Limit of Detection (LOD)
Definition
The lowest concentration of analyte that produces a signal reliably distinguishable from the instrument noise, conventionally three times the standard deviation of the blank. LOD sets the floor for what an instrument can find; ICP-MS sits in the parts-per-trillion range for most metals, GC-MS in the low nanogram per millilitre range, and a UV-Vis spectrophotometer in the low microgram per millilitre range.
- TLC (Thin Layer Chromatography) LOD
- Around five micrograms per spot
- HPLC/GC LOD range
- Nanogram quantities
- LC-MS/MS LOD
- Picogram quantities
- ICP-MS LOD for metals
- Parts-per-trillion range
- Anti-human haemoglobin ELISA LOD
- Sub-nanogram per millilitre range
Common questions
What does LOD actually mean in forensic testing?+
LOD is the lowest concentration of a substance that an instrument can reliably detect above the noise the machine itself produces. If a result is below the LOD, you cannot report it as a confirmed finding, even if the instrument gives a signal. It sets the hard floor on what a test can find.
How is LOD actually defined numerically?+
LOD is the concentration that produces a signal three times stronger than the standard deviation of the blank (background noise). This 3:1 signal-to-noise ratio is the conventional threshold. Different instruments have wildly different LODs because they have different noise floors.
Why does the LOD differ so much between instruments?+
Instrument sensitivity varies enormously. LC-MS/MS can reach picograms, while TLC spots sit around five micrograms. ICP-MS for metals goes down to parts-per-trillion, but a UV-Vis spectrophotometer is only good to low micrograms per millilitre. The more sensitive the machine, the lower the LOD.
Related terms
- Limit of Quantitation (LOQ)
- The lowest concentration at which the assay can produce a quantitative measurement with acceptable precision and accuracy, typically defined as the concentration...
- 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...
- Confirmatory Technique
- An instrumental method capable of identifying a specific molecule with sufficient selectivity to be admissible in court. The accepted gold standards are...
- Cross-Reactivity
- The capacity of an antibody raised against one analyte to bind structurally related compounds. In RIA, cross-reactivity is the main driver of...
- Cross-Reactivity Matrix
- A table documenting the assay's optical density response to blood from multiple non-target species, expressed as a percentage of the human blood...
- Epitope Degradation
- The destruction or alteration of the molecular site on an antigen that an antibody recognises, caused by hydrolysis, oxidation, UV radiation, or...
- Horseradish Peroxidase (HRP)
- The most common enzyme label in immunoassays. When the substrate TMB (3,3'-5,5'-tetramethylbenzidine) is added, HRP converts it to a blue product that...
- Hyphenated Technique
- A single workflow that couples a separation method to a spectroscopic detector, so a complex mixture is resolved and each component identified...
- Instrumental Technique
- An analytical method that uses an instrument to detect, identify or quantitate an analyte, distinguished from classical wet-chemistry methods such as titration,...
- 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...
- ISO 17034
- The international standard specifying general requirements for the competence of reference material producers; an ISO 17034-accredited producer has demonstrated the technical competence...
- Limit of Quantification (LOQ)
- The smallest concentration that can be quantified with acceptable precision and accuracy, typically corresponding to a signal-to-noise ratio of 10:1 or an...
Explained in these topics
- Anti-Human Haemoglobin ELISA and Modern ImmunoassaysThe minimum concentration of haemoglobin that produces a signal statistically distinguishable from background noise in the assay. The LOD for anti-human Hb ELI...
- Chromatographic Techniques: TLC, HPLC, HPTLC and GLCThe lowest concentration of analyte the method can reliably distinguish from baseline noise. TLC sits around five micrograms per spot; HPLC and GC reach nanogr...
- Introduction and Scope of Instrumental Techniques in Forensic ScienceThe lowest concentration of analyte that produces a signal reliably distinguishable from the instrument noise, conventionally three times the standard deviatio...
- Method Validation and Fitness for PurposeThe smallest quantity of an analyte that can be distinguished from background signal with a defined level of confidence, typically estimated as the blank mean...
- Reference Standards, Certified Reference Materials and the Quantification BaselineThe smallest concentration of an analyte in a sample that can be detected with reasonable certainty, typically corresponding to a signal-to-noise ratio of 3:1;...
- Sensitivity, Specificity, and Quality Controls in Forensic ImmunoassaysThe lowest concentration of antigen that can be distinguished from background noise with a defined statistical confidence, typically 95% or 99%. Below the LOD...