I index is not an accurate indicator of icteria in conjugated hyperbilirubinemia

Serum indices have become a standard in assessing degree of endogenous interferences in serum and plasma samples. The aim of this study was to evaluate accuracy of I index in comparison with total bilirubin concentration in icteric samples with ranging amount of conjugated bilirubin.

This study retrospectively analyzed data from laboratory information system. Total, conjugated bilirubin and I index are measured on Abbott Architect c8000 (N=900). Agreement between total bilirubin and I index in subgroups according to proportion of direct bilirubin (<50% and ≥50%) was investigated using Bland-Altman analysis.

In samples where percentage of direct bilirubin accounts for <50% of total bilirubin there was no statistically significant constant difference, while small proportional difference was observed (2.5%) between total bilirubin and I index. In samples where percentage of direct bilirubin accounts for ≥50% of total bilirubin, significant constant (26.6) and proportional difference (22.2%) were observed between total bilirubin and I index.

I index is not accurate indicator of icteria if >50% of bilirubin is conjugated. Manufacturers should declare icteria interference with both, bilirubin concentration and value of I index.