While there are many factors that influence drug transfer to an infant in lactation, some calculations can help you measure transfer of a particular drug you are considering. The below are some of the most commonly used.
Relative Infant Dose (RID)
This is a particularly useful value as it provides a standardised way to relate the dose of drug the infant is exposed to via breastmilk to the dose administered to the mother. Multiple sources suggest that for a healthy, term breastfeeding infant drugs with an RID of <10% can be considered low risk in general. However it is important to consider all drug, maternal and infant factors for each particular breastfeeding dyad when considering using medications in lactation. More information on these considerations can be found here.
RID = Absolute Infant Dose (mg/kg/day) / maternal dose (mg/kg/day)
Multiply your answer by 100 to express the RID as a percentage.
Absolute Infant Dose (AID)
The AID is a measure of the amount of drug an infant is exposed to via breastmilk per day. It takes into account the volume of breastmilk consumed by the infant per day and is expressed in mg/kg/day.
It can be compared to the therapeutic dose of that drug when administered to that particular infant. A drug with an AID of <10% of the therapeutic dose of a drug is considered to be low risk for adverse events related to drug transfer in lactation but as above, with all medications all factors must be taken into account for the individual circumstances of each dyad.
The AID is also used in calculating the RID.
AID = Concentration of drug in milk x Volume of milk consumed per day
Divide the answer by the weight of the infant to obtain the result per kg
Ensure the unit of volume used for both values is consistent, i.e. if Cmilk is expressed as mg/L then volume of milk should be expressed as L also. You may use either of the average concentration of drug in milk or the maximum concentration for this calculation.
Medications and Mother’s Milk; a manual of lactational pharmacology 2012,
fifteenth edition, (author: Thomas W Hale ), Hale Publishing, LP, ISBN: 978-
Rowe H et al, Maternal medication, drug use, and breastfeeding, Pediatr ClinN Am 2013;60:275-294