A-111 There is More to Diluting Icterus Interference than Meets the Eye
Authors: Lasho M A , Baumann N A , Block D R 
Published: September 2023

Background

Total protein is measured in serum and body fluids as part of routine evaluations of general nutritional status and differentiating exudative and transudative effusions. Spectral interference thresholds for bilirubin (e.g., icterus) is set by assay manufacturers to prevent inaccurate results from being reported. The Roche Cobas serum total protein assay package insert states no significant interference from bilirubin up to an icterus (I) index of 20. Body fluids tend to have lower total protein concentrations; therefore, thresholds were derived during prior validation studies and set to I = 10, a threshold that is exceeded routinely. Laboratories can mitigate some interferences by recollecting (e.g., hemolysis) or ultracentrifuging (e.g., lipemia) specimens. Serial dilutions are another option used; however, it is imperative that the laboratory understands the mechanism of interference and can demonstrate that it is mitigated by sample dilution. The aim of this study was to investigate whether bilirubin interference can be mitigated by sample dilution in pleural fluid, peritoneal fluid, and serum specimens.

Methods

Residual clinically ordered pleural fluid (n = 3), peritoneal fluid (n = 3), and serum (n = 3) samples submitted for protein testing were split into two equal volume aliquots with concentrations ranging 2.7 to 4.3 g/dL in pleural fluids, 3.9 to 5.4 g/dL in peritoneal fluids, and 3.8 to 11.0 g/dL in serum. One aliquot was spiked (&lt;10% by volume) with a bilirubin conjugate solution (∼1000 mg/dL) to surpass the icterus index threshold (spiked). The second aliquot was spiked with an equal volume of 0.9% saline (control). Total protein was measured on the Roche Cobas c701 instrument in both samples to calculate difference (Spiked-Control). The mean(SD) difference was calculated for each sample type. Each spiked sample was serially diluted (2-fold, 4-fold and 8-fold) with 0.9% saline. Total protein was measured for each diluted sample. Mean(SD) difference was calculated (DilutedX-Control), where X = 2-fold, 4-fold, 8-fold. Linear regression analysis was performed by plotting the measured (x-axis) vs expected (y-axis) total protein concentration. Slope, y-intercept, and R2 were determined from serially diluted samples where the expected concentrations were derived using the spiked concentration to replicate the practice of diluting an icteric sample.

Results

Bilirubin spiking (average icterus index = 33.5) into pleural fluid, peritoneal fluid, and serum, respectively demonstrated mean(SD) bias (g/dL) = −1.0(0.2), −0.9(0.0), and −0.7(0.0). Subsequent 2-fold dilution demonstrated mean(SD) bias (g/dL) = −1.0(0.2), −1.0(0.0), and −0.8(0.1), 4-fold dilution with mean(SD) bias (g/dL) = −1.1(0.2), −1.0(0.0), and -1.0(0.1), and 8-fold dilution with mean(SD) bias (g/dL) = −1.3(0.2), −1.0(0.0), and −1.4(0.1). Diluting pleural, peritoneal, and serum samples, respectively spiked with bilirubin resulted in slope = 0.99, 1.00, and 0.99; y-intercept = 0.04, 0.01, and 0.07; R2 = 0.99, 0.99, 0.99.

Conclusion

The dilutions exhibited a linear dilution response, however the dilution results never returned to baseline (non-spiked) concentration. Laboratories should exercise caution when conducting serial dilutions to diminish bilirubin interference in samples when measuring total protein as the mechanism of interference does not appear to be reversible by dilution. This behavior was demonstrated in both body fluids and serum.