Regulations and guidelines sometimes make it difficult to perform linearity testing and calibration verification in a coherent and useful way. 

By Robert J. Janetschek – MS, MT (ASCP)

CLIA, along with other regulatory requirements, now mandates that all clinical laboratories perform specific testing procedures to confirm the continued accuracy of their clinical testing systems. These requirements apply to every laboratory and testing site in the U.S., regardless if the site is performing only a few basic tests as part of routine physical examination, or hundreds of thousands of tests.1 Although the CLIA directives have been in effect for quite some time, the unfortunate fact is that lab personnel and experienced administrators still express confusion when it comes to linearity, reportable range, and analytical measurement range (AMR).

Frankly, it is not too surprising this has occurred—CLIA uses one set of terms and definitions, while other industry agencies and regulatory bodies use different ones. And it is important to understand the differences between the three criteria as it impacts laboratory inspections and compliance1-2. In fact, one of the top five reasons for a laboratory to be served with a noncompliance citation has been reported as “ . . . not performing calibration verification according to the manufacturer’s instructions including: the number, type and concentration of materials to be used; use of materials at low, medium, and high values within the reportable range as determined by the laboratory; acceptable limits for calibration verification, once every 6 months or more often, if required by laboratory procedures.”

Defining Linearity Testing

When we come to see how linearity enters the mix, let us first designate what it is. Linearity in the clinical laboratory—as initially defined by the National Committee for Clinical Laboratory Standards (NCCLS), and now known as the Clinical Laboratory Standards Institute (CLSI) per their EP06-A Guideline—is the ability, within a given range, to provide results that are directly proportional to the concentration of the component in the test sample3. One must also keep in mind that the analyte concentration versus the measurement signal is not always a straight line linear reaction. Now, looking at the clinical importance and diagnostic history behind this concept, it really is quite surprising that the term “linearity” does not appear anywhere within the CLIA regulations, and it is not separately designated by CLIA. Instead, the concept of “reportable range” attempts to address this.  

The reportable range, per CLIA, is defined as the span of test results, low end to high end, that the laboratory can accurately verify the results of a clinical test system for a given assay. An important concept 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 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 additionally complicate things, the College of American Pathologists (CAP) further delineates reportable range as being the “analytical measuring range” (AMR), 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 4-6.

Clarifying Differences

The most important distinction between CLIA and CAP is how each entity refers to reportable range. CLIA utilizes 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 AMR to describe the range of analyte values that a method can directly measure on the specimen without any dilution, concentration, or other pretreatment not part of the usual assay process. Furthermore, CAP segments AMR from calibration verification requirements through a separate AMR verification process, while CLIA captures both requirements within calibration verification, while defining linearity as a straight-line relationship between observed values and expected values. In a nutshell, linearity is taken as the method employed to determine the AMR verification 7-9. Hopefully, this explanation better defines these concepts and provides the reader with a clearer understanding of the current terminology.

In summary, it is safe to state that these testing procedures for clinical systems are here to stay. As discussed, the purpose of this testing (PHOTO 1 – Linearity Testing Materials), aside from CLIA compliance, is to support and bolster overall laboratory quality assurance and help ensure that clinical diagnostic systems are reporting accurate test results resulting in improved clinical testing, enhanced diagnosis, and better patient care.

Featured Image: An example of linearity testing materials from Verichem. Photo: Verichem


Robert J. Janetschek, MS, MT (ASCP), is a veteran of the clinical laboratory, with over 40 years of relevant involvement. Aside from his direct clinical experiences, he has also held marketing, business development, and technical leadership positions with several in-vitro diagnostics organizations. He is currently affiliated with Verichem Laboratories Inc., an industry recognized leader within the field of calibration verification testing, as the company’s director of Business Development.

The author wishes to thank Elizabeth A. Pasini of Harborside Press, LLC – Huntington, NY, for her assistance in the editing of this article.


  1. CLIA ’88: Section 493.2 – Definitions.
  2. CLIA Brochure No. 3:
  3. CLSI Document EP6-A: Evaluation of the Linearity of Quantitative Measurement Procedures – A Statistical Approach. Approved Guideline.
  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 Register 2003;68(163):50722–50725]. Fed Regist. 2003; 68(16):3707–3714. Codified at 42 CFR §493.2.
  7. Killeen AA, Long T, Souers R et al. Verifying Performance Characteristics of Quantitative Analytical Systems. Arch Pathol Lab Med 2014;138:1173-1181.
  8. 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.
  9. College of American Pathologists, Commission on Laboratory Accreditation. Chemistry and Toxicology Checklist. Northfield, IL: College of American Pathologists; 2012.