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Relative Infant Dose (RID)

The Relative Infant Dose (RID) estimates infant drug exposure via breast milk. The RID uses a known milk concentration and compares it to either an infant therapeutic dose or the weight-adjusted maternal dose when an infant dose is not well established.
In general, breastfeeding is considered acceptable when the relative infant dose is <10%. Additional considerations would include the gestational and postnatal age of the infant, the actual amount of milk being ingested (less in the first couple days of life and when weaning), properties of the specific maternal medication, medical conditions of the infant, and medications the infant is receiving therapeutically.
When calculating the RID, the milk concentration is used to estimate the amount of drug ingested by a nursing infant. Note: Dosing units [eg; mcg, units, grams] may be changed as appropriate in the following equations:
Estimated daily infant dose via breast milk (mg/kg/day) = drug concentration in breast milk (mg/mL) x volume breast milk ingested (mL/kg/day)a
aWhen the actual volume of milk ingested by the infant is not known, 150 mL/kg/day is used in the calculation (the amount assumed to be ingested by an exclusively breastfed infant)
This daily dose received via breast milk is then compared to the dose used therapeutically for an infant of the same age:
Relative infant dose (%) = estimated daily infant dose via breast milk (mg/kg/day) / infant therapeutic dose (mg/kg/day) x 100
When the medication is not used in infants or does not have an accepted infant dosage, a weight-adjusted maternal dose is used:
Relative infant dose (%) = estimated daily infant dose via breast milk (mg/kg/day) / maternal dose (mg/kgb/day) x 100
bNote: Calculation uses maternal dose per kg, not fixed dose; if maternal weight is not known, 70 kg is being used in the calculations.

References

Anderson PO, Sauberan JB. Modeling drug passage into human milk. Clin Pharmacol Ther. 2016;100(1):42-52.[PubMed 27060684]

Bennett PN. Use of the monographs on drugs. Drugs and Human Lactation. Bennett PN. Elsevier, Amsterdam; 1996;67-74.

Ito S. Drug therapy for breast-feeding women. N Engl J Med. 2000;343(2):118-126.[PubMed 10891521]

Lexi-Comp, Inc. (Pediatric & Neonatal Lexi-Drugs®). Lexi-Comp, Inc.; January 25, 2018.