Why Lab Results Can Change Over Time

Understanding why laboratory reports—especially Certificates of Analysis (COAs) for THC-free CBD products—can differ between tests or over time helps consumers, retailers, and manufacturers make informed decisions and manage risk.

What this page covers

Key reasons lab results change (scientific and practical).
Definitions and testing concepts relevant to THC-free CBD (COA, LOD, LOQ, ISO accreditation).
Practical guidance for consumers, retailers, and manufacturers to reduce surprises.
Common pitfalls and a short FAQ addressing real-world concerns, including drug-testing risks.

What this page doesn’t cover

Detailed analytical chemistry methods (e.g., full method validation protocols).
Medical advice or legal advice about specific workplace drug-testing outcomes.
Comprehensive product reviews or brand rankings; see site hub pages for those topics.

Key definitions

Certificate of Analysis (COA): A lab report showing measured analytes (cannabinoids, residual solvents, pesticides, etc.) for a specific product lot. A COA should list the lot number, test date, methods used, and detection limits.
Limit of Detection (LOD): The lowest concentration that the laboratory’s method can reliably detect as present (but not necessarily quantified precisely).
Limit of Quantitation (LOQ): The lowest concentration that can be measured with acceptable accuracy and precision. Results reported below LOQ are often labeled “below quantifiable limit” or “non-quantified.”
ISO/IEC 17025 accreditation: A recognized standard indicating that a lab operates quality management and technically competent testing processes. Accreditation reduces—but does not eliminate—inter-lab variability.

Why lab results change: core causes

1. Sample heterogeneity and representativeness

Products are not perfectly uniform. Small differences between sampled units or between sub-samples from the same container can lead to different results. Proper sampling protocols and lot-level testing reduce this cause.

2. Stability, degradation, and chemical changes

Cannabinoids and other compounds can change over time due to light, heat, oxygen, or solvent interactions. For example, acids can decarboxylate or compounds can degrade, shifting measured values between tests done at different times or after different storage conditions.

3. Method differences and lab instrumentation

Different labs may use different extraction procedures, chromatography columns, detectors (GC-MS, LC-MS/MS, HPLC), or calibration standards. These method and instrument differences produce systematic variation.

4. LOD/LOQ boundaries and rounding

Many apparent changes are artifacts of being near the lab’s LOD or LOQ. A trace amount near the LOQ can be reported as “non-detect” in one run and a small quantified value in another. Rounding and reporting conventions add variability.

5. Reference standards and calibration

Accuracy depends on certified reference materials and calibration curves. Different lots of standards, or differences in calibration procedures, introduce variability.

6. Inter-laboratory variability and proficiency

Even accredited labs can differ. Proficiency testing and accreditation reduce variability but do not eliminate it; instrument condition, operator technique, and method interpretation all matter.

7. Cross-contamination and handling errors

Improper sample handling, shared equipment without adequate cleaning, or lab mistakes can introduce or eliminate trace analytes between tests.

8. Lot-to-lot manufacturing variation

Differences in raw materials, blending, or post-processing mean one production lot may legitimately test differently than another, even for products from the same brand and formulation.

Practical guidance: how to interpret and respond

For consumers

Check the COA lot number and test date before assuming a COA applies to your bottle. COAs apply to specific lots—don’t accept a generic COA without lot matching.
Look for LOQ/LOD values on the COA. If results are reported as “ND” (non-detect) or “< LOQ,” understand those terms aren’t absolute zeros.
If avoiding THC for legal or employment reasons, realize that trace levels near the LOQ can fluctuate; ask the manufacturer for recent, lot-specific COAs and stability data.

For retailers and labs

Require lot-specific COAs and chain-of-custody documentation from suppliers. If a COA changes, verify the lot and testing date before updating product pages or advising customers.
When comparing COAs from different labs, compare methods, LOD/LOQ, accreditation, and date tested—differences in any of these can explain result shifts.

For manufacturers

Adopt good sampling plans and stability testing across shelf life. Maintain batch records and include retention samples to support repeat testing.
Use accredited third-party labs for routine testing and confirmatory testing when results are unexpected. Document method versions and reference standards.

Pitfalls to avoid

Misreading “ND” or “< LOQ” as absolute proof of zero—these are method-dependent limits.
Comparing COAs without confirming lot numbers, dates, and methods.
Assuming single-test permanence—one test does not prove future results or lot conformity.
Over-interpreting small numerical differences that fall within expected analytical uncertainty.

Frequently asked questions (FAQ)

1. My product once tested “non-detect” for THC and later showed a trace. Does that mean the company lied?

Not necessarily. Small changes can be due to different sampling, storage, or testing near the LOQ. Confirm whether COAs refer to the same lot and check the LOQ/LOD on both reports before concluding intentional mislabeling.

2. Can storage or shelf life make THC appear or increase over time?

Storage conditions can change the chemical profile through degradation or conversion of compounds, potentially shifting low-level cannabinoid measurements. Proper stability testing and storage guidance limit these effects.

3. If two labs give different numbers, which is right?

Both can be correct within method uncertainty. Compare methods, LOQ/LOD, accreditation status, and testing dates. If necessary, run a third-party confirmatory test with a clearly documented chain of custody.

4. Could a small measured THC amount cause a workplace drug test to fail?

Drug-test outcomes depend on the test type (urine, hair, blood), cutoff levels, metabolism, and timing. Trace amounts near analytical limits in a product increase uncertainty—consult occupational health or a testing expert about specific risk.

5. How often should companies retest products?

Regulatory requirements vary. Best practice includes testing each production lot and performing stability testing over the product’s claimed shelf life. Retesting is advisable when analytical anomalies or customer concerns arise.

6. What should I ask for when a COA looks different than expected?

Request the lot number, test date, full COA PDF (including method details and LOD/LOQ), chain-of-custody, and any available stability data. If the lab is different from the prior COA, ask why the lab changed.

Compliance and caution

This page is educational and not medical or legal advice. Laws, workplace policies, and testing technologies vary by jurisdiction. For legal or medical questions tied to specific test outcomes, consult qualified legal counsel, occupational health services, or clinical professionals.

Want products and information focused on THC-free formulations? Learn more about THC‑free options and access lot-specific testing details at THC-Free at Cannagea.