Urine protein electrophoresis, or UPEP, separates proteins in urine and can detect and measure monoclonal free light chains, historically called Bence Jones proteins. These light chains are produced by a single plasma-cell clone and may spill into urine when their production exceeds the kidney’s ability to reabsorb them. UPEP is used in the evaluation and monitoring of multiple myeloma and related plasma cell disorders, especially when urinary monoclonal protein is measurable or kidney involvement is a concern. It is different from a routine urine dipstick: standard dipsticks mainly detect albumin and can miss substantial free light-chain protein. UPEP also differs from urine immunofixation, which is more sensitive for confirming whether a urinary protein is monoclonal and whether it is kappa or lambda. Modern serum free light chain testing has reduced reliance on urine electrophoresis for some screening situations, but urine studies still provide important information in selected patients. Correct interpretation requires the collection type, total urine protein, immunofixation, serum studies, kidney function, and the diagnosed plasma cell disorder.
- Bence Jones protein means monoclonal free kappa or lambda light chains are present in urine, usually from a plasma-cell or B-cell clone.
- A routine urine dipstick can be negative despite light-chain proteinuria, because dipsticks are much more sensitive to albumin than to free light chains.
- A 24-hour urine collection is often used when accurate monoclonal protein excretion in mg/24 h is needed, particularly for formal myeloma assessment or monitoring.
- UPEP is less sensitive than urine immunofixation for tiny monoclonal proteins, so a negative electrophoresis does not always rule out light-chain disease.
- Serum free light chains and serum protein studies are usually interpreted alongside UPEP, because no single test detects every plasma-cell disorder reliably.
Table of Contents
- What UPEP Measures
- What Bence Jones Protein Means
- How to Collect a UPEP Sample
- How to Read UPEP Results
- UPEP vs Urine Immunofixation and Serum Tests
- UPEP in Myeloma Diagnosis and Monitoring
- Abnormal Results, Kidney Risk, and Next Steps
What UPEP Measures
UPEP separates urinary proteins according to their electrical charge and migration characteristics. The pattern can show albumin, other filtered proteins, and abnormal restricted bands. When a plasma-cell clone produces large amounts of one free light chain, UPEP may show a narrow monoclonal band that can be quantified.
The test has two related roles. First, it can help identify an unusual protein pattern. Second, if a monoclonal component is measurable in a timed urine collection, the laboratory can estimate how much is excreted over 24 hours.
The result may be reported as a concentration and as an amount per day. For example, the laboratory can combine the monoclonal fraction with the total 24-hour urine volume to report monoclonal protein in mg/24 h. This is different from serum electrophoresis, where a measurable M protein is usually reported in g/dL or g/L.
UPEP can also reveal non-monoclonal kidney patterns. A large albumin fraction may suggest glomerular protein loss, while other patterns can reflect tubular proteinuria. The electrophoresis interpretation therefore provides information beyond Bence Jones protein alone.
UPEP does not directly identify the light-chain type. If a restricted band is suspicious, urine immunofixation can determine whether it is monoclonal kappa or lambda. The serum free light chain test provides a separate quantitative view of circulating free kappa and lambda and is now central to evaluating light-chain plasma cell disorders.
What Bence Jones Protein Means
Bence Jones proteins are monoclonal immunoglobulin free light chains in urine. Plasma cells normally make both heavy and light chains, and small amounts of free light chains circulate and are handled by the kidneys. A clonal plasma-cell disorder can produce a very large excess of one light-chain type, kappa or lambda.
Free light chains are small enough to pass through the glomerular filter. Proximal tubular cells normally reabsorb and metabolize most filtered light chains. When production becomes excessive—or when tubular handling is impaired—larger amounts appear in urine.
A confirmed Bence Jones protein is evidence of a monoclonal process, but it does not by itself establish multiple myeloma. Urinary monoclonal light chains can occur in light-chain MGUS, smoldering myeloma, active myeloma, AL amyloidosis, monoclonal gammopathy of renal significance, and some B-cell disorders.
The amount also matters but has to be interpreted with the rest of the disease. A person with a modest urine monoclonal protein can still have clinically important AL amyloidosis or kidney injury. Conversely, a large urinary light-chain burden can occur in active myeloma and may increase concern for cast nephropathy when serum free light chains are markedly elevated and kidney function is worsening.
One practical point is easy to miss: standard urine dipsticks are poor screening tests for Bence Jones protein. They are designed mainly to react with albumin. A patient can therefore have a negative or low dipstick reading while electrophoresis, immunofixation, or free light-chain testing shows substantial monoclonal light-chain disease.
How to Collect a UPEP Sample
UPEP can be performed on a random urine sample or a timed collection, but a 24-hour collection is often preferred when the goal is to quantify total monoclonal protein excretion accurately.
A typical 24-hour collection works like this:
- On the morning the collection starts, urinate into the toilet and note the time. That first void is not saved.
- Collect all urine for the next 24 hours in the provided container, including overnight samples.
- At the same time the next morning, collect the final void into the container.
- Keep the container stored as instructed by the laboratory, which may mean refrigeration or another cool condition.
- Return the complete collection and provide the start and stop times if requested.
Missing even one substantial void can make the total 24-hour protein result falsely low. Collecting longer than 24 hours or including urine before the official start can make it falsely high. If a collection is incomplete, it is better to tell the laboratory or clinician than to assume the result is accurate.
There is usually no need to fast. Hydration should generally be normal unless the clinician has given specific fluid instructions. Extremely unusual fluid intake can change urine concentration, although a correctly collected 24-hour result accounts for total volume better than a spot concentration does.
Random samples can be useful for detection and may be more convenient, but they are less suitable for formal 24-hour excretion calculations. The requested collection type should match the clinical question.
How to Read UPEP Results
UPEP reports vary. Some list total urine protein, fractions, the percentage represented by a monoclonal band, and an estimated monoclonal protein amount. Others provide a narrative interpretation such as “monoclonal protein detected” or “no monoclonal protein identified.”
| Finding | Usual meaning | Important follow-up |
|---|---|---|
| No restricted band | No measurable monoclonal urine protein seen by UPEP | Consider urine IFE and serum free light chains if suspicion remains |
| Restricted monoclonal band | Possible Bence Jones or other monoclonal protein | Confirm/type with urine immunofixation |
| Monoclonal protein reported in mg/24 h | Quantified daily urinary M-protein excretion | Compare with prior values using the same collection method |
| Predominantly albumin pattern | May suggest glomerular protein loss rather than isolated light chains | Interpret with albuminuria and kidney evaluation |
| Mixed protein pattern | May reflect combined monoclonal protein and kidney disease | Correlate with kidney function, total protein, and IFE |
There is no single “normal range” for an M protein because normal urine should not contain a monoclonal immunoglobulin. For monitoring, the useful value is the amount of confirmed monoclonal protein and how it changes over time.
A negative UPEP is not absolute proof that no monoclonal light chain exists. Small quantities may fall below electrophoresis sensitivity. Urine immunofixation is more sensitive for detecting and typing low-level bands, while serum free light chain testing can detect circulating light-chain excess even when urine findings are limited.
UPEP vs Urine Immunofixation and Serum Tests
The tests are complementary because each answers a different question.
UPEP shows the urinary protein pattern and can quantify a measurable monoclonal component, especially in a 24-hour collection.
Urine immunofixation (UIFE) is more sensitive for confirming a small monoclonal protein and identifying whether it is kappa or lambda. It is mainly qualitative rather than the primary quantitative tool.
Serum free light chains (sFLC) measure free kappa and lambda in blood and calculate the ratio. They are highly useful for detecting and following light-chain disease, but kidney impairment can raise both light chains and complicate interpretation.
SPEP and serum immunofixation detect intact or serum-predominant monoclonal proteins. A serum protein electrophoresis test can quantify a measurable serum M spike, while serum immunofixation identifies its isotype.
Because serum free light chain testing is sensitive and convenient, many diagnostic algorithms no longer require 24-hour urine electrophoresis as the only or primary screening tool for every patient. Urine testing remains important, however, when a urinary monoclonal protein must be documented or quantified, in selected AL amyloidosis and renal evaluations, and when formal disease or response criteria require it.
Modern mass spectrometry can detect monoclonal proteins with greater sensitivity than traditional electrophoresis in some settings, but access varies and it has not made all conventional urine and serum tests obsolete.
UPEP in Myeloma Diagnosis and Monitoring
In multiple myeloma, UPEP can help characterize secretory disease at diagnosis and provide a measurable marker for follow-up. This is particularly relevant in light-chain myeloma, where serum SPEP may show little or no M spike.
Current myeloma diagnosis relies on a combination of clonal plasma-cell evidence and myeloma-defining events rather than on UPEP alone. The evaluation commonly includes a multiple myeloma test panel, bone marrow examination, cytogenetics, and imaging.
Response criteria traditionally use changes in serum and/or urine M protein when those markers are measurable. A correctly collected 24-hour urine is important because a concentration in mg/L can change with hydration even when daily excretion is unchanged. Reporting mg/24 h allows a more meaningful serial comparison.
The role of urine has become more nuanced as serum free light chain assays and more sensitive methods have improved. Serum markers often respond faster and are easier to collect, but urine remains useful when it is the established measurable disease marker or when kidney excretion itself is clinically relevant.
A key modern diagnostic nuance is that very high involved free light chains and an extreme serum free light chain ratio can be a myeloma-defining biomarker in the correct context. Updated diagnostic frameworks continue to refine how urine monoclonal protein is incorporated with these serum findings. Clinicians should therefore apply the current criteria rather than interpreting an older cutoff from a laboratory handout in isolation.
Abnormal Results, Kidney Risk, and Next Steps
A new monoclonal urine protein usually leads to serum testing and assessment for the underlying clone. Common next steps include SPEP, serum and urine immunofixation, serum free light chains, complete blood count, calcium, creatinine/eGFR, quantitative immunoglobulins, and review of symptoms. Bone marrow biopsy and imaging may follow if a plasma cell neoplasm is suspected.
Kidney function deserves particular attention because free light chains can directly contribute to renal injury. In myeloma cast nephropathy, very high circulating monoclonal light chains reach the tubules and form obstructing casts, often causing acute or rapidly progressive kidney dysfunction. Other monoclonal-protein kidney diseases can damage glomeruli or deposit abnormal immunoglobulin material in tissues.
Urgent evaluation is appropriate when a known or suspected plasma cell disorder is accompanied by rapidly rising creatinine, markedly reduced urine output, dehydration, severe weakness, confusion, high calcium symptoms, or other signs of acute illness. The goal is not to wait for the next routine UPEP if organ function is changing quickly.
For monitoring known disease, consistency improves interpretation. Use the same type of collection, complete the full 24 hours when requested, and compare results with serum markers and clinical status. A falling urine M protein usually supports response, but a small fluctuation from one collection to another can reflect collection quality as well as biology.
The most useful interpretation is therefore not simply “Bence Jones positive.” It is: Which light chain is present? How much is excreted per day? Are serum free light chains high? Is kidney function stable? Is this a new finding, a treatment response marker, or evidence of progression? Those answers determine what the UPEP result actually means for the patient.
Urine protein patterns can also help separate light-chain overflow from other kidney disease. In classic overflow proteinuria, a monoclonal free light chain may make up a large share of urinary protein while albumin is relatively modest. In glomerular disorders, albumin is often the dominant component. Myeloma patients can have both processes at once, so the pattern sometimes guides whether nephrology evaluation or kidney biopsy is needed.
The relationship between serum and urine light chains is not linear. Kidney filtration and tubular reabsorption change with renal function, and urinary excretion can fall even while serum free light chains remain high if kidney function worsens. That is one reason clinicians do not use a falling UPEP value in isolation to assume that the plasma-cell clone is improving. Serum free light chains, creatinine, urine volume, and the full clinical picture must move together.
Collection quality is a common source of misleading trends. A 24-hour result can look dramatically better if several voids were missed or if the collection ended early. Creatinine excretion and total volume may give the clinician clues about completeness, but they do not rescue a clearly incomplete specimen. When a value is unexpectedly inconsistent with serum markers or symptoms, repeating the collection is often more useful than making a treatment decision from a questionable sample.
Patients with known light-chain myeloma should also understand that kidney injury can develop quickly. Acute illness, dehydration, certain medications, hypercalcemia, and very high free light-chain production can worsen renal function. Rapidly rising creatinine or sharply reduced urine output requires prompt evaluation even if the scheduled UPEP is weeks away.
Not every urinary monoclonal light chain is large enough to quantify accurately. A report may state that a band is present by immunofixation but is too small to measure on UPEP. In that situation, serum free light chains or another established marker may be more useful for serial response assessment than forcing a numerical urine value below the assay’s reliable quantification limit.
Urine results can also lag behind serum changes because renal handling influences excretion. For that reason, formal response assessment uses defined criteria and the marker that is measurable for the individual patient rather than assuming every urine decrease represents the same degree of tumor reduction.
References
- Recommendations for the study of monoclonal gammopathies in the clinical laboratory. A consensus of the Spanish Society of Laboratory Medicine and the Spanish Society of Hematology and Hemotherapy. Part I: Update on laboratory tests for the study of monoclonal gammopathies 2023 (Consensus)
- Management of multiple myeloma-related renal impairment: recommendations from the International Myeloma Working Group 2023 (Recommendations)
- Urine immunofixation electrophoresis and serum free light chain analyses benefit diagnosis of multiple myeloma in orthopedic patients with normal serum total proteins, creatinine, calcium, and hemoglobin 2024
- Diagnosis and Management of Monoclonal Gammopathy of Undetermined Significance: A Review 2025 (Review)
- Multiple Myeloma: A Structured and Multidisciplinary Approach to Diagnosis 2026 (Review)
- Multiple Myeloma: 2026 Update on Diagnosis, Risk-Stratification and Management 2026 (Review)
Disclaimer
UPEP results should be interpreted with urine immunofixation, serum monoclonal-protein tests, kidney function, and the clinical diagnosis. A negative UPEP does not exclude low-level light-chain disease, and a positive Bence Jones protein does not by itself prove active multiple myeloma. Seek prompt medical care for rapidly worsening kidney function or other acute symptoms.
