From the Guidelines

Precautions for a prolactin laboratory test are crucial to ensure accurate results and diagnosis.

Stress and prolactin pulsatility: Consider investigating modestly elevated serum prolactin levels by serial measurements over time to exclude the effect of stress and prolactin pulsatility 1.
Age-specific and sex-specific reference ranges: The diagnosis of hyperprolactinaemia requires age-specific and sex-specific prolactin reference ranges and the exclusion of confounding conditions such as hypothyroidism, renal and/or hepatic impairment, and use of medications that cause hyperprolactinaemia 1.
High-dose hook effect: Perform serial dilutions of serum for prolactin measurement in patients with large pituitary lesions and normal or mildly elevated prolactin levels to avoid the high-dose hook effect 1.
Confounding conditions: Exclude confounding diseases such as hypothyroidism, renal and/or hepatic impairment, and use of medications that cause hyperprolactinaemia 1.
Macroprolactin levels: Assess baseline macroprolactin levels where serum prolactin is found to be mildly or incidentally elevated to detect the possible presence of macroprolactin 1.

Prolactin secretion rises in response to stress, and sampling can be repeated on a different day with two or three samples at 20–60 min intervals to differentiate stress-related hyperprolactinaemia from organic disease 1.
Severe primary hypothyroidism can be accompanied by hyperprolactinaemia, and care should be taken to distinguish such pituitary enlargement from a true prolactinoma 1.
Medications are one of the most common causes of hyperprolactinaemia in adults and children, and medication-induced hyperprolactinaemia should be considered in the diagnosis 1.

From the Research

To ensure accurate results from a prolactin laboratory test, several precautions should be taken:

Blood should be drawn in a fasting state, preferably in the morning, to minimize the effects of dietary habits and stimulants on prolactin levels 2, 3.
Sampling should be done after a period of rest to avoid the impact of stress, which can amplify prolactin secretion 2, 4.
It is essential to consider the pulsatile secretion profile of prolactin and its diurnal cycle, with maximum levels typically occurring during sleep 2.
The presence of macroprolactin, a polymeric form of prolactin with diminished receptor-binding specificity, should be taken into account, as it can lead to elevated prolactin levels without corresponding clinical symptoms 2, 5.
Serial prolactin sampling may be useful to confirm true hyperprolactinemia and avoid unnecessary additional tests and treatments 4.
Laboratory findings should always be interpreted cautiously, considering the clinical context and potential sources of error during sampling and assessment 2, 3.
Reference intervals for serum prolactin concentrations should be established by each laboratory, taking into account the variability between assay manufacturers and the physiological causes of variation in prolactin levels, such as age, sex, and menopause 5.