Easing Regulatory Concerns about Linearity Testing in Clinical Chemistry Labs

Robert J. Janetschek, MSc, MT(ASCP), is a veteran in the clinical laboratory space with over 40 years of relevant experience. Aside from direct clinical involvement, he has also held marketing, business development, and technical leadership positions with several in vitro diagnostics organizations. Janetschek is currently director of business development at Verichem Laboratories Inc., an industry-recognized leader in the field of clinical reference materials and calibration verification testing.

Current legislative and regulatory directives now require all clinical laboratories to perform specific testing protocols under 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 certain terminology found in the regulations. Here are answers to a few common questions about linearity testing in clinical labs.

Q: Is it true that clinical laboratories are still cited for noncompliance issues related to CLIA and other industry-related regulations?

A: Yes. Unfortunately, noncompliance issues rank as one of the top five reasons for a laboratory to be served a citation. Failure to perform the aforementioned testing protocols, or performing them incorrectly, is reported as the primary reason. In fact, many experienced administrators and lab personnel still express confusion when it comes to linearity, analytical measurement range, and reportable range.

This could be because federal legislation under the Clinical Laboratory Improvement Amendments (CLIA) uses one set of terms and definitions, while other industry and regulatory bodies use others. Laboratorians need to understand the differences between all these terms and definitions as this directly impacts laboratory inspections and compliance.

Q: Linearity is not directly referenced in any of the CLIA regulations, yet labs are required to perform testing equivalent to a linearity check. Why is that?

A: Based on the clinical significance and diagnostic history behind the concept, it is quite surprising that the term “linearity” does not appear in the CLIA directives and is not separately defined. Instead, the term and concept of “reportable range” attempts to address this, and like linearity, refers to the relationship between the final analytical result for a measurement and the concentration of the analyte being measured.

Linearity in the clinical laboratory is still defined as the ability to provide results that are directly proportional to the concentration of an analyte in the test sample within a given range. However, the analyte concentration versus the measurement signal is not always linear.

Q: The term “analytical measuring range” is used when discussing linearity. What is the difference between reportable range and analytical measuring range?

A: 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.

The College of American Pathologists (CAP) further delineates reportable range as the analytical measuring range, i.e., the range of numeric results a method can produce via the normal measuring process and without any special specimen pretreatment, such as a dilution.

Q: Why is there a lack of consistency in all of these terms?

A: It comes down to those who authored these regulations within the different organizations while attempting to integrate both scientific and legal aspects. 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 “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 ranges 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.

Q: Why is it technically and clinically important for the laboratory to perform linearity testing as part of calibration verification?

A: Linearity testing is still required by CLIA and that likely won’t change for some time. It also serves a purpose. For example, if the assay calibration changes, patient test result values will also change. This type of testing can detect problems earlier than QC or proficiency testing, including problems with reagents, specimen handling, or analyzers.

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