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A Journal on Forensic Medicine
Minerva Medicolegale 2000 September;120(3): 143-50
The determination of narcotic substances. Problems of procedure and interpretation with special reference to the assay for cannabinoids in the urine
Giusiani M., Viviani P.
Background. The aim of this study was to evaluate a given number of urine samples and analyse them using two immunochemical screening methods.
Methods. We used a more reliable method to obtain a critical analysis of the validity of the ''first analysis'' methods used. Samples were taken from 165 urines and were examined using both the FPIA technique (ADx ABBOTT) and the EIA technique (random 120 Bracco). Confirmation was obtained using HPLC.
Results. When the threshold was altered from 25 to 100 ng/ml the EIA technique is more severely affected by changes in reference values compared to FPIA. The FPIA intra-technique comparison between ADx and TDx shows that ADx is confirmed in 80% of cases at all three cut-offs. TDx appears to be more variable given that only 20% of cases are confirmed at the 25 ng/ml cut-off (with 50 ng/ml and 100 ng/ml the percentages vary from 70 to 80%).
Conclusions. We can affirm that the immunochemical techniques used are reasonably valid, although they both are affected by the choice of threshold which is essential at low values to distinguish between positive and negative. In this respect, based on the evaluation made both intra-technique and between different techniques at the various thresholds examined (25, 50, 100 ng/ml), the value of 50 ng/ml appears to be the best since it gives fewer discrepancies both intra- and extra-technique.