Comparative determinations of non SHBG-bound serum testosterone, using ammonium sulfate precipitation, Concanavalin A binding or calculation in men

BackgroundTestosterone (T) circulates tightly bound to sex hormone-binding globulin (SHBG) and weakly to albumin. Non-SHBG-bound T is considered as the bioavailable T (BT) and was recommended for the evaluation of androgen disorders. Two methods, BT calculating from T, SHBG and albumin or BT measurement using ammonium sulfate precipitation of [SHBG-T] complex have been widely used. Using SHBG separation with Concanavalin-A (ConA) was recently proposed as a more specific method. The aim of this work was to compare these three methods in male patients.MethodsSerum samples of 131 consecutive untreated men (15–81 years) referred for suspicion of hypogonadism were collected. Total T was measured by GC–MS, SHBG by immunoradiometric assay. Level of BT was assayed using ammonium sulfate precipitation, ConA separation and calculated using the Issam web calculator.ResultsOnly few differences were found between ammonium sulfate or ConA BT measurements. However, we found much higher serum calculated BT than assayed BT with an increasing bias when BT levels increased.ConclusionsMeasurement of BT using ConA separation could be recommended. The results are equivalent to those obtained using ammonium sulfate precipitation. This method eliminates the possible non specific albumin precipitation that can occur with ammonium sulfate when the assay conditions are not rigorously controlled.

► We confirm the reliability of the bioavailable testosterone (BT) assay using ammonium sulfate precipitation.
► The recently proposed method for BT assay based on Concanavalin A binding shows similar results.
► Calculated BT is higher than the two preceding assay methods, with increasing bias as BT increases.