The milk/plasma (M/P) ratio for drugs has been measured and reported for a number of medications. The milk plasma ratio is the concentration of the drug in the milk at the same moment that the concentration in maternal plasma is measured. It assumes that the relationship that between the two remains constant, but in most cases it does not. Therefore, the M/P ratio is often calculatcd from the average concentrations in the blood and milk over a longer period of several hours. These average concentrations are the area under the respective concentration curves (area under curve, or AUC), which are constructed from individual concentrations reported over the course of the time interval. The latter method is preferred in the newer studies because the M/P values they establish are more representative. Nevertheless, to some extent there are considerable variations in the M/P ratios calculated, not only between different studies and subjects, but also with the same mother; the colostrum has different concentrations than the milk some weeks later, and the first milk of a breastfeed is different from a sample taken later in the same feed. Thus, the M/P ratios citcd in the following chapters should be viewed only as approximate values. They represent the mean values of present experience, and are useful only for a rough comparison with other medications.
The M/P ratio is not suitable for comparison of drug risks. A milk/plasma ratio of 1 assumes that the levels are the same in both plasma and milk. If, however, the level is very low in the plasma, it will also be low in the milk, even though the milk plasma ratio is 1. Low M/P ratios (<1) indicate that there is no accumulation in the mother's milk. However, significant concentrations in the milk can be reached even with low M/P ratios when there is a high maternal plasma value. On the other hand, relevant or even toxic amounts of medication cannot necessarily be assumed from a high M/P ratio with those drugs where the concentration in the maternal serum is very limited because of a high distribution volume typical for the particular medication. In such a case, even an M/P ratio of 8, which indicates a relative accumulation in the milk compared to the maternal plasma, means only a limited concentration of the mcdication in the milk, and consequently only a limited relative dosage (see above).
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