL fetal RBCs | ### Timing and Indications | Scenario | Dose | Timing | |---|---|---| | Delivery of Rh+ baby (unsensitised mother) | 500 IU IM | Within 72 hours | | Abortion/miscarriage (first trimester) | 250 IU IM | Within 72 hours | | Amniocentesis/CVS | 250–500 IU IM | Within 72 hours | | Antenatal prophylaxis (28 weeks) | 500 IU IM | Routine | **Clinical Pearl:** The Kleihauer–Betke test or flow cytometry is used to detect excessive FMH. The dosing ratio of **500 IU per mL of fetal RBCs** (Option A) is the accepted standard in UK BCSH guidelines and referenced in major obstetric texts. Option D (100 IU/mL) is a derived mathematical ratio from an older approximation and does not represent the clinical dosing standard. **Mnemonic:** **500-1** — 500 IU anti-D covers 1 mL of fetal red cells. [cite: BCSH Guidelines for the Use of Anti-D Immunoglobulin, 2014; Obstetrics by Ten Teachers 21e Ch 15; Williams Obstetrics 26e]
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