This assay was originally developed for the World Health Organization. It has been widely used in clinical trials and pharmacokinetic studies of contraceptive steroid preparations and has also been used by major pharmaceutical companies for studies of the effects of other drugs on the pharmacokinetics of steroid contraceptives. The method is an extraction RIA and not subject to the well-documented interference problems associated with direct serum/plasma immunoassays.

Levonorgestrel in the sample (200µL of serum or plasma) is extracted with diethyl ether. The extract is dried and redissolved in assay buffer. The redissolved extract is assayed for Levonorgestrel by 3H-RIA (tritium labelled radioimmunoassay with charcoal separation). All samples, standards and quality control specimens are assayed in duplicate.

The assay is controlled through the use of at least 3 Internal Quality Control specimens in every run. In addition the performance of every run is closely monitored for various parameters including imprecision, antibody binding of tracer, non-specific binding and reproducibility of the standard curve.

• Standard Curve. The range of the standard curve is 0 to 7500 pmol/L (2344 pg/mL).
• Precision. The between-run precision of the assay is generally less than 10% CV.
• Sensitivity. The assay reliably measures Levonorgestrel concentrations as low as 150 pmol/L (47 pg/mL).

The cross-reactions of the assay (estimated as the relative potency causing a 30% displacement of zero binding) for each substance tested are shown below:

Substance % Cross Reaction Levonorgestrel 100 5a-dihydrolevonorgestrel 26 3b,5a-tetrahydrolevonorgestrel 12 3a,5b-tetrahydrolevonorgestrel 0.6

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