Endocrine Abstracts

Introduction: Current guidelines recommend a single prolactin sampling for the diagnosis of hyperprolactinaemia. Nonetheless, in some patients, prolactin levels may normalize in a subsequent sampling or if prolactin is collected through a venous catheter some time after puncture. We aimed to assess the percentage of patients in which prolactin remained elevated in repeated sampling and to determine the best prolactin cutoff associated with persistent hyperprolactinaemia.

Methods: We retrospectively studied patients referred to the endocrinology clinic of the Centro Hospitalar do Tâmega e Sousa from 2006 to 2019 due to hyperprolactinaemia (referral prolactin – rPRL). We included those that underwent repeated prolactin samples by a peripheral venous catheter. The first sample (PRL0’) was collected immediately after catheter insertion, the second sample 20 – 30 min after catheter insertion and the third sample 40 – 60 min after. The lowest value of these last two samples was defined as the nadir prolactin (nPRL). Prolactin was considered normal if within the reference range for sex and menopausal status. We excluded patients treated with dopamine receptor agonists. The endpoint under analysis was persistently elevated PRL0’ and nPRL. Receiver-operating characteristic (ROC) curves were used to determine the best rPRL cutoffs to predict elevated PRL0’ and nPRL. A logistic regression analysis was used to test the association between the rPRL cutoffs and persistent hyperprolactinaemia.

Results: We studied 53 patients (three males), mean age 34 ± 3 years, and 51.0% treated with possible hyperprolactinemic drugs. Median rPRL: 48.0 ng/ml (39.5 – 72.5), PRL0’: 34.3 ng/ml (18.0 – 50.8) and nPRL: 29.5 ng/ml (11.4 – 44.4). PRL0’ was elevated in 35 (66.0%) patients and in 7 of them a normal nPRL was reached; therefore 28 (52.8%) patients presented persistent hyperprolactinaemia. The areas under de ROC curves for the association between rPRL and elevated PRL0’ and nPRL was 0.70 (95% CI : 0.56–0.85) and 0.70 (95% CI : 0.56 – 0.84), respectively. The best cutoff for elevated PRL0’ was 53.4 ng/ml: 51.4% sensitivity, 88.9% specificity, 83.8% positive predictive value (PPV) and 62.0% negative predictive value (NPV). The best cutoff was the same for elevated nPRL prediction: sensitivity = 53.6%, specificity = 80.0%, PPV = 75.0% and NPV = 60.6%. Patients with rPRL > 53.4 ng/ml had an OR of elevated PRL0’ and nPRL of 5.29 (95% IC: 1.30–21.59, P = 0.02) and 3.65 (95% CI : 1.12 –11.90, P = 0.03), respectively.

Conclusions: Approximately half of the patients referred due to hyperprolactinaemia reach normal levels in repeated sampling. Patients with prolactin > 53.4 ng/ml (2 times the upper reference level) have 80% probability of having real hyperprolactinaemia.