Several medications, their metabolites, and even the physical appearance of a urine sample can trigger a positive bilirubin reading on a dipstick test when no clinically meaningful bilirubin is actually present. The most common culprits are drugs that produce deeply colored urine or that chemically mimic bilirubin during the test’s diazo coupling reaction. Understanding these interference sources matters because a false positive can set off unnecessary workups for liver disease, while dismissing a real positive can mean missing an early sign of biliary or hepatic trouble.
How the Dipstick Detects Bilirubin
Standard urine dipstick tests detect bilirubin through a diazo coupling reaction. The test pad contains a diazonium salt that reacts with bilirubin to produce an azo dye, changing the pad’s color. A technician or automated reader compares that color change to a reference scale. Because the reaction depends on color chemistry, anything that adds unexpected color to the urine or that reacts with the diazonium salt in a way that resembles bilirubin can produce a falsely positive result. Automated strip readers reduce some of the subjectivity of visual interpretation, but they are not immune to chemical interference from substances that genuinely trigger the reaction pad.
Medications That Trigger False Positives
Drugs are the single most common source of false positive urine bilirubin. They interfere in two broad ways: by producing metabolites that chemically react with the test pad, or by coloring the urine so intensely that the color change on the strip is misread.
Etodolac
Etodolac, a nonsteroidal anti-inflammatory drug once widely prescribed for arthritis pain, is one of the best-documented offenders. Research has shown that the 6-hydroxylated and 7-hydroxylated metabolites of etodolac are primarily responsible for triggering false positive bilirubin readings on dipstick tests. The reaction is not caused by the parent drug itself but by these breakdown products excreted in urine.1PubMed. Effects of urinary metabolites of etodolac on diagnostic tests of bilirubin in urine If you are taking etodolac and a routine urinalysis flags bilirubin, your provider should consider the drug as a likely explanation before ordering liver imaging or blood work.
Phenazopyridine
Phenazopyridine, sold over the counter as Azo or Pyridium, is the bladder-pain reliever famous for turning urine bright orange. That intense pigmentation alone can fool both human readers and some automated analyzers into calling the bilirubin pad positive. The drug’s metabolites may also react directly with the diazonium salt on the strip. Because phenazopyridine is so commonly used for urinary tract infection symptoms, it is one of the most frequent real-world causes of false positive bilirubin results, and one of the easiest to catch on history alone.
Chlorpromazine and Other Phenothiazines
Chlorpromazine and related phenothiazine antipsychotics have long been recognized as a source of dipstick interference. These drugs and their metabolites can react with the diazo reagent on the bilirubin pad, producing a color change that mimics a true positive. The effect is well enough established that most laboratory reference guides list phenothiazines among the standard interferents for urine bilirubin. For patients on these medications, a single positive dipstick should be interpreted cautiously and confirmed before assuming liver pathology.
Other Medications Worth Knowing About
A handful of other drugs appear on interference lists for urine bilirubin, though the documentation for each is thinner than for etodolac or phenazopyridine. Large doses of certain B vitamins, particularly riboflavin, can deeply color urine and occasionally cause visual misreads. Nitrofurantoin, an antibiotic commonly prescribed for urinary tract infections, produces dark yellow to brown urine and has been flagged as a possible interferent. When any drug that noticeably changes urine color is in the picture, the bilirubin reading deserves an extra layer of skepticism.
What About Rifampin?
Rifampin is worth singling out because it is a source of real confusion. The antibiotic, widely used for tuberculosis, turns urine a vivid orange-red that looks alarming. Many clinicians assume it would interfere with the bilirubin pad, and it does interfere with several other dipstick parameters, including protein, leukocytes, ketones, and nitrite readings. However, a study evaluating rifampin and its metabolite desacetyl-rifampin found that their interference was independent of the bilirubin pad on urine dipstick tests.2PubMed Central. Oral rifampicin interferes with urine dipstick tests for patients with pulmonary tuberculosis In other words, while rifampin wreaks havoc on several dipstick readings, bilirubin is not one of them at the chemical level. That said, an inexperienced reader eyeballing an intensely orange strip might still overcall the bilirubin pad visually, so the distinction between chemical interference and visual misinterpretation matters here.
Specimen Handling and Non-Drug Causes
Not every false positive traces back to a medication. The way a urine sample is collected, stored, and processed introduces its own set of problems.
Highly Concentrated or Colored Urine
Dehydration produces concentrated, dark-amber urine. The background color of that urine can make the bilirubin pad look slightly positive even when no bilirubin is present, especially when a technician reads the strip by eye under imperfect lighting. This is more of a visual misread than a true chemical false positive, but from the patient’s perspective the result is the same: an unexplained positive on the report.
Indican and Dietary Pigments
Indican is a substance that appears in urine when intestinal bacteria break down tryptophan in certain patterns, sometimes seen with bacterial overgrowth or malabsorption. It has been reported to cross-react with bilirubin test pads. Heavily pigmented foods like beets produce colored urine (beeturia) but generally do not chemically trigger the diazo reaction; they can, however, contribute to visual misreads on the strip.
Delayed Reading and Light Exposure
Bilirubin is sensitive to light. Extended exposure of a urine specimen to sunlight or fluorescent lighting breaks bilirubin down, which actually causes false negatives rather than false positives. But the reason this matters in a discussion of false positives is that some of bilirubin’s degradation products, particularly biliverdin, may behave differently on certain strip formulations. The more practical point is that any delay between collection and testing introduces variability, so timing matters for test accuracy in both directions.
How Common Are Unexpected Positive Results?
Most urine bilirubin tests come back negative, and most positives are genuinely positive. A large retrospective review spanning about 20 months and nearly 242,000 urine bilirubin tests found that only about 0.3% of all tests yielded a positive result. Of those positives, roughly 60% came from patients who already had abnormal liver function tests in the preceding two weeks, meaning the bilirubin finding was expected and clinically consistent. The remaining 40% were considered “unexpected positives,” cases where the clinician had no prior reason to suspect liver or biliary disease.3PubMed. Are unexpected positive dipstick urine bilirubin results clinically significant? A retrospective review
Here is where it gets interesting: of those unexpected positives, about 80% had liver function tests ordered within two weeks afterward, and roughly 85% of those tests came back abnormal.3PubMed. Are unexpected positive dipstick urine bilirubin results clinically significant? A retrospective review That is a remarkably high rate. It suggests that while false positives certainly exist, a positive urine bilirubin result is not something to wave away casually, even when it is unexpected. Many of those “surprise” positives turned out to be catching real hepatobiliary problems that had not yet been investigated.
How to Tell a False Positive From a Real One
The dipstick is a screening tool, not a final answer. When a positive bilirubin shows up and the clinical picture does not immediately explain it, a few steps can help sort things out.
The most direct approach is to check serum bilirubin levels with a blood test. A normal total bilirubin and direct (conjugated) bilirubin in the blood makes it extremely unlikely that the urine finding reflects genuine bilirubinuria. Only conjugated bilirubin is water-soluble enough to pass through the kidneys, so if serum conjugated bilirubin is normal, the urine result is almost certainly a false positive.
Historically, a tablet confirmatory test called the Ictotest was used. It employed a more concentrated diazo reagent on an absorbent mat and was considered more specific than the dipstick. The Ictotest could help distinguish a true positive from some forms of drug interference because it used a different chemical concentration and reading method. However, the Ictotest has become harder to find in clinical practice as manufacturers have discontinued it, so most confirmatory testing now defaults to serum chemistry.
A simple medication review is often the fastest path to an answer. If the patient is on etodolac, phenazopyridine, chlorpromazine, or another known interferent, repeating the test after stopping the drug (when clinically appropriate) can confirm that the medication was responsible.
Why Only Conjugated Bilirubin Shows Up in Urine
To understand false positives, it helps to know why the test exists at all. Bilirubin circulates in two forms: unconjugated (bound to albumin, not water-soluble) and conjugated (processed by the liver, water-soluble). Only the conjugated form is small and soluble enough to be filtered by the kidneys and appear in urine. This is why urine bilirubin is specifically a marker of liver or biliary problems that impair the normal excretion of conjugated bilirubin into the gut, such as bile duct obstruction, hepatitis, or infiltrative liver disease. Conditions that elevate unconjugated bilirubin, like hemolytic anemia or Gilbert syndrome, do not produce bilirubinuria because the unconjugated form cannot pass the kidney’s filtration barrier.
This distinction also explains why the dipstick test has clinical value despite its susceptibility to interference: when it is truly positive, it points specifically toward conjugated hyperbilirubinemia, narrowing the diagnostic possibilities. A false positive muddies that signal, which is why understanding interference sources is more than an academic exercise.
Automated Readers Versus Visual Reading
Whether the dipstick is read by a human or by a machine affects the likelihood of certain types of false positives. Early evaluations of automated urine analysis systems found high agreement with manual strip reading. One study comparing an automated instrument with manual dipstick results across nearly 3,000 paired tests found about 98% agreement on whether individual parameters were positive or negative.4Clinical Chemistry. Automatic System for Urine Analysis. II. Evaluation of the System That 2% disagreement rate includes all tested parameters, not just bilirubin, but it illustrates that no reading method is perfect.
Automated readers generally reduce the problem of intensely colored urine being visually overcalled, because they use reflectance photometry rather than the human eye. However, they are equally vulnerable to chemical interference, since the problem is the reagent reaction itself, not how the color is interpreted. If a drug metabolite triggers the diazo reaction on the pad, the machine will read the resulting color change as bilirubin just as faithfully as a technician would. The practical upshot: automated systems help with some visual artifacts but do not solve the medication interference problem.
Common Misconceptions About Urine Bilirubin
A few misunderstandings circulate widely enough to be worth addressing directly.
One is that vitamin C causes false positive bilirubin. It does not. Ascorbic acid is actually a well-known cause of false negatives on several dipstick parameters, including bilirubin. High urinary ascorbic acid can suppress the diazo reaction, causing a truly positive specimen to read as negative. If anything, vitamin C makes you less likely to see a positive, not more.
Another misconception is that a positive urine bilirubin is a reliable standalone indicator of liver disease. The data from the large retrospective study discussed earlier show that while most unexpected positives do correlate with abnormal liver function when checked, about 15% of the time the follow-up liver tests came back normal. A single dipstick result should prompt further evaluation, not a diagnosis.
A third confusion involves urobilinogen, the other bilirubin-related analyte on many dipstick strips. Urobilinogen is a breakdown product of bilirubin that is produced by gut bacteria and partially reabsorbed. It is always present in small amounts in normal urine. Some people assume that a positive urobilinogen means the same thing as a positive bilirubin, but they point in different diagnostic directions. Elevated urobilinogen without bilirubin suggests increased bilirubin production (as in hemolysis) or impaired hepatic uptake. Positive bilirubin without elevated urobilinogen suggests biliary obstruction. The two analytes are complementary, not redundant, and confusing them can lead to misinterpretation of what the dipstick is actually saying.
Practical Steps When You See an Unexpected Positive
If your urinalysis report shows positive bilirubin and you have no known liver or gallbladder condition, here is a reasonable approach. First, review your medications and supplements, paying special attention to anything that changes the color of your urine. Over-the-counter bladder analgesics like phenazopyridine are easy to forget to mention. Second, consider whether the specimen was collected under good conditions: was it a fresh sample, protected from prolonged light exposure, and tested promptly? Third, ask your provider whether confirmatory testing makes sense. In most cases, a simple set of liver function tests (a blood draw checking enzymes like ALT, AST, alkaline phosphatase, and serum bilirubin) will either confirm a real problem or rule it out quickly.
The worst response to an unexpected positive bilirubin is to ignore it entirely. The evidence suggests that the majority of even unexpected positives, when investigated, do turn out to reflect genuine hepatobiliary abnormalities.3PubMed. Are unexpected positive dipstick urine bilirubin results clinically significant? A retrospective review A brief medication review and a blood test can sort out the false positives from the clinically important ones without much cost or effort.
Substances That Cause False Negatives Instead
Because the topic of urine bilirubin accuracy inevitably raises the opposite question, it is worth a brief mention. False negatives, where bilirubin is genuinely present but the test misses it, are caused by a different set of factors. Prolonged light exposure degrades bilirubin in the specimen before testing. High concentrations of ascorbic acid inhibit the diazo reaction. Very dilute urine may have bilirubin levels below the test’s detection threshold even when serum conjugated bilirubin is mildly elevated. Outdated or improperly stored dipstick strips can also lose reagent sensitivity over time. For someone being monitored for liver disease, a negative dipstick does not guarantee the absence of bilirubinuria if any of these conditions apply. A serum bilirubin level remains the gold standard when precision matters.