American Association for Clinical Chemistry
Better health through laboratory medicine
NACB - Scientific Shorts
NACB - Scientific Shorts (formerly NACB Blog)
By Angela M. Ferguson, Ph.D., DABCC, FACB
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Clinical Impact of the BuBc Slide RecalibOrtho Clinical Diagnostics’ method of measuring bilirubin is unique when compared to other commonly used methods.  The difference between the methods is the way different bilirubin fractions are measured or calculated.  Total bilirubin is measured with the TBIL slide that measures both conjugated (including delta bilirubin covalently bound to albumin) and unconjugated bilirubin species.  Most other methods calculate the concentration of unconjugated bilirubin by subtracting the direct reacting fraction containing the conjugated species from the total value of bilirubin.  Ortho platforms use a separate slide, BuBc, that directly measures both conjugated (excluding delta bilirubin) and unconjugated fractions.  The BuBc slide also has an ultrafiltration layer that excludes large molecules, such as hemoglobin, minimizing interference from hemolysis.  The ultrafiltration layer also excludes albumin, which is why the BuBc slide does not measure delta bilirubin, but it allows the calculation of delta bilirubin (TBIL-BuBc). 

Since bilirubin concentrations can be measured using both the TBIL and BuBc slides, there are different options for how values are reported.  To match with other bilirubin methods, our institution reported the total bilirubin value from the TBIL slide, the unconjugated bilirubin value from the BuBc slide, and a calculated conjugated value derived from subtracting the unconjugated value from the total (measured Tbil-measured Bu= Bc (calculated).)  This allows any delta bilirubin that may be present in the patient to be accounted for in the conjugated value.  

In May of 2012, Ortho notified customers that due to complaints about a positive bias in proficiency testing results, there would be an adjustment in the calibrator values for the BuBc slides.  There was no accompanying change to the TBIL slides.  After the recalibration of the BuBc slides, we began receiving calls from clinicians stating that they were seeing an increase in patients, especially neonates, with elevated conjugated bilirubin that necessitated consults with gastrointestinal specialists to rule out conditions such as biliary atresia.  The week prior to the recalibration, 3.8% of conjugated bilirubin results were elevated (>0.6 mg/dL), but the week after recalibration, the rate increased to 35%.  It was determined that these calculated values were falsely elevated due to the lowering of the values from the BuBc slide with no change in the values reported from the TBIL slide. 

Ortho Diagnostics pulled bilirubin data from our analyzers before and after the recalibration occurred, examined the median Bu values before and after the recalibration, and compared that to the change that they expected to see due to the recalibration. Our data showed that the change in measured Bu values was similar to what was expected for total bilirubin values from 0-18 mg/dL, but much more than they had expected in the range of total bilirubin 18-27 mg/dL, with an observed change of 7.7 mg/dL compared to the 2.1 mg/dL expected change. 

During our investigation and consultation with Ortho Diagnostics, the laboratory began to report out all measured values from the two slides.  The new reporting strategy led to concerns from our hepatology team, since delta bilirubin was no longer included with the conjugated bilirubin value, the values were lower, and this complicated the interpretation of results in long-standing patients.  In addition to comments from clinicians that the conjugated bilirubin results were too low, we also received comments that the bilirubin values did not match those from outside hospitals, and the sum of the conjugated and unconjugated bilirubin no longer equaled the total bilirubin value.

Working together with both the neonatologists and the gastroenterologists, the laboratory devised a new reporting scheme for bilirubin results based on the age of the patient.  Bilirubin values for neonates 21 days and younger were reported as follows: Measured Bu + measured Bc= Total (calculated).  For patients older than 21 days, we returned to using our previous formula: Measured Tbil-measured Bu= Bc (calculated). This strategy has solved the problem of falsely elevated direct bilirubin values in neonates and bilirubin values are more in line with those from outside hospitals.  We have also continued to share data with Ortho Diagnostics to determine if the recalibrated BuBc slides had been adjusted too much. 

Is this the best solution for reporting of pediatric bilirubin from the Ortho platform?

 

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