When should I order the serum free light-chain assay, and how should I interpret mild kappa or lambda elevations?

1. ORDER SERUM FLC ASSAY ONLY WITH CLEAR CLINICAL INDICATIONS

Use of the assay as a routine screening test in patients who are asymptomatic or at low risk for plasma cell disorders contributes to overdiagnosis (eg, light-chain MGUS in up to 5% to 10% of older adults).⁶ Reserve the test for scenarios where a plasma cell disorder is clinically suspected:

• CRAB features (elevated calcium > 11 mg/dL, renal impairment with serum creatinine > 2 mg/dL or estimated glomerular filtration rate [eGFR] < 40 mL/ min/1.73 m², anemia with hemoglobin < 10 g/dL, and bone lesions on imaging)⁴

• Recurrent bacterial infections

• Peripheral neuropathy

• Organ dysfunction suggestive of amyloidosis

• Unexplained fatigue accompanied by cytopenia

• Elevated total protein

• Rouleaux formation (aggregation of red blood cells into linear structures resembling stacked coins) on peripheral smear.

Indiscriminate use in patients at low risk (eg, vague fatigue without CRAB features) often yields false-positive results, leading to unnecessary referrals and costly procedures such as bone marrow biopsy and imaging studies.

The serum FLC assay is critical for initial workup of suspected multiple myeloma, amyloidosis, Waldenström macroglobulinemia, and light-chain deposition disease.^2,3 In light-chain deposition disease, serum FLC testing detects “light-chain–only” diagnoses that can be missed by serum protein electrophoresis. Light chain multiple myeloma, characterized by production of only immunoglobulin light chains, accounts for 15% to 20% of newly diagnosed cases of multiple myeloma.⁷

The assay is also critical for monitoring established MGUS, smoldering multiple myeloma, and treated multiple myeloma to track progression or treatment response. For example, a rising kappa:lambda ratio in a patient with known MGUS or treated multiple myeloma signals progression or hematologic relapse and warrants specialist reevaluation. Recent IMWG and National Comprehensive Cancer Network updates⁴ favor serum FLC testing combined with serum protein electrophoresis and immunofixation for response assessment; stringent complete response requires a normalized kappa:lambda ratio.

2. PRIORITIZE THE KAPPA:LAMBDA RATIO OVER ABSOLUTE VALUES TO ASSESS CLONALITY

Mild elevations in isolated kappa (eg, 20–30 mg/L; reference range < 19.4 mg/L) or lambda (eg, 30–40 mg/L; < 26.3 mg/L) light chains with a normal kappa:lambda ratio (0.26–1.65, according to original reference intervals, but see next section for discussion of revised reference values) are often benign and polyclonal.² The kappa:lambda ratio detects imbalances in one type of light chain resulting from clonal plasma cell proliferation. An abnormal ratio (> 1.65 if kappa is the involved light chain or < 0.26 if lambda is the involved chain) indicates potential monoclonality even if absolute levels are modestly elevated.

In contrast, absolute values fluctuate with non-malignant factors like hydration, inflammation (eg, rheumatoid arthritis), or infections, making them unreliable in isolation.¹ A patient with chronic kidney disease might show elevated levels of both light chains (kappa 25 mg/L, lambda 30 mg/L) but have a normal kappa:lambda ratio (0.83), reflecting reduced clearance rather than clonality.

Serial monitoring of the kappa:lambda ratio outperforms absolute values for detecting response to therapy and relapse. Studies show that relying on the kappa:lambda ratio reduces overreferrals by 50% to 70%.⁶ In tracking serial kappa:lambda ratios for trends, stable mild deviations rarely progress, but doubling over 6 to 12 months warrants a consultation.

3. USE REVISED, CONTEXT-ADJUSTED FLC RATIO REFERENCE RANGES FOR INTERPRETATION, RISK STRATIFICATION

Using legacy reference intervals for serum FLC ratios leads to significant overdiagnosis of light-chain MGUS in healthy populations. The Iceland Screens, Treats, or Prevents Multiple Myeloma (iStopMM) study^8,9 used population-based screening for MGUS to identify candidates for early treatment of multiple myeloma. Population data from this study showed that an isolated mild kappa elevation (approximately 25 mg/L) alongside a normal kappa:lambda ratio and preserved eGFR (> 60 mL/min/1.73 m²) typically reflects transient fluctuation rather than clonal disease. Data from the iStopMM cohort⁸ were used to revise serum FLC reference intervals (Table 1),^9,10 reducing false positives and unnecessary interventions in low-risk groups. These adjustments reclassify about 80% of mild elevations as normal.⁶

Using the revised reference intervals allows clinicians to focus resources on patients with high-risk features. For example, a ratio of involved FLCs to uninvolved FLCs greater than 100 in patients with smoldering (asymptomatic) multiple myeloma predicts an 80% risk of progression to multiple myeloma or light-chain amyloidosis within 2 years.² Furthermore, incorporating the revised reference intervals supports personalized follow-up: those with low-risk MGUS (normal kappa:lambda ratio) need annual laboratory studies only; the IMWG advises against routine follow-up bone imaging in low-risk MGUS unless clinical or biological signs of progression emerge.^4,6,8

4. INTERPRET MILD ELEVATIONS IN THE CONTEXT OF PATIENT FACTORS AND PRETEST PROBABILITY

Slight deviations (eg, kappa 22 mg/L with normal lambda and normal kappa:lambda ratio) often reflect benign causes. Such deviations are common in adults older than 70 and in patients with chronic kidney disease (eGFR < 60 mL/min/1.73 m²), and are often due to age-related polyclonal increases or to impaired renal excretion that prolongs light-chain half-life in chronic kidney disease.⁶

Clinical context is important, and the values need to correlate with clinical history. In a 70-year-old patient with osteoarthritis and mild anemia, an isolated mild light-chain elevation might be incidental, while the same finding in a patient with bone pain and hypercalcemia should prompt further workup.

Very high levels (involved light-chain level > 100 mg/L) should prompt a hematology referral regardless of the kappa:lambda ratio, as levels greater than 100 mg/dL have a positive predictive value for multiple myeloma and amyloidosis greater than 90%.² If the kappa:lambda ratio is borderline (Table 1), then retesting after 3 to 6 months is indicated rather than immediately escalating the workup, such as urgent bone marrow biopsy or advanced imaging. Newer tools like mass spectrometry can clarify ambiguity by detecting monoclonal-protein signatures missed by immunoassays, reducing false positives by 30%.¹¹

Avoid overinterpreting mild elevations in isolation by integrating serum FLC results with serum protein electrophoresis, complete blood cell count, chemistry panel results, and imaging. Isolated mild changes rarely progress without red flags.

5. BE VIGILANT FOR ASSAY LIMITATIONS THAT CAN MIMIC OR MASK MILD ABNORMALITIES

A falsely low reading in diagnostic testing can occur when there is an overwhelming excess of antigen in high-burden disease. Known as the hook effect, the excess antigen saturates the immunoassay capture and detection antibodies, reducing the formation of antigen-antibody complexes. If suspicion for plasma cell disorder is high despite apparently normal or mild results, then systematic dilution of the sample is warranted.

Also, variability in testing platforms affects antigen thresholds. Assays should not be considered interchangeable, and it is advisable to use the same assay consistently to ensure accurate trends.

THE BOTTOM LINE

Order serum FLC testing judiciously when there is clinical suspicion for a plasma cell disorder. Interpret mild kappa or lambda elevations through the lenses of the kappa:lambda ratio, patient context, and age- and eGFR-adjusted reference ranges (Table 1).⁹

Prompt hematology referral is warranted in the following settings:

• The involved light-chain level exceeds 100 mg/L, regardless of kappa:lambda ratio (positive predictive value > 90% for multiple myeloma and amyloidosis)

• The kappa:lambda ratio is less than 0.02 or greater than 100

• New CRAB features emerge in a patient known to have MGUS

• Criteria for smoldering multiple myeloma are met (per IMWG⁴: serum monoclonal protein ≥ 30 g/L or urine monoclonal protein ≥ 500 mg/24 hours or bone marrow plasma cells 10%–60%, without myeloma-defining events).

In established MGUS, rising or doubling of the kappa:lambda ratio over 6 to 12 months should prompt reevaluation. Isolated mild elevation with a normal kappa:lambda ratio without red flags can be monitored with serial laboratory studies using revised reference ranges. Assess urinary light chains (Bence Jones protein) when kidney involvement is suspected or serum electrophoresis, serum FLC findings, or both are discordant. This approach aligns with value-based care, minimizes unnecessary tests and referrals, and ensures timely detection of actionable disease.

DISCLOSURES

Dr. Parmar reports no relevant financial relationships which, in the context of their contributions, could be perceived as a potential conflict of interest.