Clinical and Laboratory Standards Institute guideline EP07—Interference Testing in Clinical Chemistry is intended to promote uniformity in the evaluation of interference characteristics of medical laboratory measurement procedures. EP07 describes procedures to screen potential interferents, quantify interference effects, and confirm interference in patient samples. This guideline also describes procedures for medical laboratories to verify interference claims and investigate discrepant results caused by unsuspected interferents. Detailed examples are given. EP07 also contains background information on interference testing concepts. Tables of recommended test concentrations for potential interferents can be found in the supplement, CLSI document EP37.

This guideline replaces the previous edition of the approved guideline, EP07-A2, published in 2005. Several changes were made in this edition, including:

• Improved the process for conducting drug screening and characterization to make it simpler and easily performed

• Reviewed and updated the statistics used in determining interference

• Updated the appendixes, including clarifying their purpose and function

• Moved former Appendixes C (Interferent Test Concentrations) and D (Interference Test Concentrations for Endogenous Analytes) to the new supplement, CLSI document EP37, so they may be updated more frequently

This guideline is intended for manufacturers and medical laboratories, for two purposes:

• Assist manufacturers and other developers of laboratory measurement procedures in characterizing the effects of potential interferents on measurement procedures results by providing information on:

– Relevant interferents and concentrations to be tested

– Likely effects of the interferent on the concentration of the measurand of interest (ie, no effect, positive effect, or negative effect)

– Scientifically valid experimental designs

– Appropriate data analysis and interpretation

– Stating meaningful interference claims

• Assist medical laboratories in investigating discrepant results that may be due to interferents by:

– Defining a systematic investigation strategy

– Specifying data collection and analysis procedures

– Promoting greater cooperation between laboratory scientists and manufacturers so that new interferents are identified, disclosed, and ultimately eliminated

Any measurement procedure, quantitative or qualitative, may be subject to interference. This guideline is written for a broad spectrum of measurement procedures and measuring systems, with primary focus on quantitative methods and qualitative methods with interpretation based on numeric values. Modification may be necessary to accommodate the particular characteristics of the procedure being evaluated. Measurement procedures that use serum, plasma, whole blood, cerebrospinal fluid, urine, and most other body fluids can be evaluated for interferents using this guideline.

EP07 and its supplement, CLSI document EP37, are not meant to include a complete list of interferents to be tested and do not stipulate that all potential interferents included in CLSI document EP37 are to be tested. However, EP07 and CLSI document EP37 are intended to provide a solid starting point for assessing interference effects. This guideline is limited to testing potential interference from chemical substances that may be exogenous (eg, drugs) or endogenous changes in concentrations of substances caused by disease processes (eg, bilirubin, lipoproteins).

This guideline does not cover potential interference from physiological conditions (eg, pregnancy, diurnal effects) or effects from environmental conditions (eg, heat, sunlight).