What Is LoD (Limit of Detection)?
Direct answer: LoD (limit of detection) is the lowest analyte concentration a test can reliably distinguish from zero — the smallest amount the assay can actually "see". It is an analytical property of the assay, distinct from the clinical cut-off used to call a result positive.
LoD vs cut-off
| LoD | Cut-off | |
|---|---|---|
| What it is | Analytical detection floor | Clinical decision threshold |
| Question it answers | How little can the test detect? | What counts as positive? |
| Relationship | Usually below the cut-off | Usually set above the LoD |
How LoD is determined
LoD is established experimentally by measuring replicate blank and low-concentration samples and determining the concentration that yields a signal reliably above background. It is a validation parameter, not a fixed universal number — it varies by assay and matrix.
Why it matters
LoD determines a test's ability to catch low analyte levels — critical for early detection and low-viral-load cases. A lower LoD means higher analytical sensitivity, which is one reason molecular (PCR) tests detect earlier than antigen rapid tests.
Related terms
- Sensitivity — the proportion of true positives detected (clinical, not analytical).
- Limit of quantification (LoQ) — the lowest concentration measured with acceptable precision (quantitative assays).
Frequently Asked Questions
Is LoD the same as cut-off? No — LoD is the analytical detection floor; cut-off is the clinical threshold for a positive call.
Why does a lower LoD matter? It lets the test detect lower analyte levels, improving early detection.
Related knowledge
References
- CLSI EP17-A2: Evaluation of detection capability. Clinical and Laboratory Standards Institute.
By Progene Bio Regulatory Affairs Team · Medical Device QA · Medical disclaimer · Last reviewed: 2026-09-12