Practical measurement of total and bioavailable estradiol in female macaques.

BACKGROUND: Commercially available, direct assay kits provide a simple and rapid means for measuring estradiol. Similarly, ammonium sulfate precipitation of SHBG-bound steroids is a reliable alternative to equilibrium dialysis or ultrafiltration for assessing bioavailability. However, while these techniques are useful for humans, they have yet to be evaluated systematically for macaques-species which are frequently used to model estrogen effects on women's health.
METHODS: A reference assay (which included chromatography) and two modified versions of a human kit (one in which monkey serum was assayed directly, and another in which an extraction step was added) were used to measure estradiol in matching samples. Ammonium sulfate precipitation and an established ultrafiltration technique were used to assess bioavailability.
RESULTS: Values from both kit modifications correlated significantly with those from the reference assay. Although both modifications underestimated values, the addition of the extraction step resulted in far more useful estimates due to the consistency of underestimation across the standard curve. Measures of bioavailability from ammonium sulfate precipitation and ultrafiltration were strongly correlated and consistent across all concentrations.
CONCLUSIONS: Simplified techniques developed for humans can be used in macaques, although the addition of an extraction step markedly improves the performance of direct assay kits.