Investigation into the linearity of the Roche c 702 carbamazepine assay.

Background: Carbamazepine is an anticonvulsant drug which is monitored in patients due to toxic side effects. At Manchester University NHS Foundation Trust (MFT), carbamazepine is measured using Roche's Kinetic Interaction of Microparticles in Solution (KIMS) method on the c 702 platform. The assay has an upper limit of linearity of 20 mg/L. Samples with concentrations above this limit should be identified and manually diluted. However, a poor EQA return from UK NEQAS for Tox and TDM Distribution 456 has highlighted an issue with the Roche KIMS assay. Sample A of the distribution had a carbamazepine concentration of 36 mg/L but was underreported by several Roche users. This indicated that the assay was not consistently identifying high concentration samples which required a dilution.

Method: In this investigation, fresh frozen plasma was spiked with carbamazepine concentrations ranging from 15 to 40 mg/L. The spiked samples and EQA material were analysed at two clinical laboratories using the Roche KIMS assay.

Results: Samples spiked with concentrations 20-30 mg/L were not consistently identified for dilution by the analyser. This was observed at both hospital sites. Spike samples and EQA with concentrations >30 mg/L were correctly identified at both sites.

Conclusion: The manual dilution policy has been changed at MFT, so all samples with a carbamazepine level ≥15 mg/L will be manually diluted. The problem was reported to Roche who are investigating the issue further. We would suggest that other laboratories look at validating their dilution protocols.