It travels through the bloodstream to the liver, where it is processed and eventually excreted from the body, primarily in stool. A bilirubin urine test is often performed to check liver function or to investigate conditions that cause an excess of bilirubin to accumulate in the body. While a positive result typically suggests a health concern, sometimes the test can show a “false positive,” meaning bilirubin appears to be present in the urine when it is not.
The Bilirubin Urine Test
The body continuously recycles old red blood cells, a process that yields bilirubin as a byproduct. This unconjugated bilirubin then binds to albumin and travels to the liver. In the liver, it undergoes conjugation, becoming water-soluble, and is then excreted into bile, which aids in digestion before being eliminated in feces. Under normal circumstances, conjugated bilirubin does not appear in urine because it is efficiently cleared by the liver and kidneys.
The presence of conjugated bilirubin in urine, a condition known as bilirubinuria, usually indicates a problem with the liver’s ability to process bilirubin or an obstruction in the bile ducts. Urine tests, often using dipsticks, detect bilirubin through a chemical reaction, typically a diazo reaction, which produces a color change when bilirubin is present. A positive result generally prompts further medical investigation to identify the underlying cause. However, various factors can interfere with this chemical reaction, leading to an inaccurate positive reading.
Medications and Supplements
Certain medications and over-the-counter supplements can interfere with the chemical reagents used in bilirubin urine tests, leading to false positive results. Phenothiazines, a class of antipsychotic drugs including chlorpromazine, are known to react with the diazo reagents on test strips, producing a color change that mimics the presence of bilirubin. Similarly, some tricyclic antidepressants can also cause such interferences. Rifampin, an antibiotic primarily used to treat tuberculosis, can also cause a false positive reading for bilirubin. This medication is known to impart a reddish-orange color to urine, which can be mistaken for a positive reaction or directly interfere with the colorimetric test pad. Other compounds, such as certain indoxyl sulfates, can also contribute to false positives by reacting with the test strip’s components.
Dietary Factors and Sample Contamination
Non-medicinal substances, including certain dietary components, can also contribute to false positive bilirubin results in urine. While less common for bilirubin than for other urine analytes, highly pigmented foods or food dyes, if excreted in the urine, could theoretically cause a color change on the test strip that is misinterpreted. For example, large consumption of beets can lead to reddish urine, but direct interference with the specific bilirubin reaction is rare. External contaminants introduced into a urine sample are a more significant source of false positives. Cleaning agents, disinfectants, or other chemicals present in an improperly rinsed collection container can react with the test strip’s reagents. Even trace amounts of bleach or peroxide, if present, can oxidize the test pad, leading to an incorrect color development. Therefore, ensuring the collection container is clean and free of foreign substances is important for accurate results.
Collection and Laboratory Errors
Improper urine sample collection techniques are a common source of inaccurate results, including false positives for bilirubin. If the urine sample is not a “clean catch,” meaning it collects bacteria, skin cells, or other contaminants from the genital area, these substances can potentially interfere with the test. Proper hygiene before collection helps minimize such issues. Errors during the handling or laboratory analysis of the urine sample can also lead to false positive readings. If test reagents on the dipstick or in the laboratory are expired or have been improperly stored, their chemical integrity can be compromised. This degradation can lead to non-specific reactions that yield a positive result even in the absence of bilirubin. Additionally, incorrect technique by laboratory personnel, such as reading the test strip outside the specified time window, can result in misinterpretation of the color change.