Clinical Chemistry Blog: Understanding A1C Linearity

Why does linearity matter in measuring A1C—and why don’t other at-home A1C tests talk about it?

Linearity refers to the relationship between test results and analyte concentration. Linear results mean that doubling the analyte concentration exactly doubles the test result. Linear relationships must be confirmed through both calibration and actual A1C sample testing.

Non-linear relationships commonly occur in Laboratory Developed Tests (LDTs) that use immunoassay methods, due to binding-site saturation.

Understanding linearity is essential for reliable laboratory test results in clinical testing, such as A1C. Methods with limited linearity offer limited clinical significance.

LDT companies distributing at-home dried blood (blood spot) A1C tests claim to be CLIA-approved. However, unless they demonstrate full linearity across the test's reportable range, they fail CLIA’s regulatory requirements. All labs must perform bi-annual linearity testing and publish the results to meet CLIA regulations.

Many factors can affect linearity. What CLIA and the general public may not realize is that the linearity results reported by LDT labs don’t actually use the at-home A1C kits they sell. Instead, they typically use venous blood samples tested using a laboratory method of their choosing. The actual at-home test procedure often hasn’t been verified for linearity at all. Furthermore, none of these LDT tests have ever undergone FDA compliance validation.

An LDT test with limited linearity cannot accurately detect the full range of A1C values. This omission can cause tests to miss clinically important conditions such as anemia or abnormal hemoglobin variants (also known as silent variants). Using an LDT means very low or very high A1C values may go undetected, leading to inaccurate therapeutic guidance from healthcare professionals.

The term "rate and direction of change" only applies if linearity covers the entire A1C range. If your actual A1C is 15.9%, but your test can only read up to 12.0%, your physician can’t accurately monitor if your current therapy is effective, potentially resulting in the misapplication of insulin or other treatments.

Why won’t LDT companies publicly demonstrate their linearity? The truth is, their linearity is substandard, and they don’t want it exposed. They struggle with accuracy due to a distorted linearity range. Without proving linearity, accuracy claims are baseless. As a result, their A1C readings may be misleading or completely incorrect.

Screening for diabetes should never rely on methods with limited linearity, as critical diagnostic points could be missed. Therapeutic decisions must be based on accurate A1C measurements with proven linearity.

AccuBase is Different—Here’s Why That Matters

The AccuBase A1C At-Home Test is the only FDA-approved OTC home A1C test. AccuBase has demonstrated a linearity range of 3.6% to 19.6% to CLIA, ensuring accuracy across nearly any possible A1C value.

Additionally, the AccuBase analytical method (HPLC-IE) also screens for abnormal hemoglobin variants such as HbC, S, D, E, and F. These variants cannot be detected by standard LDT, dried blood spot, or disposable A1C kits. Undetected variants can result in falsely reported A1C values.

Final Thoughts

Managing diabetes is challenging enough without worrying about the accuracy of your chosen A1C kit—or one mandated by a managed care provider. Choose accuracy. Choose proven linearity.

If you have any questions or thoughts, please share them below in the blog comments.

Written By Arthur G. Williams August 2, 2025