Anti-D Immunoglobulin Dose Calculator
Anti-D Immunoglobulin Dose Calculator
Calculate the anti-D dose for a measured fetomaternal haemorrhage under either convention. The UK figure is per mL of fetal red cells and the US figure per mL of fetal whole blood — crossing them under- or over-doses.
Anti-D Immunoglobulin Dose
UK 125 IU/mL vs US 300 µg vials8 mL of fetal red cells, UK (BSH) convention
Formula — two conventions, two denominators
US (AABB): vials = round(mL of fetal WHOLE BLOOD ÷ 30) + 1, then dose (IU) = vials × 1,500
1 µg anti-D = 5 IU, so 300 µg = 1,500 IU and 100 µg = 500 IU
- 125 IU/mL fetal red cells
- the BSH figure. A 500 IU dose is stated to cover a haemorrhage of up to 4 mL of fetal red cells, and above that the top-up is 125 IU per additional mL — which is the same as 125 IU per mL overall
- one 300 µg vial per 30 mL
- the AABB figure, written against fetal WHOLE BLOOD. Thirty mL of whole blood is 15 mL of red cells, so the US vial covers 15 mL where the UK convention would charge 1,875 IU for the same volume
- round, then add one
- the AABB rule rounds the vial count to the nearest whole number and then adds one extra vial as a safety margin. It is round-to-nearest, not round-up: 0.4 vials becomes 1, not 2
- reading the answer
- divide by 5 for micrograms and by 1,500 for 300 µg vials. UK ampoules come in 500 IU, 1,500 IU and 2,500 IU, so the calculated requirement is rounded up to whole ampoules at issue
Worked example
8 mL of fetal red cells, UK (BSH) convention
8 mL of fetal red cells is about 16 mL of fetal whole blood at a fetal haematocrit of 0.5
UK: 125 × 8 = 1,000 IU, which is 200 µg — issued as two 500 IU ampoules, or one 1,500 IU ampoule where that is the stocked presentation
The first 4 mL is covered by the standard 500 IU dose, so 500 IU of this is the top-up
US on the same bleed: 16 ÷ 30 = 0.53 vials, rounded to 1, plus one extra = 2 vials = 600 µg = 3,000 IU
The two conventions differ threefold here — which is why the convention has to be chosen deliberately, not inherited from whichever paper was read last
The two conventions side by side
| UK — BSH | US — AABB | |
|---|---|---|
| Dose is written per | mL of fetal RED CELLS | mL of fetal WHOLE BLOOD |
| Standard quantum | 125 IU (25 µg) per mL | One 300 µg (1,500 IU) vial per 30 mL |
| Equivalent per mL of fetal red cells | 125 IU | 100 IU — 1,500 IU ÷ 15 mL |
| Rounding | Calculated requirement rounded up to whole ampoules of 500, 1,500 or 2,500 IU | Vial count rounded to the nearest whole number, then one extra vial added |
| A bleed of 8 mL fetal red cells (16 mL whole blood) | 1,000 IU (200 µg) | 2 vials — 600 µg (3,000 IU) |
Timing, follow-up and what this page does not cover
| Step | UK (BSH) practice |
|---|---|
| When to give it | As soon as possible, and always within 72 hours of the sensitising event |
| If more than 72 hours have passed | Still give it — partial protection is better than none — and document the delay |
| Follow-up sample when the bleed exceeds 4 mL of fetal red cells | 48 hours after an intravenous dose, 72 hours after an intramuscular one, to confirm fetal cells have cleared; repeat the dose if they have not |
| Routine antenatal prophylaxis | 1,500 IU between 28 and 30 weeks, or 500 IU at 28 and 34 weeks — a fixed regimen, not calculated from a bleed, and outside the scope of this page |
| Potentially sensitising events before 20 weeks | A minimum dose regimen applies and a Kleihauer is not usually required — again outside this calculator |
Same drug, same bleed, two different arithmetics
Anti-D immunoglobulin works by clearing fetal RhD-positive red cells from the maternal circulation before the mother’s immune system can respond to them, so the dose has to be matched to how many fetal cells arrived. Both major conventions do that, and both are correct within their own system — but they express the dose in different units and, more dangerously, against different denominators. The United Kingdom figure is 125 IU per mL of fetal red cells. The United States figure is one 300 µg vial per 30 mL of fetal whole blood, which is 15 mL of red cells.
Set side by side, the UK convention charges 125 IU per mL of red cells and the US convention effectively 100 IU, before the extra vial the AABB rule adds. That is close enough that neither is unsafe, and far enough apart that the answers differ. What is genuinely unsafe is crossing them: a laboratory that reports 30 mL of fetal whole blood, fed into the UK formula as though it were red cells, produces 3,750 IU where 1,875 IU was needed; the same mistake in the other direction halves a dose. This calculator asks separately what was measured and which convention applies, precisely so that the two cannot be crossed by accident.
The rounding rules differ too. In the UK the calculated requirement is rounded up to the ampoule sizes stocked — 500, 1,500 and 2,500 IU. The AABB rule instead rounds the vial count to the nearest whole number and then adds one extra vial: 0.4 vials becomes one, and 1.6 becomes three. It is round-to-nearest followed by a safety margin, not a straight round-up, and treating it as a round-up gives an extra vial to every small bleed.
Two things sit outside the arithmetic and matter more than it. Anti-D should be given as soon as possible and always within 72 hours of the sensitising event, and where that window has been missed it should still be given, because partial protection is worth having. And where the bleed exceeded 4 mL of fetal red cells, a follow-up maternal sample is required — 48 hours after an intravenous dose, 72 hours after an intramuscular one — to confirm that the fetal cells have gone, with a further dose if they have not. This page does not cover routine antenatal prophylaxis, which is a fixed schedule rather than a calculated dose.
Frequently asked questions
How much anti-D is needed per mL of fetal red cells?
In the UK, 125 IU — that is 25 µg — per mL of fetal red cells, with a minimum of the standard 500 IU postnatal dose, which itself covers a haemorrhage of up to 4 mL. In the US the equivalent works out at about 100 IU per mL of red cells before the extra vial is added.
How many 300 µg vials do I give?
Divide the volume of fetal WHOLE BLOOD by 30, round to the nearest whole number, then add one extra vial. A 16 mL bleed gives 0.53, which rounds to 1, plus one, so two vials. Note this is round-to-nearest and then add one, not round up and then add one.
Why do the UK and US doses differ for the same bleed?
Because they use different denominators. The UK dose is written per mL of fetal red cells and the US dose per mL of fetal whole blood, which is roughly twice the volume. The AABB convention also adds a spare vial. Mixing the two units under- or over-doses by a factor of two or more.
How quickly must anti-D be given?
As soon as possible and always within 72 hours of the sensitising event. If more than 72 hours have passed it should still be given, since partial protection is better than none, and the delay documented.
Does this calculator cover routine antenatal prophylaxis?
No. Routine antenatal anti-D prophylaxis is a fixed regimen — 1,500 IU between 28 and 30 weeks, or 500 IU at 28 and 34 weeks — not a dose calculated from a fetomaternal haemorrhage. This page sizes the dose for a measured bleed only.
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References
- Qureshi H, Massey E, Kirwan D, et al. BCSH guideline for the use of anti-D immunoglobulin for the prevention of haemolytic disease of the fetus and newborn. Transfus Med. 2014;24(1):8–20.
- Cohn CS, Delaney M, Johnson ST, Katz LM, eds. Technical Manual. 20th ed. AABB; 2020.
- British Committee for Standards in Haematology. The estimation of fetomaternal haemorrhage. Transfus Med. 1999;9(1):87–92.