Icteric human samples: Icterus index and method of estimating an interference-free value for 16 biochemical analyses

Abstract

Background: Hemolysis, Icterus, and Lipemia constituting the HIL index, are the most common causes of interference with accurate measurement in biochemistry. This study focuses on bilirubin interference, aiming to identify the analyses impacted and proposing a way to predict nominal interference-free analyte concentrations, based on both analyte level and Icterus Index (I_ict ).

Methods: Sixteen common analytes were studied: alanine aminotransferase (ALT), albumin (ALB), alkaline phosphatase (ALP), amylase (AMY), aspartate aminotransferase (AST), total cholesterol (CHOLT), creatinine (CREA, enzymatic method), fructosamine (FRUC), gamma-glutamyl transferase (GGT), HDL cholesterol (HDLc), total iron (Iron), lipase (LIP), inorganic phosphorus (Phos), total protein (PROT), triglycerides (TG), and uric acid (UA). Both the traditional 10% change in concentrations from baseline and the Total Change Level (TCL) were taken as acceptance limits. Nineteen pools of sera covering a wide range of values were tested on the Cobas® 6000 (Roche Diagnostics). I_ict ranged from 0 to 60.

Results: Eight analytes increased (FRUC and Phos) or decreased (CHOLT, CREA, HDLc, PROT, TG, and UA) significantly when I_ict increased. FRUC, HDLc, PROT, and UA showed a linear relationship when I_ict increased. A non-linear relationship was found for TG, CREA, and for CHOLT; this also depended on analyte levels. Others were not impacted, even at high I_ict .

Conclusions: A method of estimating an interference-free value for FRUC, HDLc, PROT, Phos, UA, TG, and CREA, and for CHOLT in cases of cholestasis, is proposed. I_ict levels are identified based on analytical performance goals, and equations to recalculate interference-free values are also proposed.

Keywords: bilirubin; biochemistry; correction; icterus index; interference.

Figure 1

Relation between I _ict and TBIL (μmol/L; see abbreviation in text). A linear relation is observed between I _ict and TBIL. As I _ict is always available (and not TBIL), a relation between I _ict and the interference on the analytes should be studied

Figure 2

Relation between I _ict and analytes. Observed variation before D (%) and after correction Z (%), allowable cut‐off as ±TCL (%) and equations between I _ict and D (%) are shown for PROT (A), AU (B), Phos (C), HDLc (D),TG (E), CREA (F), CHOLT (G) and FRUC (H). CHOLT is subdivided from <3 mmol/L to >5 mmol/L. The interference with CHOLT depends on level of this analyte. For CHOL (G), Z (%) is valid only when CHOLT >4 mmol/L and I _ict≤40. In this last case (G), equation is 8.38 × 10⁻³ − 5.41 × 10^−3x (see abbreviation in text)