Limit of Detection (LOD) and Limit of Quantitation (LOQ)
Definition
LOD is the lowest concentration the instrument can reliably distinguish from blank, defined as three times the standard deviation of the blank or a signal-to-noise ratio of 3:1. LOQ is where the response is reliable enough to put a number on, ten times SD of blank or S/N of 10:1. A case result below LOQ is reported as 'detected, below LOQ', not as a number.
- LOD threshold
- 3x SD of blank, or signal-to-noise 3:1
- LOQ threshold
- 10x SD of blank, or signal-to-noise 10:1
- Below-LOQ report wording
- Detected, below LOQ, not a number
- Context
- Instrument calibration in sample preparation
Common questions
Why can't a lab report a specific number for a result between LOD and LOQ?+
Below LOQ the measurement's precision and accuracy have not been validated, so any number would overstate confidence. The honest report is a qualitative detected statement instead.
Does a result below LOD mean the substance is absent?+
No. It means any amount present, if there is any, is too low for the instrument to distinguish from background noise, so it is reported as not detected rather than proven absent.
Related terms
- Bracketing Standards
- Calibration standards or quality control samples injected before, during and after a batch of case samples. They track instrument drift through the...
- Calibration Curve
- In RIA, a set of standards containing known concentrations of unlabelled analyte that are run alongside test samples. Bound radioactivity is measured...
- Certified Reference Material (CRM)
- A sample with one or more analyte concentrations established by an accredited body and traceable to a national metrology institute. NIST SRM...
- Extraction
- The first step in sample preparation. The analyte is pulled out of its original matrix (blood, urine, viscera, soil, paint chip) into...
- Internal Standard (IS)
- A compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sample at a known...
- Recovery
- The fraction of analyte that survives the extraction and clean-up workflow, measured by spiking a known amount of analyte into a blank...