We read with interest the article by Onwuchuruba et al on the transplacental passage of vancomycin and its implications on vancomycin dosing for group B streptococcus (GBS) prophylaxis. Results from the study suggested that the standard dosage of 1 g intravenously every 12 hours, as recommended by guidelines from the Centers for Disease Control and Prevention, may achieve only subtherapeutic levels while higher weight-based dosages are more likely to attain levels in the therapeutic range. However, we contend that the results are over interpreted because of the definition of “therapeutic levels” that was based on an outdated understanding of vancomycin pharmacodynamics.

Current guidelines on vancomycin use the ratio of area under the concentration-time curve to the minimum inhibitory concentration (AUC/MIC) of the target organism as the pharmacodynamic index for dosage guidance. This reflects the latest expert consensus that vancomycin does not exhibit simple time-dependent killing. Unfortunately, the suggested “therapeutic levels” in the study are derived from studies and guidelines before the adoption of the AUC/MIC concept.

In adults, vancomycin trough level is correlated with the AUC and indeed may be used as a surrogate marker in therapeutic drug monitoring. However, this correlation is based on adult data and may not hold for exposure to the drug in utero. Moreover, commonly cited therapeutic targets for vancomycin, whether AUC/MIC ratio or trough level, were derived mainly from studies on treatment of methicillin-resistant Staphylococcus aureus infections in adults. It is uncertain whether they are applicable in the setting of neonatal GBS prophylaxis.

Although we concur with the need for a better and more evidence-based dosage recommendation, much more work must be done before one can be made. The present results are insufficient to refute the utility of standard vancomycin dosage or suggest superiority of higher dosages in GBS prophylaxis. Unnecessarily high doses will lead to more infusion-related reactions and potentially higher risks of drug toxicity without any definite benefit to neonatal outcome.