Legion Diagnostics

Limit of Quantitation

The Limit of Quantitation (LoQ) is the lowest concentration at which a measurement procedure can reliably quantify the measurand with acceptable precision. Unlike the Limit of Blank (LoB) and Limit of Detection (LoD), which are purely statistical constructs based on measurement noise, the LoQ reflects the point at which results meet a predefined accuracy goal — typically expressed as a maximum acceptable CV%.

A result below the LoQ may be detectable but cannot be reported with confidence in its numeric value. Understanding the LoQ is essential for tests that require quantitative precision at low concentrations — such as therapeutic drug monitoring, endocrine panels, tumor markers, and infectious disease viral loads.

Legion Diagnostics calculates the LoQ using a power model regression per CLSI EP17-A2. The model characterizes how CV% changes with concentration across the low-end region of the measuring interval, then solves for the concentration where CV% equals the acceptance threshold. When multiple reagent lots are tested, the maximum LoQ across lots is reported as the overall result.

When to Use This Module

LoQ evaluation is required or strongly recommended in the following situations:

  • New method or instrument implementation — Before a measurement procedure enters routine use, the LoQ must be verified to confirm that the instrument can quantify results with acceptable precision across the low-end range in your laboratory's environment.
  • Analytes requiring low-end quantitative precision — For tests where the medical decision point is near zero — such as troponin, PSA, thyroid hormones, viral loads, or low-concentration therapeutic drugs — the LoQ defines the lower boundary of the reportable range where numeric results are reliable.
  • Establishing or verifying the reportable range — The LoQ is the lower bound of the analytical measurement range for quantitative reporting. It should be established as part of any comprehensive method evaluation that includes low-end performance characterization.
  • Reagent lot changes — Lot-to-lot variability can shift precision performance at low concentrations. Evaluating the LoQ across multiple lots ensures the reported value is conservative across manufacturing variation.
  • Accreditation requirements — CAP and CLIA require documentation of detection capability for non-waived quantitative tests. The LoQ establishes where the measurement procedure transitions from detection to reliable quantitation.