In IgG patients specifically, persistence of small amounts of monoclonal IgG is partly the result of a prolonged half-life due to recycling by FcRn receptors, causing IFE to remain positive after the tumour has been eradicated. This is an issue previously reported by Waldmann et al.  (Chapter 3) and demonstrated in Figure 18.15. Consistent with this, discrepancies between IFE and bone marrow immunophenotypic responses can be observed during follow-up .
HLC ratios are not affected by the variable metabolism of IgG and for the detection of residual disease, they demonstrate a better agreement than sIFE with flow cytometry results (Figure 18.16) . Others have similarly found a good correlation between HLC and flow cytometry during serial monitoring  including those patients treated with novel agents .