Which lab test should be ordered for a patient on risperidone who has amenorrhea?

For a patient on risperidone who is experiencing amenorrhea, measuring the prolactin level is the appropriate laboratory test to order. Risperidone, an antipsychotic, is known to increase prolactin levels due to its dopamine antagonist properties, particularly at D2 dopamine receptors. Elevated prolactin, or hyperprolactinemia, can lead to a variety of side effects, including menstrual irregularities such as amenorrhea.

By assessing the prolactin level, clinicians can determine if the amenorrhea is potentially linked to hyperprolactinemia induced by risperidone. This is an important assessment, as managing the elevated prolactin levels can help resolve the amenorrhea and prevent additional complications such as infertility or bone density loss due to prolonged hyperprolactinemia.

The other lab tests listed would not provide relevant or direct information regarding the cause of the amenorrhea in this context. For instance, a lumbar puncture may be indicated for certain neurological conditions but does not address the issue of amenorrhea or the effects of risperidone. Similarly, checking a risperidone level may be helpful in cases of toxicity but is not useful for understanding menstru