Evaluation of the online immunoassay for propoxyphene: comparison to EMIT II and GC-MS

A study was conducted to compare the clinical sensitivity of the OnLine and EMIT II assays for propoxyphene (PPX) use in human urine. A total of 5138 random clinical samples were evaluated by both OnLine and EMIT II. Samples that were positive for each immunoassay were confirmed for PPX and norpropoxyphene (NPPX) by gas chromatography-mass spectrometry (GC-MS). There were 14 samples that were identified positive by both immunoassays and confirmed positive by GC-MS. An additional six samples were positive by OnLine, negative by EMIT II, and confirmed positive by GC-MS. There was one unconfirmed positive sample identified by each immunoassay, and 5116 samples were identified as negative by both immunoassays. The increased sensitivity by OnLine can be attributed to the cross reactivity of the OnLine antibody, which is higher than the cross reactivity of the EMIT II antibody for NPPX (77% versus 7%, respectively). The high concentrations of NPPX, relative to those of PPX, found in all of the clinical samples suggest that laboratories that currently confirm for PPX should confirm for NPPX in order to obtain a better correlation between immunoassay results and GC-MS confirmations.