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7.5.1. Instruments with automated antigen excess checking

Chapter 7

Freelite sFLC assays available on some instruments include prozone parameters (Section 37.2.3). For example, the Binding Site SPAPLUS instrument monitors the initial reaction kinetics of each sample at three separate time intervals (Figure 7.6.) and compares the results with defined reaction limits established by testing of an extensive library of myeloma and normal samples (Section 37.3.3). Samples detected as being in antigen excess are automatically flagged by the instrument and retested at a higher sample dilution. The Binding Site Optilite has three different methods of antigen excess detection, including; control addition, sample addition and review of the reaction kinetics in a similar method to the SPAPLUS. Whilst the incidence of Freelite antigen excess on the SPAPLUS and Optilite is very low (see Section 7.5.3), some samples may generate such unusual kinetics that the reaction does not prompt a flag, and may result in undetected antigen excess. Such undetected antigen excess is a rare event but cannot be excluded. Therefore, it is recommended that the following statement accompanies all FLC results “Undetected antigen excess is a rare event but cannot be excluded. If these free light chain results do not agree with other clinical or laboratory findings, or if the sample is from a patient that has previously demonstrated antigen excess, the result must be checked by retesting at a higher dilution”.

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