A Bence Jones protein urine test looks for monoclonal immunoglobulin free light chains—either kappa or lambda—that are produced by an abnormal clone of plasma cells or related B cells and excreted into urine. These proteins are strongly associated with plasma-cell disorders such as multiple myeloma and light-chain amyloidosis, but a positive urine result does not by itself establish a cancer diagnosis. Modern testing usually relies on urine protein electrophoresis and immunofixation, often alongside serum protein electrophoresis, serum immunofixation, and a serum free light chain assay. Bence Jones proteins matter for kidney health because large amounts of free light chains can injure proximal tubules and, in multiple myeloma, can form obstructing casts that cause acute kidney injury. A standard urine dipstick can miss this pattern because it is much more sensitive to albumin than to free light chains. When a monoclonal light chain is found, the type, quantity, kidney function, blood findings, and clinical context determine the next steps.
- Bence Jones proteins are monoclonal free kappa or lambda immunoglobulin light chains found in urine.
- A routine protein dipstick can be negative or only weakly positive despite substantial light-chain proteinuria because dipsticks mainly detect albumin.
- Urine immunofixation is used to confirm whether a monoclonal light chain is present; urine electrophoresis can help quantify and characterize the protein pattern.
- A positive result can occur with multiple myeloma, light-chain amyloidosis, monoclonal gammopathy, and related disorders, so diagnosis requires additional blood, bone marrow, and imaging evaluation as appropriate.
- New kidney dysfunction with very high free light chains is clinically urgent because myeloma cast nephropathy can cause rapid and sometimes severe kidney injury.
Table of Contents
- What Bence Jones Proteins Are
- How Urine Testing Detects Light Chains
- What Positive and Negative Results Mean
- Multiple Myeloma and Other Causes
- How Light Chains Can Damage the Kidneys
- Collection and Testing Limitations
- Follow-Up After a Bence Jones Result
What Bence Jones Proteins Are
Bence Jones proteins are free immunoglobulin light chains produced by one abnormal cell clone. Normal antibodies contain two heavy chains and two light chains. Plasma cells usually make a small excess of free light chains, which circulate in blood, are filtered by the kidneys, and are largely reabsorbed and broken down by proximal tubular cells.
In a monoclonal plasma-cell or B-cell disorder, one clone can produce a very large amount of a single light-chain type. The light chain may be kappa (κ) or lambda (λ). When production exceeds the kidney’s reabsorptive capacity, monoclonal free light chains spill into urine and can be detected as Bence Jones protein.
The term is historical. Today, laboratories do not rely on the old heat-precipitation behavior that gave Bence Jones protein its name. Modern testing uses protein electrophoresis, immunofixation, and increasingly sensitive serum free light chain assays. A urine free light chain test may also be used in some laboratories, but testing strategies vary.
A key point is that Bence Jones protein is not albumin. Standard urine dipsticks mainly react with albumin, so they can substantially underestimate light-chain proteinuria. A person may have a surprisingly low dipstick reading while a quantitative total urine protein measurement is much higher. That discrepancy is one reason clinicians consider monoclonal proteins.
Bence Jones protein also differs from polyclonal free light chains. In infection, inflammation, or reduced kidney filtration, both kappa and lambda light chains can increase. A monoclonal pattern means one light-chain population is being produced disproportionately and requires evaluation for an underlying clonal disorder.
How Urine Testing Detects Light Chains
The most informative urine tests are urine protein electrophoresis (UPEP) and urine immunofixation electrophoresis (uIFE). They answer related but different questions.
UPEP separates proteins according to their physical properties and can show a concentrated monoclonal band or peak. It also helps estimate how much monoclonal protein is being excreted when a timed urine collection is used. The urine protein electrophoresis test is especially useful when total urine protein is high but the protein type is uncertain.
Urine immunofixation uses antibodies to identify the immunoglobulin type. It is more sensitive for confirming a small monoclonal light-chain population and can show whether the protein is kappa or lambda. A urine immunofixation test is therefore often used to verify that an abnormal electrophoretic band is monoclonal.
For suspected multiple myeloma, urine testing is only one part of the laboratory evaluation. Common blood tests include:
- serum protein electrophoresis;
- serum immunofixation;
- serum free kappa and lambda light chains with an involved/uninvolved ratio;
- complete blood count;
- calcium, creatinine, and eGFR;
- quantitative immunoglobulins in selected cases.
The serum free light chain assay has become especially important because it is sensitive, fast, and does not depend on a complete 24-hour urine collection. Nevertheless, urine studies still provide distinct information about monoclonal protein excretion and remain recommended in several myeloma assessment settings, particularly when kidney involvement is being evaluated.
A result may be qualitative—monoclonal light chain present or absent—or quantitative, such as mg/24 h. The exact reporting format depends on the laboratory and whether a random or timed urine specimen was used.
What Positive and Negative Results Mean
A positive Bence Jones result means monoclonal free light chains have been identified in urine. It does not, by itself, say why they are present or how aggressive the underlying disorder is.
The significance depends on several features:
- which light chain is present, kappa or lambda;
- how much is being excreted;
- whether serum immunofixation or serum free light chains are abnormal;
- whether there is anemia, high calcium, kidney dysfunction, or bone disease;
- whether bone marrow shows a clonal plasma-cell population;
- whether there are features of amyloidosis or another monoclonal gammopathy.
A negative urine test does not completely exclude a plasma-cell disorder. Small amounts of monoclonal protein may be below the detection limit, production may occur mainly in blood, or the disease may produce an intact immunoglobulin rather than abundant urinary free light chains. This is why serum and urine tests are often combined.
A low-level monoclonal light chain also does not automatically mean symptomatic multiple myeloma. Monoclonal gammopathy of undetermined significance (MGUS), smoldering myeloma, light-chain MGUS, AL amyloidosis, monoclonal gammopathy of renal significance, lymphoma-related disorders, and other conditions can all enter the differential diagnosis.
The laboratory term “Bence Jones proteinuria” therefore describes a protein finding, not a final disease label. A hematologic diagnosis requires integration of monoclonal protein studies with clinical findings and, when indicated, bone marrow or tissue evaluation.
Multiple Myeloma and Other Causes
Multiple myeloma is the best-known cause of Bence Jones proteinuria. In myeloma, clonal plasma cells in the bone marrow produce a monoclonal immunoglobulin, free light chains, or both. Some patients have light-chain myeloma, in which the dominant secreted product is a free light chain rather than a complete antibody.
Current myeloma diagnosis is not based on a urine light chain alone. Clinicians evaluate bone marrow plasma cells or a biopsy-proven plasmacytoma and look for myeloma-defining events, including organ damage attributable to the plasma-cell disorder or validated high-risk biomarkers. Kidney impairment is one of the classic organ-damage features, but it must be linked to the plasma-cell process rather than assumed from a coincidental creatinine elevation.
Other conditions associated with monoclonal light chains include:
- AL amyloidosis, in which misfolded light chains form amyloid deposits in organs such as the kidneys and heart;
- monoclonal gammopathy of renal significance, where a small clone causes clinically important kidney injury despite not meeting criteria for overt myeloma or lymphoma;
- smoldering multiple myeloma and precursor monoclonal gammopathies;
- selected B-cell lymphoproliferative disorders that produce monoclonal immunoglobulin components;
- other plasma-cell dyscrasias.
The pattern of urine protein can provide useful clues. Myeloma cast nephropathy often involves large amounts of free light chains with a relatively smaller albumin fraction. By contrast, AL amyloidosis or monoclonal immunoglobulin deposition disease can damage the glomerular barrier and produce substantial albuminuria. When urine protein is mainly albumin rather than free light chains, clinicians may consider a different kidney lesion and sometimes kidney biopsy.
How Light Chains Can Damage the Kidneys
Monoclonal free light chains can affect the kidneys in several ways. The most urgent is light-chain cast nephropathy, sometimes called myeloma kidney. Large quantities of filtered light chains reach the distal nephron, interact with proteins such as uromodulin, and form obstructing casts. This can trigger inflammation, tubular injury, and a rapid fall in kidney function.
Free light chains can also directly injure proximal tubular cells. In some patients they cause a Fanconi-type syndrome, with inappropriate urinary loss of glucose, phosphate, uric acid, amino acids, and bicarbonate. Other monoclonal proteins deposit in glomeruli or vessel walls, producing albuminuria, nephrotic syndrome, hematuria, or chronic kidney disease.
Kidney injury may be worsened by dehydration, high calcium, infection, nonsteroidal anti-inflammatory drugs, contrast exposure in selected settings, and other stressors that reduce kidney perfusion. This is why new acute kidney injury in someone with suspected or known myeloma is taken seriously.
International Myeloma Working Group recommendations for myeloma-related renal impairment include serum creatinine, eGFR, serum free light chains, and a 24-hour urine assessment with total protein, electrophoresis, and immunofixation. The balance between albumin and monoclonal light chains can help determine whether the injury looks like cast nephropathy or another renal lesion.
A conventional 24-hour urine protein test can quantify total loss, but it cannot identify light chains by itself. Electrophoresis and immunofixation are what establish the monoclonal nature of the urinary protein.
Collection and Testing Limitations
Traditional Bence Jones quantification often uses a 24-hour urine collection, but this method has practical weaknesses. Missing even one void can underestimate monoclonal protein excretion, while collecting beyond the required period can overestimate it. Incomplete collections are common enough that the quality of the sample must be considered whenever results do not fit the clinical picture.
Random urine specimens can be convenient for qualitative detection, and newer approaches are being studied to reduce dependence on 24-hour collections. Serum free light chain testing also reduces the need to rely on urine alone. However, guidelines and laboratory protocols have not completely abandoned timed urine testing, especially for formal disease assessment and response criteria in some settings.
Analytical limitations also matter. Different total-protein methods do not detect all protein species equally. Free light chains can behave differently from albumin in colorimetric or turbidimetric assays, and a standard dipstick can be misleadingly low. This is one reason the combination of total protein, electrophoresis, immunofixation, and serum testing is stronger than any single method.
Kidney function affects serum free light chains too. As eGFR falls, both kappa and lambda concentrations may rise because renal clearance decreases. Laboratories and clinicians therefore interpret serum light chain ratios with the kidney function context in mind rather than labeling every elevation monoclonal.
Sample contamination, very dilute urine, poor timing, and delayed transport can further complicate interpretation. If a result is discordant with blood studies or clinical findings, repeating the sample or using a complementary method is often appropriate.
Follow-Up After a Bence Jones Result
A confirmed monoclonal urinary light chain usually triggers a hematologic and kidney-focused evaluation, not just a repeat urine dipstick. The exact workup depends on the quantity, symptoms, kidney function, and accompanying blood tests.
Common next steps include serum protein electrophoresis, serum immunofixation, serum free light chains, complete blood count, calcium, creatinine and eGFR, and assessment for bone pain, fractures, recurrent infections, neuropathy, edema, or symptoms of amyloidosis. Bone marrow biopsy and imaging may be needed when the findings suggest multiple myeloma or another clonal plasma-cell disorder.
If kidney function is reduced, the clinician also evaluates whether the protein pattern is predominantly light chain or albumin. Very high free light chains with acute kidney injury raise concern for cast nephropathy and require prompt specialist management. If albumin makes up a large proportion of the protein or the serum involved free light chain level is not extremely high, a kidney biopsy may be considered to identify amyloidosis, monoclonal deposition disease, or another lesion.
Seek urgent medical evaluation for markedly reduced urine output, rapidly worsening kidney function, severe dehydration, confusion, severe weakness, significant hypercalcemia symptoms, or other signs of acute illness in the setting of known or suspected plasma-cell disease.
For monitoring an established disorder, trends matter. A falling monoclonal protein burden may accompany treatment response, while reappearance or increase can suggest persistent or recurrent disease. The exact monitoring strategy should follow the hematology team’s protocol because serum and urine tests are not interchangeable in every patient.
Why Serum and Urine Light-Chain Tests Can Disagree
Serum free light-chain testing and urine Bence Jones testing examine the same monoclonal process from different physiologic angles, so the results do not have to move in parallel. Free kappa and lambda light chains are produced in blood, filtered by the glomeruli, and then largely reabsorbed and broken down by the proximal tubules. Urinary excretion therefore depends not only on how much light chain is produced but also on kidney filtration, tubular handling, hydration, and collection quality.
A patient can have a clearly abnormal serum free light-chain concentration with relatively little measurable urinary light chain. Reduced filtration may limit the amount reaching urine, and tubular uptake can remove a substantial fraction until the reabsorptive capacity is exceeded. Conversely, heavy urinary light-chain excretion can occur when production is high enough to overwhelm tubular reabsorption. The urine result should therefore not be used as a simple surrogate for the serum concentration.
Testing methods also matter. Urine protein electrophoresis can show a monoclonal band and estimate the amount of monoclonal protein, while urine immunofixation is more sensitive for identifying the light-chain type. Serum immunofixation and serum free light-chain assays provide complementary information. In current myeloma evaluation, clinicians generally use a combination of blood and urine studies rather than relying on the historical heat-precipitation concept of a “Bence Jones test” alone.
The pattern of kidney injury adds another layer. Myeloma cast nephropathy often occurs with high circulating free light chains and can cause abrupt kidney failure. Other monoclonal gammopathy-related lesions can produce albumin-predominant proteinuria because the glomerulus is affected. That is why the International Myeloma Working Group recommends assessing serum creatinine, eGFR, serum free light chains, 24-hour total urine protein, electrophoresis, and immunofixation in patients with myeloma and renal impairment.
A negative urine study is reassuring only within that broader context. It does not by itself exclude a plasma-cell disorder, and a positive study does not establish multiple myeloma without evidence of a clonal plasma-cell or B-cell process and the appropriate diagnostic criteria. The purpose of urine light-chain testing is to characterize monoclonal protein and kidney involvement, not to replace the rest of the hematologic evaluation.
How Urine Testing Is Used During Follow-Up
Once a monoclonal protein disorder is diagnosed, urine testing may be used to document the amount and type of monoclonal protein and to follow response, particularly when urine protein was measurable at baseline. The most useful comparison is generally with the same method over time, because electrophoresis, immunofixation, immunochemical light-chain measurements, and total-protein assays do not produce interchangeable numbers.
Kidney function must be followed at the same time. A falling urine light-chain measurement is encouraging, but creatinine, eGFR, serum free light chains, calcium, blood counts, and the broader myeloma assessment determine whether treatment is controlling the disease and whether renal injury is recovering. Persistent proteinuria may also reflect a glomerular monoclonal-protein lesion rather than ongoing cast nephropathy alone.
References
– Management of multiple myeloma-related renal impairment: recommendations from the International Myeloma Working Group 2023 (Guideline) – Multiple myeloma: 2024 update on diagnosis, risk-stratification, and management 2024 (Review) – The dark side of current analytic methods for Bence Jones Proteinuria 2022 (Review) – Evaluation of Proteinuria in Plasma Cell Disorders: Shortcomings of Measurements Based on 24-Hour Collections and Alternative Approaches 2025 (Review) – Novel Approach to Rule-Out Unnecessary Urine Bence Jones Protein Testing: A Serum Free Light Chain Algorithm 2025
Disclaimer
This article is for general education and does not diagnose multiple myeloma or any other plasma-cell disorder. A positive or negative urine light-chain result must be interpreted with serum studies, kidney function, symptoms, and specialist assessment. New kidney injury or severe illness with suspected myeloma warrants prompt medical evaluation.
