7.5.2. Instruments without automated antigen excess checking

Chapter 7

Freelite sFLC assays are also available on some other instruments, such as the Siemens BNII, which do not include automatic antigen excess checks. On these instruments, samples should be tested for antigen excess following advice given in the product insert. This involves retesting the sample at a higher dilution (known as the antigen excess check dilution). These protocols minimise reagent usage and ensure consistency of sample measurement. Whilst these protocols will ensure that the vast majority of samples in antigen excess are detected, Rassner et al. [1158] reported a rare sample (~1/10,500 samples) with monoclonal κ sFLCs that was still in antigen excess when tested at a higher dilution. However, a number of other findings were present that were suggestive of MM disease (λ sFLCs were undetectable, the κ/λ sFLC ratio was highly abnormal, κ urinary Bence Jones protein was detected, and renal failure and osteolytic lesions were present). The authors conclude that this case highlights that “clinical decisions should not be based on a single assay…”.

Binding Site recommends that when the result obtained at the antigen excess check dilution is more than 4-fold greater than the result obtained at the initial sample dilution, the sample should be considered to be in antigen excess and the result obtained at the higher dilution should be reported. On the other hand, if the result from the antigen excess check dilution is less than 4-fold greater than that from the initial dilution, the sample should be considered non-linear and the value at the initial dilution should be reported. Examples of the use of this guidance to distinguish between antigen excess and non-linearity are shown in Table 7.7.
Dilution Result
Sample 11/100 95.3
1/2000 202
Dilution results ratio* 2.1
Sample 21/100145
Dilution results ratio*3.2
Sample 31/10078.1
Dilution results ratio*55
Antigen excess

Table 7.7. Examples of κ FLC non-linearity and antigen excess on a Siemens BNII. *Results from antigen excess check dilution (1/2000) divided by those from the initial dilution (1/100). The results in bold were reported.