Individual variations of serum testosterone in patients with prostate cancer receiving androgen deprivation therapy

To analyse individual variations in serum testosterone level, the cumulative rate of 'breakthrough' increases over castrate levels, and to evaluate whether the increases can be predicted. Serum testosterone levels were determined every 6 months over 3 years in 73 consecutive patients with prostate cancer who were medically castrated, prospectively enrolled in a single tertiary academic centre. Patients recruited for this study were being treated with a 3-monthly depot of luteinizing hormone-releasing hormone agonist over 6-48 months. Serum testosterone was measured using a chemiluminescent assay with a lower sensitivity level of 15 ng/dL and interassay coefficient of variation of 25% at low testosterone concentrations. Individual variations could not be explained by the interassay variation coefficient in 26% of the patients. The rate of breakthrough increases > 50 ng/dL increased from 12.3% at the first determination to 24.7% at the third, then remaining stable. The rate of breakthrough increases of 20-50 ng/dL increased from 27.4% at the first determination to 31.5% at the second, and then remained stable. A first determination of < 20 ng/dL provided an 11.4% probability for future increases of > 50 ng/dL, with a 5.7% probability if two consecutive determinations were < 20 ng/dL and a null probability when three consecutive determinations were < 20 ng/dL. Individual variations in serum testosterone level cannot be explained by the coefficient of variation of the assay in a quarter of patients who are medically castrated. Breakthrough increases over castrate levels increase over time and those of > 50 ng/dL can be predicted from the previous levels.