Serum free light chain tests for the diagnosis and monitoring of MM

The detection of urine free light chains (FLCs) has been used to diagnose Multiple Myeloma (MM) since 1847. However, current guidelines request measurement of whole immunoglobulin and FLCs in both blood and urine. Studies have shown that measurement of serum FLC is more sensitive compared to urine. Hence sFLC testing has been integrated into the International Myeloma Working Group (IMWG) guidelines for the diagnosis and management of all plasma cell dyscrasias. Two assays are predominantly used in practice: Freelite and Seralite. Both assays measure a κ:λ ratio, which is a sensitive diagnostic marker for MM.

A comparison of these two assays in clinical use was presented by Jennifer Heaney from the Institute of Immunology and Immunotherapy, University of Birmingham, UK. Data from this study was published in the British Journal of Haematology in March 2017.

Table 1. A comparison of the Freelite and Seralite assays

Key points:

• Diagnosis of light chain only (LCO) and oligosecretory (OS) myeloma using sFLC (serum FLC) tests (Freelite vs Seralite) at disease presentation:
  • N = 325 patients (pts)
  • Abnormal ratios for oligosecretory (OS) pts: 72 patients (pts) vs 69/72 pts
  • Abnormal k: λ ratio for all light chain only (LCO, n = 253); 100% vs 100% in both assays (complete concordance)
  • Good clinical concordance for the diagnosis of myeloma in pts without an intact M-protein in both assays 
  • Seralite levels were significantly lower compared to Freelite (P < 0.001)
  • Freelite involved FLC (iFLC) concentrations were at least 5 times higher than results generated by Seralite although the ratio was the same from both assays 
  • Receiver operating characteristic (ROC curve) analysis showed that the Seralite dFLC (difference in concentration between iFLC and uFLC) could accurately identify pts with >100 mg/l on Freelite, AUC = 0.85 (95% CI, 0.63–0.1; P < 0.05)
• When monitoring individual patients with a monoclonal κ FLC using both assays:
  • Freelite returned higher values than Seralite at all time points, especially at diagnosis, but not for all pts
  • In therapy, both tests show a decrease in dFLC values and a similar percentage reduction
  • During progression of the disease, both tests show an increase in dFLC

Table 2. Serum FLC parameters and response criteria for LCO and OS patients assessed by Freelite and Seralite at the maximum response (adapted from Heaney J. et al.)

N.B: *Significantly higher than Seralite, P < 0.01 (comparisons made within patient subgroup).

• When assessing LCO patients at maximum response:
  • The absolute levels of FLC difference (sFLC = involved minus uninvolved) were significantly lower on Seralite vs Freelite
  • Percent reductions in dFLC were not significantly different in both methods
  • Both Seralite and Freelite show little difference in the classification of pts with a VGPR (83% vs 88%)
  • Lower sFLC levels were observed on Seralite at all time points, but overall there was broad clinical concordance in response to therapy in the pt cohort as a whole
• Assessing survival outcome and sFLC response assessment:
  • Both Freelite and Seralite tests show that pts with a good response (VGPR or CR) had a significantly better OS vs pts with PR or stable disease (SD) at maximum response
  • Pts with ≥ VGPR and assessed by Freelite had a 63% reduced risk of death
  • Pts with ≥ VGPR and assessed by Seralite had a 46% reduced risk of death
• Use of Seralite for the diagnosis of MM and non-MM acute kidney injury:
  • Important to diagnose AKI quickly to prevent irreversible damage and quick diagnosis of myeloma or ruling out myeloma to refer the pt for a kidney biopsy
  • Speed is important in the diagnosis of AKI and Seralite could be a suitable test
• sFLCs were retrospectively analyzed using Seralite and Freelite in pts with AKI at stage 3:
  • Confirmed MM diagnosis in 45/99 pts
  • κ:λ ratio provided by Seralite accurately discriminated MM pts and all non-MM related AKI pts
  • Seralite has the same level of performance as Freelite

This study demonstrated that both assays return different values, possibly due to the anti-FLC antibodies having different affinities for an individual patient’s monoclonal FLC. However, despite the differences in quantification, both methods provided similar information regarding disease activity, changes in disease and prognosis. It is important to note that due to the differences in quantification, it is not possible to use both methods interchangeably. Overall, Seralite could make sFLC testing more easily accessible, and the quick speed at which the test can be carried out could be particularly beneficial for urgent samples (e.g. AKI).