7.5.3. Incidence of antigen excess

Chapter 7

7.5.3. Incidence of antigen excess

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 7.5.2. 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.

Investigations have also been carried out into the incidence of antigen excess on Binding Site analysers with automated antigen excess checks. Bossuyt et al. [1046] evaluated automatic antigen excess detection by the Freelite Optilite assays. A total of 730 samples (n=525 MGUS and n=205 MM) were measured at two dilutions: the standard dilution and a 10-fold higher dilution. Initially, 3 κ and 5 λ samples were in antigen excess, but as a result of this study, the antigen excess detection protocol was optimised, and an improvement in antigen excess detection was subsequently reported [1046]. These improved parameters have been applied to all subsequent kit lots. The authors also stated that a number of samples were in antigen excess by the N Latex FLC assays, which is further discussed in Section 8.4.4. Hitchcock et al. [1086] studied the incidence of antigen excess flags generated against 1452 serum samples analysed on the SPAPLUS. Flags were generated against 1.1% (16/1451) samples, and the authors concluded that the incidence of antigen excess was low. Coley et al. [1091] reviewed the incidence of antigen excess flags on the Optilite for over 10,000 sample results. The proportion of samples that demonstrated antigen excess flags on the κ and λ Freelite assays were 11% and 6%, respectively, and these were correctly identified in at least 98% of cases. A separate study by Walker et al. [1092] investigated the Optilite performance across over 18,000 κ and λ measurements and reported a very similar incidence of antigen excess flags, and antigen excess was confirmed in 95 and 99% of κ and λ cases, respectively. These authors observed only one instance of undetected antigen excess during the 1 year study period. These results confirm that the antigen excess detection for Freelite Optilite assays is robust and that undetected antigen excess is a rare event, but laboratories should remain vigilant if other laboratory tests results or clinical findings are inconsistent with Freelite results.