Simple SummaryMedical castration is the most frequent form of androgen deprivation therapy given as primary treatment for hormone-sensitive metastatic prostate cancer (PCa) and as neo-adjuvant treatment in radiation therapy for intermediate and high-risk localised and locally advanced PCa. PCa guidelines recommend determining serum testosterone levels during castration to assess its efficacy and define castration resistance. Some studies suggest other biochemical serum compounds may assess the effectiveness of castration, such as free testosterone and luteinising hormone (LH). This systematic review aims to analyse the current evidence of biochemical candidates to properly assess medical castration efficacy. We found low-quality evidence supporting the current method to measure serum testosterone during medical castration. All reported studies used immunoassays to quantify the serum testosterone levels of men undergoing medical castration instead of the appropriate liquid chromatography with tandem mass spectrometry (LC-MSMS) to measure low testosterone concentrations. Longitudinal studies are needed to analyse the prognostic value of the serum testosterone measured with LC-MSMS and measure serum-free testosterone or LH.AbstractMeasuring serum testosterone determination during medical castration is recommended by prostate cancer (PCa) guidelines to assess its efficacy and define castration resistance. It has been suggested that other biochemical compounds, such as free testosterone or luteinising hormone (LH), could also assess castration efficacy. We aimed to analyse the current evidence for serum biochemical compounds that could be appropriate candidates for evaluating medical castration efficacy. A systematic review was conducted after two investigators independently searched the literature in the PubMed, Cochrane Library, and EMBASE databases published between January 1980 and February 2023. Their searches used the medical subject headings ‘prostatic neoplasms’, ‘testosterone and androgen antagonists’, ‘gonadotropin-releasing hormone/analogues and derivatives’, ‘free testosterone’, and ‘luteinising hormone’. Studies were selected according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses criteria, and their eligibility was based on the Participants, Intervention, Comparator, and Outcome strategy. The search was limited to original articles published in English. Among the 6599 initially identified titles, 15 original studies analysing the clinical impact of serum testosterone levels in PCa patients undergoing androgen deprivation therapy (ADT) were selected for evidence acquisition. The risk of bias in individual studies was assessed using the Quality Assessment of Diagnostic Accuracy Studies 2 tool. All selected studies used immunoassays to measure serum testosterone, although only methods based on liquid or gas chromatography and mass spectrometry are recommended to measure low testosterone concentrations. The reported series were not uniform in clinical stage, ADT types, and the time or number of serum testosterone measurements. Only some studies found low serum testosterone levels (<20 or <32 ng/dL) associated with greater survival free of biochemical progression and castration resistance. We conclude that little current evidence justifies the measurement of serum testosterone during ADT using no appropriate methods. No reported longitudinal studies have examined the clinical impact of serum testosterone measured using liquid chromatography with tandem mass spectrometry (LC-MSMS), free testosterone, or LH in PCa patients undergoing medical castration. We conclude that well-designed longitudinal studies examining the clinical impact of serum testosterone measured with LC-MSMS, serum-free testosterone, and LH on biochemical progression and castration resistance in PCa patients undergoing neo-adjuvant castration in radiation therapy or continuous castration are needed.