To the Editor: Zachary N. Stowe, M.D., and colleagues expressed concern in their article (1) that the manner in which we have reported serum levels might be misleading: "The reporting of ‘0 ng/ml’ in infant serum…could be misinterpreted by clinicians as suggesting a complete absence of infant exposure to medication." We welcome the opportunity to clarify the terms used to describe small amounts of drug in breast-fed infants’ sera, as we have done previously (2). Assays have a limit of quantifiability. In the report cited (1), it was 2 ng/ml, which means that the limit of the assay for reliable quantification was 2 ng/ml (not truly a limit of detection). Levels below 2 ng/ml frequently are detectable but not are reliably quantified by analytical readouts. Our report of 0 ng/ml means that no amount was detected above the baseline (within instrumental limitations). We use three categories of exposure in our developmental studies of infants (0, not detectable; <2 ng/ml, less than reliably quantifiable but detectable; and ≥2 ng/ml, a reliably quantifiable numeric amount). We have stated that the unknown neurochemical effects of even these small amounts of drug or metabolite remain a concern (3).