NOTE: While the timeliness of this exact model may be limited to a narrow window because of changes in instruments or assays, the concept of different levels of cross-reactivity in 2 different assays can be applied to other drugs and assays.
Carbamazepine 10,11-epoxide is an active metabolite of barbamazepine. It cross-reacts with the parent compound in immunoassays.
Assays:
(1) fluorescent polarization immunoassay (FPIA) on the AxSYM
(2) turbidometric assay on the ADVIA 1650 analyzer
Specimen: A single serum sample is tested using both assays and the result compared.
|
Assay
|
Percent Cross-Reactivity of the Epoxide
|
|
FPIA
|
32%
|
|
turbidometric
|
16%
|
Two equations were developed:
concentration by the turbidometric assay in µg/mL =
= (concentration of carbamazepine in µg/mL) + (0.16 * (concentration of epoxide in µg/mL)
concentration by the FPIA assay in µg/mL =
= (concentration of carbamazepine in µg/mL) + (0.32 * (concentration of epoxide in µg/mL)
Solving both equations simultaneously:
concentration of the epoxide in µg/mL =
= 6.25 * ((concentration by the FPIA assay) - (concentration by the turbidometric assay))
concentration of the carbamazepine in µg/mL =
= (2 * (concentration by the turbidometric assay)) - (concentration by the FPIA assay)
Performance:
• The authors did not give a complete analysis of the performance. In the example given the estimated carbamazepine concentration was 3% below the actual level. The estimated epoxide was slightly higher than the actual level.
