Modest sFLC ratio elevations (but no monoclonal FLC) are not exclusive to patients with renal impairment but are also observed in patients with polyclonal inflammatory and autoimmune conditions (Section 6.2 and Chapter 35), suggesting a possible increased bias towards κ sFLC production in these instances [163][164][161] Indeed, application of the renal range has been found to improve specificity when screening general populations [129][164]. Whilst borderline sFLC results can be problematic, it should be remembered that urine electrophoresis can produce greater numbers of equivocal results [510]. Analysis of a screening population by McTaggart and colleagues [511] led them to conclude that addition of UPE into their screening panel decreased the specificity through the identification of more “false-positive” results.

Apart from application of the renal reference range, other authors have similarly explored the influence on specificity/positive predictive value (PPV) of categorising patients into groups with different degrees of ratio abnormality. Hill et al. [163] found that κ/λ sFLC ratios of >3 or >5 were successively more predictive of finding a monoclonal gammopathy (by sIFE) or diagnosing a B-cell disorder. Vermeersch and colleagues [170] proposed the use of different sFLC ratio intervals to give likelihood ratios as an aid for interpreting screening results. In the screening population studied by Piehler et al. [323] sFLC ratios of <0.05 or >10 were found to be 100% specific for haematological disorders. However, as with any diagnostic test, sFLC results should always be interpreted in the context of the patient’s symptoms and other test results (Chapter 7).