Abnormal Free Light Chains in Individuals with African Ancestry Screened for Monoclonal Gammopathy: Definition of New Reference Range Accounting for Renal Function and Race

Introduction Serum free light chain (sFLC) are important for diagnosis and prognosis of plasma cell dyscrasias and are known to be affected by kidney function and inflammatory conditions. However, African Americans (AA) have been reported to have significantly higher Kappa (κ) and Lambda (λ) sFLC levels than European Americans (EA) in a biobanked population, even after adjusting for kidney function (El-Khoury et al., Lancet Haem 2022). Data on race-adjusted reference ranges for sFLC and sFLC ratio (sFLCr) remain limited. We hypothesize that an adjustment in sFLC reference ranges for individuals of African and European ancestry is needed.

Methods We analyzed serum samples from 738 healthy Black South African (SA) and 3402 US individuals (AA+EA) as part of an international study screening for individuals at high risk of multiple myeloma (MM) and precursor conditions. Self-identification as Black or having family history of hematologic cancer or MM precursor were key eligible criteria. sFLC levels were measured using the Optilite® Freelite assay (The Biding Site). All participants were screened for Monoclonal Gammopathy of Undetermined Significance (MGUS; M protein concentration ≥ 0.2 g/L) by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Serum Cystatin C (Optilite®) was used to calculate estimated Glomerular Filtration Rate (eGFR) with CKD-EPI Cystatin C Equation (2012) formula for 752 individuals (568 SA, 184 AA). Clinical data was collected using validated questionnaires at study enrollment. Group medians were compared with Wilcoxon tests. Abnormal sFLCr was defined as any value outside the normal reference range with the involved sFLC above the manufacturer's normal reference range. We used 95% central interval (CI) values to compute new sFLCr reference values for each group.

Results After excluding individuals with Heavy Chain-MGUS, our study population consisted of 654 SA, 326 AA, and 2,551 EA. Median age was 52 (IQR 46-59), 56 (IQR 50-65, and 58 (IQR 50-64) for the three groups, respectively. There was a female predominance in the AA (74%) and EA (73%) groups compared to SA (52%). Chronic kidney disease (CKD) was self-reported in 0.2%, 1% and 1% of the three groups, respectively. Calculated median eGFR was 118 ml/min (IQR 106-134) for SA and 112 (IQR 98-123) for AA. HIV positivity was higher (23%) in SA than the other groups (<1%) and 99% were under treatment (Tenofovir 92%, which is known to be nephrotoxic). The median free κ was higher in SA (32 mg/l, IQR 24.8-46) than AA (18.1 mg/l, IQR 14.4-23, +80%, p<0.01) and EA (13.7 mg/l, IQR 10.8 - 17.1, +133%, p<0.01). Similarly, median free λ was higher in SA (24.6 mg/l, IQR 18.8-33.4) than AA (15 mg/l, IQR 11.7-19.3, +64%, p<0.01) and EA (11.9, mg/l IQR 9.1-15.5, +106%, p<0.01). 92% of SA, 43% of AA, and 16% of EA had κ values above the normal range. Similarly, 43% of SA, 10% of AA, and 3% of EA had λ values outside the normal range. Median sFLCr was higher in SA (1.34, IQR 1.13-1.65) than AA (1.25, IQR 1.03-1.48, p<0.01), and EA (1.18, IQR 0.99-1.39, p<0.01) (Fig 1). Using the standard manufacturer ranges, 25% of SA, 11% of AA and 3% of EA had abnormal sFLCr and involved FLC. Decreasing eGFR, increasing age, HIV positivity, and the cohort (SA, AA, or EA) were all significant predictors of changes in sFLCr in univariate analysis. In multivariate logistic regression, increasing age, HIV positivity, and decreasing eGFR were independent predictors of abnormal sFLCr (Table 1) for SA or AA compared to EA. In SA with normal renal function (eGFR >60 ml/min), excluding those with free κ and λ outside the 95% CI, we identified a new reference (95% CI) for FLCr of 0.80-2.38.

Conclusions This is the first report of sFLC measurements in an African population screened for MG and compared to AA and EA. We observe a significantly higher sFLC for Black (SA and AA) than White individuals (EA), independent of renal function. This difference could be due to genetic background and socioeconomic and environmental factors (e.g., HIV epidemiology). In contrast with previous reports (Zemlin, et al, J Clin Pathol 2015), HIV was associated with higher FLCr, maybe with a possible role of nephrotoxic HIV drugs. Here, standard reference ranges do not apply to SA participants. While further validation is needed, we propose to use a different sFLC range for African ancestry populations with normal renal function.

Sakrikar:The Binding Site: Current Employment. Perkins:The Binding Site Group Ltd: Current Employment. Harding:The Binding Site: Current Employment, Membership on an entity's Board of Directors or advisory committees. Troske:The Binding Site Group Ltd: Current Employment. Getz:Pharmacyclics: Research Funding; Scorpion Therapeutics: Consultancy, Current equity holder in publicly-traded company, Other: Founder; IBM: Research Funding; MSMuTect: Patents & Royalties; SignatureAnalyzer-GPU: Patents & Royalties; MSMutSig: Patents & Royalties; MSIDetect: Patents & Royalties; POLYSOLVER: Patents & Royalties. Leung:Takeda Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees. Ghobrial:Sanofi: Honoraria; AbbVie: Honoraria; Janssen: Honoraria; Bristol Myers Squibb: Honoraria; Huron Consulting: Honoraria; Aptitude Health: Honoraria; GSK: Honoraria; Amgen: Honoraria; Pfizer: Honoraria; Oncopeptides: Honoraria; Celgene: Research Funding; Adaptive: Honoraria; Novartis: Research Funding; Sognef: Honoraria; Takeda: Honoraria; The Binding Site: Honoraria; Vor Biopharma: Honoraria; Menarini Silicon Biosystems: Honoraria; Veeva Systems: Honoraria; Window Therapeutics: Other: Advisory board participation.