In keeping with the results of the literature search, no apparent relationship was observed between domperidone dose and prolactin level. The domperidone doses that the patients were receiving when their prolactin was elevated were all below or within the recommended dosage range of 1.2—2.4 mg/kg per day.
Only 14% of the elevated prolactin values led to a decrease in domperidone dose or discontinuation of therapy. The inconsistency in therapeutic adjustment for patients with elevated prolactin suggests inconsistency in interpretation of these data or other unspecified factors affecting such decisions.
Except for the case of prolactinoma described above, there were no other reports of hyperprolactinemia symptoms directly attributed to domperidone therapy. In this study population, the highest prolactin level recorded was 131.9 Hg/L, with no adverse effects attributed to hyperprolactinemia. The patient was a 13-year-old female who had not reached menarche and who was receiving a subtherapeutic dose of domperidone at the time. Although the patient was asymptomatic, the domperi- done was discontinued as a result of this prolactin result. Symptoms of hyperprolactinemia were not recorded for any of the other 48 patients, but because this was not a prospective trial designed to elucidate such symptoms, their absence cannot be stated with certainty.
This chart review revealed much variability in the timing of prolactin measurement, both within and between patients. In addition to the known diurnal fluctuation in prolactin release, prolactin levels may be altered by factors such as stress, eating, and exercise. As such, it is important to consider these factors when measuring and interpreting prolactin levels. In addition, reference prolactin levels at the authors’ institution were derived primarily from adult values. Given the great variability in pediatric reference ranges in the literature, it would be useful to define pediatric reference ranges for specific institutions to help in interpreting measured values.
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The findings reported here, as well as the apparent lack of a relationship between domperidone dose and prolactin level, suggest that it may be inappropriate to alter domperidone therapy on the basis of elevated prolactin level alone. For therapeutic drug monitoring of aminoglycosides, for example, a patient’s serum drug levels can be monitored and the dose adjusted according to pharmacokinetic parameters estimated for that particular patient. In contrast, it is not clear how a domperidone dosage adjustment will affect a patient’s prolactin level or if elevated prolactin will result in symptoms of hyperprolactinemia. Further research is needed to determine if there is a dose—response relationship between domperidone and prolactin levels and if there is a relationship between prolactin levels and symptoms of hyperprolactinemia. The question of whether high prolactin levels indicate toxicity needs further exploration through prospective trials and specific objective measures of potential toxic effects. Until such questions are answered, the measurement of prolactin levels will remain of limited use in adjusting domperidone dosage.
The possible use of prolactin levels as an indicator of patient compliance deserves further investigation. The observation of low prolactin levels at the start of a hospital stay associated with higher levels at the end of the hospital stay is consistent with the suggestion that the initial levels reflect poor compliance. However, more data are needed regarding the variability of serum levels of prolactin and the factors contributing to this variation.
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This study had some limitations. First, it was a retrospective study in a small sample of patients. Data were based on a review of medical records, which might not have fully captured the decision-making process related to interpreting prolactin values. Second, this review was not designed to determine whether a dose—response relationship exists between domperidone and prolactin level. The literature evidence for such a relationship is mixed. Third, patients’ compliance with their domperidone regimen outside of the hospital stay was unknown. Finally, baseline prolactin was not determined before the start of domperidone therapy, and samples for prolactin monitoring were drawn at different times of the day, which may have contributed to the observed fluctuation of levels.
This study has brought to light the difficulties in interpreting prolactin values and the need for more information before the true role of serum prolactin in monitoring domperidone therapy and evaluating patients’ compliance can be determined.