Why Lab Results Can Change Over Time

Certificates of Analysis (COAs) are snapshots of a product at a specific moment. If you follow THC-free CBD closely, you may notice that COA values shift between batches or even when the same batch is retested later. This can be confusing, especially when you are trying to avoid THC and want consistent results.

These changes are usually explained by normal agricultural variability and the realities of analytical testing. Understanding why CBD COA changes occur helps you read reports with confidence and make informed choices.

In this guide, we focus on the testing factors behind COA variability and lab variance in CBD, and how to interpret results over time. For broader supply chain topics, see Industry, Sustainability & Traceability.

The short answer

COAs change because products and test methods aren’t static. Natural batch differences, sampling, storage conditions, method updates, instrument calibration, reporting conventions (like LOQ/LOD and rounding), and regulatory nuances can all shift reported values slightly. Most differences are expected and manageable when you know what to look for.

Factors that can change lab results over time

1) Natural batch variability

Hemp is a biological material. Climate, soil, cultivar, and harvest timing influence cannabinoid profiles. Even with careful processing, two lots can show small differences in CBD potency or minor cannabinoids. For farming and sourcing context, see Responsible Sourcing & Hemp Farming and Industry, Sustainability & Traceability.

2) Sampling and homogenization

How and where a sample is taken matters. If a large batch isn’t fully homogenized, the top, middle, and bottom can vary slightly. Small fill volumes and viscous matrices (oils, gummies) also increase sampling error. Retests that use a different portion of the same lot may read a bit higher or lower.

3) Storage, stability, and packaging

Heat, light, oxygen, and time can alter cannabinoids. For example, CBDa can convert to CBD (decarboxylation), and some cannabinoids can slowly degrade. Minor evaporation from a bottle can concentrate remaining components, nudging percentages upward. Packaging quality and headspace control influence these effects. If a later COA on a retained sample looks different, product aging is a likely factor.

THC-free context: A product listed as “ND” (non-detect) for THC at bottling may later show “

4) Lab variance and method updates

Accredited labs use robust methods, yet inter-lab and intra-lab variance still exists. Instruments are calibrated routinely, columns are replaced, and method parameters are refined. Labs may participate in proficiency testing and update methods or software that change integration baselines or signal processing. These improvements can shift results modestly and often lower LOQs, revealing minor cannabinoids that previously read as ND.

5) Reporting conventions: LOQ, LOD, ND, and rounding

LOD vs LOQ: LOD means the analyte may be present but not quantified reliably; LOQ is the lowest level confidently quantified. ND typically means “below LOQ,” not absolute zero.
Units: COAs may report % by weight, mg/g, mg/mL, or mg per unit. Comparing different units without conversion leads to perceived differences that are purely arithmetic.
Total vs neutral: “Total CBD” often means CBD + (CBDa × 0.877) to account for decarboxylation potential. Make sure you’re comparing like-for-like values.
Rounding rules: Two labs can round differently (e.g., 0.049% vs 0.05%), especially near thresholds.

6) Regulatory thresholds and interpretation

For hemp compliance, delta-9 THC limits are defined on a dry-weight basis at the plant or extract stage. For finished goods marketed as THC-free, brands typically target ND THC at a defined LOQ. As LOQs change across labs and over time, the way results are displayed (ND,

How to read and compare COAs wisely

Match details: Confirm product name, strength, lot/batch number, and sample matrix (oil, gummy, capsule) and the COA date.
Check the lab’s credentials: Prefer ISO/IEC 17025-accredited labs with proficiency testing.
Find LOQ/LOD and units: Ensure you’re comparing the same measurement basis and that “ND” is defined relative to a clearly stated LOQ.
Look for measurement uncertainty: Some COAs list an uncertainty range; small differences may be within expected error.
Expect reasonable variance: Minor shifts (often within about ±10% for potency) can be normal, depending on product type and method.
Ask about stability data: Brands that retain samples and perform periodic retests can explain time-related changes.
Storage matters: Keep products sealed, cool, and away from light; note the manufacture date and suggested use-by window.
THC-free tip: Favor products with third-party COAs showing ND THC at a low, clearly stated LOQ (often ppm-level) and a separate cannabinoid panel.

Drug testing awareness for THC-free CBD

Even when a product tests ND for THC at a low LOQ, no CBD product can guarantee you will pass a drug test. Immunoassays may react to THC metabolites, and rare cross-contamination or trace levels can occur. Risk increases with frequent, high-dose use. If testing is a concern, consider abstaining and consult your employer’s policy.

When a new COA looks different: common scenarios

CBD potency drifts slightly but remains within spec due to sampling or normal variance.
Minor cannabinoids appear because the lab lowered LOQs or improved detection.
THC changes from ND to
A retest on a retained sample shows small differences attributable to storage and aging.

What brands can do to reduce variability

Robust homogenization and sampling plans for each lot.
Use accredited third-party labs and participate in inter-lab comparisons.
Track control charts, keep retention samples, and publish consistent reporting formats.
Be transparent about LOQs, methods, and any method updates over time.

For broader supply chain context and costs, see Seed-to-Shelf Traceability, Environmental Impact of THC-Free CBD Production, Why Some Brands Avoid THC-Free Claims, and the Cost Breakdown of THC-Free CBD Products.

FAQ

Why did THC appear on a later COA when it was previously ND?
Often the lab lowered its LOQ or used an updated method, revealing a very small amount that was previously below quantification, or the product slightly concentrated over time. It doesn’t necessarily mean THC increased meaningfully.

How much variance is normal between labs?
Small differences are common due to sampling, calibration, and integration settings. Well-controlled results typically align within a modest range; ask for the lab’s measurement uncertainty and accreditation.

Does ND mean zero THC?
No. ND means the amount is below the lab’s LOQ. It is effectively “not quantifiable,” not absolute zero.

Can storage change CBD levels?
Yes. Light, heat, oxygen, and time can convert or degrade cannabinoids, and minor evaporation can concentrate remaining compounds.

Why do some COAs list total CBD and others list CBD?
Total CBD accounts for CBD plus the convertible portion of CBDa. If you compare reports, make sure you’re aligning total-to-total or neutral-to-neutral values.

Summary

COAs are living documents that reflect both the product and the evolving precision of testing. Most changes arise from normal batch variability, sampling, storage, and method differences, including LOQ and reporting conventions. By matching lot numbers, checking LOQs and units, and understanding uncertainty, you can compare results appropriately—especially when choosing THC-free CBD.

CTA: Looking for third-party tested, THC-free options? Explore products at https://www.cannagea.com/thc-free.

Disclaimer: This information is for educational purposes only and is not medical or legal advice. Product claims and testing policies vary by jurisdiction and organization. Always review batch-specific COAs and consult a professional if you have questions about your situation.