## Rh Isoimmunisation Prevention: Key Principles ### The Correct Answer: Option D is FALSE (the EXCEPT answer) **High-Yield:** The Kleihauer-Betke (KB) test or flow cytometry is **NOT mandatory before administering anti-D** in all situations. For routine antenatal prophylaxis (28 weeks) and for minor sensitising events in the first trimester (≤12 weeks), a standard dose of anti-D is given **without** prior KB testing. KB testing is indicated **after** delivery of an Rh-positive baby or after significant sensitising events (e.g., antepartum haemorrhage, abdominal trauma) to quantify fetomaternal haemorrhage (FMH) and determine whether additional anti-D is required beyond the standard dose. The statement that KB/flow cytometry is "mandatory before administering anti-D" is therefore incorrect — it is used to guide additional dosing, not as a prerequisite for every anti-D administration. ### Why the Other Options Are CORRECT **Option A — Correct:** Anti-D immunoglobulin must be given **within 72 hours** of a sensitising event for maximum efficacy. This is the universally accepted window (RCOG Green-top Guideline No. 22; FOGSI guidelines). Some residual benefit may persist up to 10 days, but 72 hours is the standard. **Option B — Correct:** The standard dose is **500 IU (100 mcg) per mL of fetal red cells** or per 1 mL of fetal whole blood. This is the basis for all anti-D dose calculations (RCOG / BCSH guidelines; KD Tripathi Pharmacology). **Option C — Correct:** Routine antenatal anti-D prophylaxis at **28 weeks** is recommended for all Rh-negative, unsensitised women. The phrase "regardless of antibody screening results" is contextually acceptable here — the key criterion is that the woman is **unsensitised** (negative indirect Coombs), and the 28-week dose is given routinely without repeating antibody screening at that exact visit in many protocols. If already sensitised (positive ICT), prophylaxis is not indicated, but the option specifies "unsensitised women," making the statement correct. ### Kleihauer-Betke Test: When Is It Required? | Scenario | KB Test Required? | |---|---| | Routine antenatal prophylaxis (28 wks) | **No** | | 1st trimester sensitising event (≤12 wks) | **No** (standard 250 IU given) | | Delivery of Rh-positive baby | **Yes** (to detect large FMH) | | Antepartum haemorrhage / trauma | **Yes** | | Invasive procedure with significant bleeding | **Yes** | **Clinical Pearl:** Per RCOG Green-top Guideline No. 22 and Harrison's Principles of Internal Medicine, the KB test is used **post-event** to quantify FMH and calculate top-up doses — it is not a prerequisite before every anti-D injection. Administering a standard dose first and then testing (or testing simultaneously) is the accepted practice. **Key Point (Mnemonic):** "500 IU per mL fetal RBCs = baseline dose; KB test tells you if you need MORE." *(References: RCOG Green-top Guideline No. 22; KD Tripathi Essentials of Medical Pharmacology, 8th ed.; Ian Donald's Practical Obstetric Problems)*
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