| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Letters to the Editor |
Department of Pathology, University of Virginia, Health System, Charlottesville, VA
aAddress correspondence to this author at: Department of Pathology, P.O. Box 800214, University of Virginia Health System, Charlottesville, VA 22908. Fax 434-924-2151; e-mail lmh5p{at}virginia.edu.
To the Editor:
Abbott Laboratories recently supplied a new reagent for total bilirubin (catalog no. 6L45-20) for use with the Architect cSystems analyzer. The reagent uses 2,4-dichlorophenyl diazonium (2,4-DCPD) and is described as minimizing interference from hemoglobin, although interference from indican was reported to be higher than with the previous reagent (1). Our initial comparison of the new and previous reagents yielded a regression equation with a slope of 1.05 (Fig. 1
, upper panel, 
) and similar imprecision (not shown) for the 2 reagents. After introduction of the new reagent into routine use, however, bilirubin results in renal dialysis patients were noted to be higher than with the previous reagent. Among 
512 predialysis samples received during a 3-day period from patients on renal dialysis, 43% had bilirubin values above the upper limit of the reference interval (3–13 mg/L). For most of these patients, bilirubin concentrations reported within the previous 1–2 months had been within the reference interval.
|
We measured bilirubin with both the old and new reagents in a group of predialysis renal patient plasma specimens (Fig. 1
, upper panel, 
). The slope of the Deming regression equation was significantly higher in the renal group than it was in the initial method-comparison study using unselected leftover laboratory specimens (1.33 vs 1.05; unpaired t-test, P <0.0001). Interestingly, with the new reagent the absorbance continued to increase after the first minute in renal dialysis samples, but not in nonrenal samples (Fig. 1
, middle panel).
The findings suggested interference from indican, a metabolite that increases in uremia (2). After addition of indican, total bilirubin results with the 2,4-DCPD and 2,5-DCPD methods were reported to increase by 50 and 33 mg/L per mmol/L of indican, respectively (2). Abbott reported(1) that, with the new reagent, the bilirubin increased by 15 mg/L for 0.25 mmol/L of added indican as compared to a 17 mg/L increase for 0.50 mmol/L of indican using the old reagent. Indican concentrations up to 0.38 mmol/L have been observed in predialysis serum samples from renal failure patients (3).
To test the effect of indican, we added indican (indoxyl sulfate; Sigma-Aldrich) to a plasma pool generated from patients with normal renal function. The time course of absorbance for the new Abbott bilirubin assay matches that seen during analysis of plasma from dialysis patients. In the absence of added indican, near-maximum absorbance was achieved with the new reagent at 0.9 min (Fig. 1
, lower panel) and remained relatively constant through the rest of the 5.1-min period during which the analyzer monitored the reaction. In the presence of 0.4 mmol/L of added indican, the absorbance was higher at 1 min and continued to increase for at least 5 min (Fig. 1
, lower panel).
As suggested by McPhaul et al. (4) in 1985, indican interference may be markedly reduced by the use of an early reading time of 1.7 min (4). As shown in Fig. 1
(middle panel), we expected that the influence of indican would be decreased by 75% if we used a reading time of 1.2–2.4 min rather than the manufacturer’s recommended time of 4.2–5.1 min (Fig. 1
, middle panel). The predialysis specimens from 223 renal dialysis patients with abnormally high bilirubin values, as measured with the new reagent (reading time, 4.2–5.1 min), were reanalyzed using both the early (1.2–2.4 min) and late (4.2–5.1 min) reading times for the new reagent. Total bilirubin values decreased by a mean (SD) of 9.0 (2.0) mg/L with the early reading time, and 97% of these values were then within the reference interval. Use of the earlier reading time did not affect day-to-day imprecision (CV, 2.9% and 2.7% at 61 mg/L for late and early reading times; n = 36).
The problem of indican interference in 2,4-DCPD and 2,5-DCPD methods for total bilirubin measurement, reported more than 30 years ago (2)(3), continues to be problematic. This report is intended to remind the laboratory community to evaluate the effect of indican in new formulations of these reagents. Fortunately, the simple solution proposed by McPhaul et al. (4) more than 2 decades ago still appears to be effective for decreasing indican interference in current bilirubin assays.
Acknowledgments
No support from the manufacturer was received for this work. Dr. Boyd has received past grant support from Abbott Diagnostics for method and instrument evaluations and has served as a speaker and consultant for Abbott Diagnostics.
References
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
