Pradip Datta PhD DABCC

Contents

1. Introduction

2. Mechanism of Interference

3. Interference of Various Agents

4. False Results Caused by Systems Issues

5. How to Detect and Correct Interferences

6. Removal of Interfering Substances

7. Conclusion

Among the endogenous interferents affecting assay results, the most common are bilirubin, hemoglobin, lipids, and paraproteins. These interferents may affect therapeutic drug monitoring (TDM), drugs of abuse (DAU) testing, and toxicology assays of any format where the sample is used directly for analysis without any pretreatment of specimen. Immunoassays are commonly used in clinical laboratories where analyte-specific antibody or binding agents are used to estimate the analyte concentration in the specimen. Some enzyme and chemistry assays are also utilized in TDM and DAU analysis. Such assays use various types of signals, the most common being colorimetry, fluorimetry, and chemiluminescence. Assays may be prone to interference depending on the format or label used. Commercial assay kits report the result of such interference in the kit inserts (up to levels of >20mg/dL bilirubin, >500mg/dL hemoglobin, and >1000mg/dL lipids). The interference is caused by the optical, fluorescent, or chemiluminescent properties of these interferents. Thus, bilirubin interferes by its absorption and fluorescence properties, hemoglobin by its absorption, fluorescence and chemiluminescence properties, and lipids interfere mostly from their light-scattering (turbidity) properties. Bilirubin and hemoglobin may also interfere because of side reactions in the assay. Modern auto-analyzers can detect all three interferents and flag the results. Flagged results should be carefully reviewed for the accuracy. Both hypo- and hyper-proteinemia can affect assay results. Paraproteins interfere in many assays by precipitating out during the sample blanking step thus producing false results. Another source of interference may be from probe (sample or reagents) or reaction cuvettes carryover contamination in random-access auto-analyzers. If the validity of test results

Summary

From: Handbook of Drug Monitoring Methods Edited by: A. Dasgupta © Humana Press Inc., Totowa, NJ

is questioned, the assay should be repeated either by removing the interferent from the sample or by using different method which is known to suffer less from that type of interference.

Key Words: Bilirubin; hemoglobin; lipids; interference; assays.

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