### Description

Since one 300 µg dose of RhIG will protect against a transplacental hemorrhage of 30 mL of Rh-positive fetal blood, the volume of fetal blood should be divided by 30 to determine the number of doses required. Calculating the volume of fetal blood in the maternal circulation is done by the Kleihauer-Betke procedure.

doses of Rhogam required =

= ((estimated volume of fetal blood in mL) / 30)

Because this test in imprecise and because the risk of overdosing is less serious than underdosing, the following rules are used:

1) If the number to the right of the decimal point is less than 5, then administer a number of doses equal to the next whole number that is higher.

2) If the number to the right of the decimal point is 5 or greater, then administer a number of doses equal to the next whole number plus 1.

Some recommend using 25 rather than 30 for the denominator, in order to compensate for the poor precision of the Kleihauer-Betke test. Using 25 results in a slightly higher dose of Rhogam.

Table of Rhogam Doses in Massive Fetomaternal Hemorrhage

A more conservative dosage schedule for Rhogam than that calculated from an estimate of fetal hemorrhage volume is as follows. This table embodies both a wider range of enumeration error and a wider margin for inconsistency in absorption or efficacy.

percent fetal cells in Kleihauer-Betke range in volume of fetal maternal hemorrhage average volume of fetal maternal hemorrhage vials of RhIG to inject
0.3 - 0.5 < 50 mL 20 mL 2
0.6 - 0.8 15 - 80 mL 35 mL 3
0.9 - 1.1 22 - 110 mL 50 mL 4
1.2 - 1.4 30 - 140 mL 65 mL 5
1.5 - 2.0 37 - 200 mL 88 mL 6
2.0 - 2.5 52 - 250 mL 115 mL 6

If the percent fetal cells is < 0.3%, then a single dose of RhIG should be sufficient.