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7.4.1. Incidence of antigen excess

Chapter 7

Several studies have evaluated the incidence of antigen excess in large numbers of consecutive patients. Murata et al. [177] studied 7,538 serum samples over a 4-month period using 1:100 and 1:400 sample dilutions on a Siemens BNII. There were nine samples with κ antigen excess but no samples with λ antigen excess giving an incidence of 1/840 (0.12%). Importantly, all the antigen excess samples had elevated FLC concentrations or abnormal κ/λ ratios at the initial dilution of 1:100 so they would not have been classified as normal. Bosmann et al. [178] studied the incidence of antigen excess in 91 patients. Samples from two patients (2.2%) exhibited antigen excess: one, a patient with λ FLC-monoclonal gammopathy of undetermined significance (Chapter 13) and the other, a κ FLC patient with a known IgAκ monoclonal gammopathy. The authors concluded that the interpretation of FLC measurements is facilitated in many cases, when combined with electrophoresis results and clinical information.

Vercammen et al. [179] studied the incidence of antigen excess in 865 patient samples using 1:100 and 1:2000 sample dilutions on a Siemens BNII. Antigen excess was defined as a greater than 4-fold difference between the results obtained at the two dilutions. This approach improves the consistency of reporting FLC values and is discussed further in Section 7.4.2 below. A total of 5.4% (44/811) and 1.2% (9/773) of κ and λ samples exhibited antigen excess respectively, which was much higher than that reported by others [177][178]. A follow on study of 3,645 samples by the same group identified sample carryover by BNII cuvettes as a cause of false antigen excess [180]. This phenomenon was reduced by batch analysis and the introduction of a cleaning and rinsing protocol, and the incidence of true antigen excess was recalculated as 0.25% and 0.03% for κ and λ sFLCs, respectively. Vercammen et al. [180] also reported that true antigen excess was not observed in samples with normal κ and λ sFLC concentrations.