Glossary term
LOD (Limit of Detection)
Also known as: LOD, limit of detection, below the limit of detection
Part of: How to read a report
- Reading time
- 3 min read
- Last reviewed
- Last reviewed January 1970
- Reading level
- Written in plain English
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- Reviewed by Medipher Editorial Team
The smallest amount of a substance a test can reliably tell apart from nothing at all.
Medipher explains medical wording in plain English — it does not diagnose or replace a clinician. If this is a medical emergency, call your local emergency number immediately.
What is this?
Every measurement has a floor. The limit of detection is the smallest amount of a substance that a test can distinguish from a blank sample with confidence.
When a result falls beneath that floor, laboratories report it as below the limit of detection, or with a less-than sign and the limit — for example, <0.1.
This is why 'below LOD' is not identical to zero: it means the amount, if any, is too small for that method to measure.
Why is it important?
It explains why a report may say a result is below a stated level rather than giving a number.
Where you'll see it: Drug-level, viral-load, hormone and specialist assay reports, often written as '<' a value.
Does an abnormal result always mean something is wrong?
Not necessarily. A number that sits outside the printed range is a prompt for interpretation — not a diagnosis.
- A single result rarely gives the full picture — clinicians interpret it alongside your symptoms.
- Your medical history, medications and other blood tests all shape the meaning of one value.
- Small movements just outside the reference range are common and often not concerning.
- Repeat testing over time is frequently more useful than any single number.
What might happen next?
- Note the limit printed on your own report, as it differs between laboratories.
- Ask the requesting service what the result means for your treatment or monitoring.
One result rarely tells the whole story. Clinicians look at your symptoms, medical history, medications and other tests together — and a single value that sits slightly outside the reference range is often normal for you. Repeat testing over time is frequently more useful than any one number. Medipher helps you understand what a result means in plain English, but it cannot diagnose medical conditions or replace advice from a doctor, nurse or pharmacist.
When should I speak to a healthcare professional?
Speak with your GP or another healthcare professional if:
- You've been asked to repeat the test.
- You're experiencing new or worsening symptoms.
- Your clinician has flagged the result as needing follow-up.
- You don't understand your results or feel unsure what to do next.
If you have severe symptoms — chest pain, severe breathlessness, fainting, heavy bleeding or feel very unwell — seek urgent help. If this is a medical emergency, call your local emergency number immediately. Medipher is designed to help explain medical information and is not an emergency service.
Frequently asked questions
Related blood tests
Terms clinicians often look at alongside LOD (Limit of Detection).
- LOQ (Limit of Quantitation)The smallest amount of a substance a test can measure with acceptable precision, as opposed to merely detect.
- Not detectedThe substance or organism the test looked for was not found at a level the test can measure.
- EquivocalA result sitting in a borderline zone where the test cannot give a clear positive or negative answer.
Related guides
Still unsure what your result means?
This page explains one medical term. If you have a blood test, GP letter or hospital report, Medipher can explain your entire document in plain English.
- what your results mean
- unfamiliar medical terminology
- what your clinician may be monitoring
- useful questions to ask at your next appointment
Sources
General patient information referenced when writing this page.
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About this page
- Author
- Omer Suliman
- Medical reviewer
- Clinical review pending
- Last reviewed
- January 1970
- Last updated
- January 1970
Evidence. Written from public patient-information sources including the NHS, NICE and the Association for Laboratory Medicine, and reviewed for plain-English accuracy before publication.
Educational content only. Medipher explains medical language in plain English. It does not diagnose, and it is not a substitute for advice from a doctor, nurse or pharmacist. Read the medical disclaimer.
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A short summary and recommended next reads are just below.
Continue learning
- Next upNot detectedThe substance or organism the test looked for was not found at a level the test can measure.
- Next upEquivocalA result sitting in a borderline zone where the test cannot give a clear positive or negative answer.
- Next upAbnormal (on a test report)A flag meaning a result differs from what the laboratory or test expected — not a statement about your health.
This glossary entry is general health information, not medical advice. Medipher explains medical wording in plain English and does not diagnose, provide treatment advice, or replace a GP, doctor, nurse or pharmacist. Always discuss your results with a qualified clinician.