Measurement of icteric index as approach to detect abnormal total bilirubin values

Abstract

We read with great interest the article by Salinas et al 1 suggesting the use of the icteric index (II) as a front-line test for the preliminary identification of blood samples with abnormal total bilirubin (TB) concentrations. Following the authors' suggestion, we were encouraged to experimentally confirm their findings in our own hospital setting. Particularly, in our laboratory we use platforms (Cobas c 501 and Integra 800) and reagents of the same vendor (Roche Diagnostics) used by Salinas et al, and we think that our findings could contribute further evidence to their results, even if obtained with instruments of different series.We evaluated all the measurements of TB requested to our laboratory between January and December 2011 and their corresponding II values. TB concentrations and II were measured on Cobas c 501 using serum samples (routine) and on Integra 800 using lithium-heparin plasma samples (stat). Both systems use for TB the same diazo-based colorimetric assay, and for the II determination, an assay based on calculations of absorbance measurements of diluted samples. Statistical evaluation, including linear regression analysis, calculations of sensitivity, specificity, positive (LR+) and (LR−) negative likelihood ratios, positive predictive value (PPV) and negative predictive value (NPV) was performed for both serum and plasma samples, separately, by applying the same cut-off values used by Salinas et al, that is, 1.2 mg/dl (20.5 μmol/l) for TB concentrations and 2 for II. The analysis was Downloaded from *TB >1.2 mg/dl and icteric index <2. †TB ≤1.2 mg/dl and icteric index ≥2. LR+, positive likelihood ratio; LR−, negative likelihood ratio; NPV, negative predictive value; PPV, positive predictive value.

Utility of icteric index in clinical laboratories: more than a preanalytical indicator

“…Regarding the second objective of this study, we set I-index cut-off with the highest specificity and lowest false positive results, finding similar results than others studies with I-index ≥ 34.2 µmol/L (16)(17)(18). Hyperbilirubinemia diagnosis could be important in these cases to know the subjacent aetiology, such as Gilbert syndrome, hepatocellular damage, biliary obstruction, haemolytic anaemia, or others (12).…”

“…To our knowledge, this is the first study that evaluates I-index on Alinity c. It is well known that the icteric index is correlated to total bilirubin but we checked it in our platform (16)(17)(18)(19). Several studies have been published with the objective of reducing number of TBil tests with two main platforms, Roche and Abbott (16)(17)(18)(19)(20)(21). An interesting study of HIL index in these two platforms showed an unacceptable comparability between them (15).…”

“…We found differences in sensitivity and FN results at the same cut-off between inpatients and outpatients. A reason why we can find these differences was the utilization of serum and plasma samples (17). In our laboratory, serum samples are usually used in primary care and outpatients while plasma samples in emergency room and inpatients.…”

Implementation of an internal quality control programme for the photometric determination of icteric index

“…An additional interesting use of the determination of II, first proposed by Salinas et al ,2 is its use as front-line test for the identification of blood samples with abnormal total bilirubin (TB) concentrations. The application of an optimal cut-off for II that reliably identifies abnormal TB concentrations allowed the accurate ‘zero-cost’ detection of samples with normal TB concentrations (ie, ≤0.012 g/L or ≤20.5 µmol/L), avoiding direct measurements in ~40% of bilirubin orders 3. In our laboratory, the use of II for the screening of hyperbilirubinaemic samples was validated in 2015 using the Abbott Architect c16000 analytical system and, since June 2016, serum and plasma samples with a request of TB determination and an II ≤0.8 are automatically reported as having a TB concentration ≤0.012 g/L (≤20.5 µmol/L), without any further measurement 4.…”