Determining standards for confirmatory testing in Urine Bilirubin requires further research
The clinical importance of urinalysis has been well documented as a useful tool in aiding with the diagnosis of various medical conditions. Urinalysis test strips and urine chemistry analyzers are now ubiquitous devices in physician’s offices and medical labs alike. However, as routine as urinalysis has become today, the use of urine confirmatory tests (such as confirming the presence of urine bilirubin) remains inconsistent in the current laboratory testing practice. [see Fig.1]
To date, there is no approved standard or guideline regarding confirmatory testing for the presence of bilirubin in urine. Neither the Clinical and Laboratory Standards Institute nor the College of American Pathologist put forth peer-reviewed and approved guidelines regarding the practice of confirmatory testing. Yet, given the serious health implications that urine bilirubin indicates, perhaps a comprehensive study of this practice is warranted in order to form a definitive standard once and for all.
Importance of Urine Bilirubin Testing
Bilirubin is formed by the breakdown of hemoglobin in the reticuloendothelial cells of the spleen and bone marrow. It is linked to albumin in the bloodstream and transported to the liver. This albumin-bound form, which is also known as indirect bilirubin, is insoluble in water and does not appear in the urine. In the liver cells, however, it is separated from the albumin and conjugated with glucuronic acid to form water-soluble conjugated bilirubin, also known as direct bilirubin. The liver cells that form the conjugated bilirubin excrete it into the bile, where it is then excreted into the intestinal tract through the bile duct. Presence of bilirubin in the urine is indicative of hepatocellular disease or intra- or extra-hepatic biliary obstruction. More simply, it is an early sign of liver disorders and therefore a useful diagnostic tool.
Currently, there is no quantitative urine bilirubin test method on the market. The only way to rule out urine-strip false-positive bilirubin results is with a confirmatory assay such as Ictotest reagent tablets.
Impact of False Positive Tests
If a positive bilirubin result occurs, a physician will typically move to another round of diagnostic testing to gather a more specified picture of the patient’s health. In the event of a false-positive, none of that additional testing would be warranted. These additional tests (such as a quantitative blood test) can unnecessarily tax healthcare resources and cause undo patient concern and inconvenience. In the event that a positive bilirubin result is obtained using a urine chemistry dipstick, for example, additional tests can be performed using the same urine sample with a confirmatory product — such as Ictotest reagent tablets, which is based on the diazotization reaction and has been used since the 1950s. Alternatively, the healthcare professional can also obtain a blood sample to achieve a fully quantitative result via a laboratory method.
Obtaining a positive urine chemistry result, but a negative bilirubin result with a confirmatory test or a lab method would indicate a false-positive. There are certain conditions under which false-positive results could occur. For example, the presence of metabolites (phenazopyridine) may mask the reaction of small amounts of bilirubin and chlorpromazine in large amounts may also give a false positive result (refer to the figure with the summary of the interferances).
More Research Needed Why Confirmatory Tests Matter
The primary reasons for ordering a semi-quantitative urine bilirubin test are that it is less invasive, less costly and most urine bilirubin tests are CLIA-waived meaning results can be interpreted by non-professionals. Moreover, semi-quantitative test can be collected at the point-of-care and a result can be obtained from a sample in about 30 seconds — meaning a physician can almost immediately discuss the result with the patient and begin outlining the next clinical and/or diagnostic steps. Semi-quantitative urine bilirubin tests are also typically performed via a urine-dipstick, which can contain 9 or 10 other parameters (e.g. pH, Albumin, Glucose, Blood, etc.) — which can provide more information about a patient’s overall condition beyond what the presence of bilirubin could on its own.
As mentioned above, confirmatory tests for urine bilirubin have existed since the 1950s, yet a literature search for studies on the topic is met with a dearth of research. No one would dispute the importance of testing for the presence of bilirubin given its role in the evaluation of liver function. However, there is no firm consensus as to the best method for confirmatory testing. What is needed then is a thorough study investigating the rate of false-positive bilirubin results to determine the necessity of a confirmatory test. An examination of the cost-benefit of semi-qualitative vs. quantitative confirmatory tests would also help to inform new guidelines around the practice of bilirubin testing. Until such data is presented, the standards for confirmatory testing will be anybody’s guess.
- Free, A.H. and Free, H.M.: Urinalysis, Critical Discipline of Clinical Science. 1972 CRC Crit. Rev. Clinical Lab Science. 3(4): 481-531
- Bonnardeax A, Somerville P. A study on the reliability of dipstick urinalysis. Clin Nephrol. 1994; 41 (3): 167-172