Redefining Linearity Testing in Today’s Clinical Laboratory

Current legislative and regulatory directives now require all clinical laboratories to perform specific testing protocols under the heading of calibration verification to ensure the accuracy of their diagnostic testing systems. Although these guidelines have been in effect for quite some time, many experienced administrators and lab personnel still express confusion when it comes to linearity, analytical measurement range, and reportable range.

This confusion is not surprising since federal legislation under the Clinical Laboratory Improvement Amendments (CLIA) uses one set of terms and definitions, while other industry and regulatory bodies use others. Thus, laboratorians need to understand how CLIA terms differ from others, as the differences can directly impact laboratory inspections and compliance. In fact, failure to perform these necessary testing protocols, or doing so incorrectly, is one of the top five reasons for a laboratory to be served with a noncompliance citation.

What is linearity testing?

According to the Clinical Laboratory Standards Institute EP06-A Guideline, linearity in the clinical laboratory is defined as the ability to provide results that are directly proportional to the concentration of the component in the test sample, within a given range. However, the analyte concentration versus the measurement signal is not always a straight-line linear reaction.

Looking at the clinical significance and diagnostic history behind this concept, it is quite surprising that the term “linearity” does not appear in the CLIA directives and is not even separately defined. Instead, the concept of “reportable range” attempts to address this.

Reportable range and analytical measuring range

Per CLIA, the reportable range is defined as the span of test results, low-end to high-end, that the laboratory can accurately verify for a given assay on a clinical testing system. An important aspect in verifying the reportable range is that a plot of measured values from test samples versus their assigned concentration or relative concentrations must be “linear” within defined acceptance criteria.

Here, linearity is outlined as the relationship between the final analytical result for a measurement and the concentration of the analyte being measured. This distinction is relevant because a plot of analyte concentration versus measurement signal from the instrument may not be linear.

To complicate things, the College of American Pathologists (CAP) further delineates reportable range as the “analytical measuring range,” meaning the range of numeric results a method can produce via the normal measuring process, and without any special specimen pretreatment, such as a dilution.

Inconsistency in defining linearity

The most important distinction between the CLIA and CAP definitions of linearity is how each entity refers to reportable range. CLIA uses the term “within calibration verification” to refer to the span of test result values over which the laboratory can establish or verify the accuracy of the instrument or test system measurement response. Conversely, CAP uses the term “analytical measuring range” to describe the range of analyte values that a method can directly measure on the specimen without any dilution, concentration, or other pretreatments not part of the usual assay process.

Furthermore, CAP segments analytical measuring range from calibration verification requirements through a separate analytical measuring range verification process, while CLIA captures both requirements within calibration verification and defines linearity as a straight-line relationship between observed values and expected values.

In a nutshell, linearity is taken as the method used to determine the analytical measuring range verification. Hopefully, better defining these concepts provides you with a clearer understanding of the current terminology.

References:

1. CLIA ’88: Section 493.2 – Definitions.
2. CLIA Brochure No. 3: www.cms.hhs.gov/clia.
3. CLSI Document EP6-A: Evaluation of the Linearity of Quantitative Measurement Procedures – A Statistical Approach. Approved Guideline.
4. www.cap.org/apps/docs/laboratory_accreditation/audio_conferences/cvl_webinar_presentation.pdf.
5. Centers for Medicare & Medicaid Services, Department of Health and Human Services. Medicare, Medicaid, and CLIA programs; laboratory requirements relating to quality systems and certain personnel qualifications; final rule [published correction appears in Fed Register 2003;68(163):50722–50725]. Fed Regist. 2003; 68(16):3707–3714. Codified at 42 CFR §493.2.
6. Centers for Medicare & Medicaid Services, Department of Health and Human Services. Medicare, Medicaid, and CLIA programs; laboratory requirements relating to quality systems and certain personnel qualifications; final rule [published correction appears in Fed Regist2003;68(163):50722–50725]. Fed Regist. 2003; 68(16):3707–3714. Codified at 42 CFR §493.1255.
7. College of American Pathologists, Commission on Laboratory Accreditation. Chemistry and Toxicology Checklist. Northfield, IL: College of American Pathologists; 2012.