Rheumatoid arthritis is an auto-immune disease that affects around 1 to 2 percent of people overall, with the incidence rising to 5 percent in women over age 55, according to rheumatologist Alan Matsumoto, M.D., of Johns Hopkins Arthritis Center. Women develop the disease two to three times more often than men. Rheumatoid arthritis affects the joints, causing inflammation and swelling that eventually destroys the joints. People with rheumatoid arthritis and other auto-immune disorders sometimes have a false-positive test for Bence-Jones proteins.
Definition
Bence-Jones proteins are small proteins not ordinarily present in the urine. Bence-Jones proteins are not seen via a regular urinalysis. Because Bence-Jones proteins are small, they can pass through the kidney and into the urine more easily than other types of protein molecules, which are usually larger.
Causes
Bence- Jones proteins occur most frequently in the urine of people with multiple myeloma, a cancer of the plasma cells, a type of white blood cell. Between 50 and 80 percent of people with multiple myeloma have a positive result. Because Bence-Jones proteins are made from plasma cells, larger numbers of the cells appear in the urine as multiple myeloma tumors progress. If you have a positive Bence-Jones protein test, it's important to differentiate between true positives and false positives.
False Positives
Around 20 percent of all positive Bence-Jones readings are false positives, "Interpretation of Diagnostic Tests" explains. Rheumatoid arthritis as well as other soft-tissue connective diseases disorders can cause a false positive result for Bence-Jones proteins. High doses of aspirin and penicillin can also cause false positive readings.
Diagnosis
Eliminating false-positive results for Bence-Jones proteins requires a special kind of urinalysis. While heating the urine causes proteins to clump, this test is not specific for Bence-Jones proteins, according to "Interpretation of Diagnostic Tests." To diagnose the exact amount of the protein in the urine, an immunoelectrophoresis is done on a urine specimen collected for 24 hours. A false-positive will not show a protein spike on the test. Around 80 percent of positive tests are actually positive, with 70 percent caused by multiple myeloma.
References
- Johns Hopkins Arhritis Center: Rheumatoid Arthritis Clinical Presentation
- Encyclopedia of Medicine: Bence-Jones Protein Test
- Medical Health Tests: Bence-Jones Protein Tests
- Gale Encyclopedia of Medicine: Bence-Jones Proteins
- "Interpretation of Diagnostic Tests:" Lippincott Williams & Wilkins; Jacques Wallach; Dec. 8, 2006
