RhIG Dosing for 1 Unit of Rh-Positive Platelets
For an Rh-negative woman of childbearing age who receives 1 unit of Rh-positive platelets, administer 20 mg (100 IU) of RhD immune globulin, which protects against 1 mL of RBCs—far exceeding the typical RBC contamination in a single platelet unit. 1
Understanding the Risk
- Platelets themselves do not express RhD antigens, but platelet preparations contain contaminating RBCs sufficient to induce Rh sensitization even in immunosuppressed patients 2, 1
- Modern apheresis platelet products contain approximately 0.00043 mL of RBCs, while whole-blood-derived products contain approximately 0.036 mL 2, 1
- The minimum RBC volume to elicit a primary anti-D immune response is only 0.03 mL, which falls within the RBC content range of some platelet products 2, 1
Specific Dosing Recommendation
The updated ASCO guideline specifies that 20 mg (100 IU) of RhIG protects against 1 mL of RBCs. 1 This single standard dose is more than adequate for one platelet unit, as even whole-blood-derived platelets (with the highest RBC contamination at 0.036 mL) contain far less than 1 mL of RBCs. 2, 1
An older guideline recommended 25 mg (125 IU) per 1 mL of RBCs, but the current standard is 20 mg (100 IU). 1
Timing Is Critical
- Administer RhIG before or immediately after the platelet transfusion for maximum efficacy 1
- RhIG remains effective if given within 72 hours of RhD-positive RBC exposure 2, 1, 3, 4
- If the 72-hour window is missed, administration up to 28 days post-exposure may still provide some benefit 4
Route of Administration
- Prefer the intravenous route if the patient is thrombocytopenic to avoid hematoma risk from intramuscular injection 1
- Some RhIG products are licensed for both IM and IV administration, while others are IM-only 2
- The FDA label confirms that RhIG administration within 72 hours reduces alloimmunization from 12-13% to 1-2% 3
Who Should Receive Prophylaxis
ASCO prioritizes RhIG administration for:
- RhD-negative children, particularly girls 1
- Women of childbearing age 2, 1
- Any RhD-negative patient with future transfusion needs where preventing alloimmunization is clinically important 1
The overall risk of alloimmunization without prophylaxis is low (1.44% in the largest study), but prevention is critical for women who may become pregnant in the future. 2, 5
Practical Considerations
- One dose of RhIG may protect against several platelet transfusions, depending on timing and RBC contamination levels 2
- Verbal or written informed consent must be obtained prior to RhIG administration, as it is a pooled biological blood product 4
- RhIG may not be acceptable to certain patient populations, such as Jehovah's Witnesses who refuse blood products 6
- Current preparations show no recent evidence of HIV, HCV, HBV, or HTLV-1 transmission due to rigorous manufacturing safeguards 6
Common Pitfall to Avoid
Do not withhold RhIG based on the small RBC volume in modern platelet products. Even apheresis platelets with only 0.00043 mL of RBCs can theoretically cause sensitization when the threshold is 0.03 mL, and the standard dose provides a wide safety margin. 2, 1