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Rh Immune Globulin (Anti-D) Dose Calculator — Kleihauer-Betke

Q: How much fetal blood does one vial of anti-D cover?

A standard 300 µg (1,500 IU) vial covers about 30 ml of fetal whole blood, or 15 ml of fetal red blood cells. Smaller 50 µg doses are used in some countries only for events before 12 weeks of pregnancy.

Q: Why add an extra vial?

The Kleihauer-Betke test is not very precise, and under-dosing can lead to sensitization that affects future pregnancies. Adding one vial after rounding gives a safety margin (AABB and ACOG guidance).

Q: When must Rh immune globulin be given?

Ideally within 72 hours of birth or of any sensitizing event such as bleeding, trauma, external cephalic version, amniocentesis, miscarriage or ectopic pregnancy. If missed, it may still give some protection up to 28 days.

Q: Do all Rh-negative mothers need a Kleihauer-Betke test?

After birth of an Rh-positive baby, a screening test (such as a rosette test) is usually done first. A quantitative test like Kleihauer-Betke or flow cytometry is needed only when the screen suggests a larger bleed than one vial covers.

Calculate the fetomaternal hemorrhage volume from a Kleihauer-Betke test and the number of 300 µg (1,500 IU) vials of Rh immune globulin (anti-D) an Rh-negative mother needs.

Fetal cells on Kleihauer-Betke test

Maternal blood volume

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How it works

Fetal blood (ml) = KB % ÷ 100 × maternal blood volume (5,000 ml) · vials = fetal blood ÷ 30 ml, rounded down if the decimal is < 0.5 and up if ≥ 0.5, then + 1 vial · 1 vial = 300 µg = 1,500 IU

One 300 µg (1,500 IU) dose of Rh immune globulin protects against about 30 ml of fetal whole blood (15 ml of fetal red cells) entering the circulation of an Rh(D)-negative mother. When screening (rosette test) suggests a larger bleed, the Kleihauer-Betke acid-elution test or flow cytometry measures the percentage of fetal cells, which is multiplied by the maternal blood volume. ACOG and the AABB recommend dividing by 30, rounding (down below 0.5, up from 0.5) and then adding one extra vial because the test is imprecise. Give it within 72 hours of the sensitizing event.

Worked example

A Kleihauer-Betke result of 0.3% with a maternal blood volume of 5,000 ml: fetal blood = 0.3 ÷ 100 × 5,000 = 15 ml. 15 ÷ 30 = 0.50, which rounds up to 1, plus 1 extra vial = 2 vials of 300 µg — a total of 600 µg (3,000 IU).

Sources: ACOG Practice Bulletin No. 181: Prevention of Rh D alloimmunization (2017) · Sandler SG et al. (2014) Use of Rh immune globulin, AABB guidance — Transfusion 54(12)

Frequently asked questions

How much fetal blood does one vial of anti-D cover?

A standard 300 µg (1,500 IU) vial covers about 30 ml of fetal whole blood, or 15 ml of fetal red blood cells. Smaller 50 µg doses are used in some countries only for events before 12 weeks of pregnancy.

Why add an extra vial?

The Kleihauer-Betke test is not very precise, and under-dosing can lead to sensitization that affects future pregnancies. Adding one vial after rounding gives a safety margin (AABB and ACOG guidance).

When must Rh immune globulin be given?

Ideally within 72 hours of birth or of any sensitizing event such as bleeding, trauma, external cephalic version, amniocentesis, miscarriage or ectopic pregnancy. If missed, it may still give some protection up to 28 days.

Do all Rh-negative mothers need a Kleihauer-Betke test?

After birth of an Rh-positive baby, a screening test (such as a rosette test) is usually done first. A quantitative test like Kleihauer-Betke or flow cytometry is needed only when the screen suggests a larger bleed than one vial covers.

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This tool supports, but does not replace, clinical judgment. Results must be interpreted by a qualified health professional together with the full clinical picture and local protocols.