Discordant carbamazepine values between two immunoassays: carbamazepine values determined by ADVIA Centaur correlate better with those determined by LC-MS/MS than PETINIA assay

Discordant carbamazepine values as determined by two different immunoassays may be due to different cross-reactivities with the active metabolite carbamazepine 10, 11-epoxide and may cause confusion in interpreting carbamazepine serum levels. In this study, we compared carbamazepine values in samples containing carbamazepine and the epoxide metabolite, as determined by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) and by two commercial carbamazepine immunoassays: the PETINIA and the ADVIA Centaur carbamazepine. Clinical specimens were used for the comparative studies wherein we determined carbamazepine concentrations using the PETINIA, ADVIA Centaur, and LC-MS/MS assays. We observed an excellent correlation between carbamazepine concentrations determined by the ADVIA Centaur and LC-MS/MS methods while carbamazepine values were overestimated using the PETINIA assay. When aliquots of drug-free serum were supplemented with clinically relevant concentrations of the carbamazepine epoxide metabolite, we observed negligible cross-reactivity of epoxide with the ADVIA Centaur assay but over 90% cross-reactivity with the PETINIA assay. We conclude that the ADVIA centaur assay accurately measures carbamazepine concentrations in plasma or serum and that the PETINIA assay significantly overestimates true carbamazepine concentration. Such discordance may cause confusion in interpreting serum carbamazepine levels.     

Effect of assay specificity on the association of urine 11‐dehydro thromboxane B2 determination with cardiovascular risk

Summary. Background: Elevated urine 11‐dehydro TXB₂, an indicator of persistent thromboxane generation in aspirin‐treated patients, correlates with adverse cardiovascular outcome and has recently been identified as an independent risk factor for vein graft thrombosis after cardiac bypass surgery in the Reduction in Graft Occlusion Rates (RIGOR) study. The polyclonal antibody‐based ELISA used to measure 11‐dehydro TXB₂ in these previous studies is no longer clinically available and has been supplanted by a Food and Drug Administration (FDA)‐cleared second‐generation monoclonal antibody‐based ELISA. Objectives: To compare the laboratory and clinical performance of the first‐ and second‐generation assays in a well‐defined study population. Methods: 11‐dehydro TXB₂ was quantified in 451 urine samples from 229 Reduction in Graft Occlusion Rates (RIGOR) subjects using both ELISA. Ultra‐performance liquid chromatography‐tandem mass spectrometry (UPLC‐MS/MS) and spiking studies were used to investigate discordant assay results. The association of 11‐dehydro TXB₂ to clinical outcome was assessed for each assay using multivariate modeling. Results: Median 11‐dehydro TXB₂ levels were higher by monoclonal antibody‐ compared with polyclonal antibody‐based ELISA (856 vs. 399 pg mg^?1 creatinine, P < 0.000001), with the latter providing values similar to UPLC‐MS/MS. This discrepancy was predominantly as a result of cross‐reactivity of the monoclonal antibody with 11‐dehydro‐2,3‐dinor TXB₂, a thromboxane metabolite present in a similar concentration but with a poor direct correlation with 11‐dehydro TXB₂. In contrast to the first‐generation ELISA, 11‐dehydro TXB₂ measured by the monoclonal antibody‐based ELISA failed to associate with the risk of vein graft occlusion. Conclusion: Quantification of urine 11‐dehydro TXB₂ by monoclonal antibody‐based ELISA was confounded by interference from 11‐dehydro‐2,3‐dinor TXB₂ which reduced the accuracy and clinical utility of this second‐generation assay.