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Why Lab Results Change Over Time

Lab results for THC-free CBD products can vary from one report to another. This page explains the common, factual reasons test results change over time, how to read and compare Certificates of Analysis (COAs), practical steps to investigate differences, and pitfalls to avoid when interpreting small or unexpected findings.

What this page covers

– Key definitions used in testing (COA, LOD, LOQ, measurement uncertainty, accreditation).

– Practical causes for changing results (sampling, degradation, method differences, reporting practices, lab variability).

– How to interpret and compare COAs and what to ask your lab or manufacturer.

What this page doesnt cover

– Detailed how-to for laboratory validation or full analytical method protocols (those belong on specialized lab pages).

– Medical, legal, or employment-advice about drug testing outcomes; see the disclaimer below for guidance on consulting professionals.

Key definitions

  • Certificate of Analysis (COA): A lab report showing measured values (e.g., cannabinoids) for a specific sample or batch.
  • Limit of Detection (LOD): The lowest concentration that the instrument can reliably distinguish from a blank, often reported as “detected” but not quantified.
  • Limit of Quantitation (LOQ): The lowest concentration that can be quantified with acceptable precision and accuracy. Values near the LOQ are inherently less precise.
  • Measurement uncertainty: A statistical range around a reported value that expresses the labs confidence in that number (often omitted on simplified COAs).
  • Accreditation: Laboratory certification (e.g., ISO/IEC 17025) indicating certain technical standards and quality systems are met.

Common reasons lab results change over time

1) Sampling and homogeneity

Lab testing reflects the specific sample submitted. If a product is not homogeneous (e.g., poorly mixed oils, unevenly coated gummies, or flower with natural variability), two samples from the same batch can produce different results. Composite sampling (pooling multiple sub-samples) versus single-point sampling also changes outcomes.

2) Batch-to-batch manufacturing variation

Manufacturing and raw material variability cause legitimate differences between batches. A later batch may use different hemp biomass or a different process, producing higher or lower trace cannabinoids.

3) Chemical changes and stability

Cannabinoids and other compounds can change over time through processes such as oxidation, volatilization, and decarboxylation. For example, non-psychoactive cannabinoid acids can convert to their neutral forms under heat or light. Poor storage or long shelf time can reduce target analytes or increase degradation products, altering COAs.

4) Instrumentation, methods and LOQ/LOD differences

Different labs or different methods in the same lab (GC-MS, LC-MS/MS, HPLC) have varying sensitivity and specificity. A newer or more sensitive method can lower the LOQ, so very low analyte amounts that previously appeared as “not detected” may later be quantified. Conversely, method changes in sample preparation or calibration can also change reported values.

5) Reporting conventions and rounding

Labs may report “ND” (not detected) below the LOD, “

6) Human error and administrative factors

Clerical mistakes, sample mislabeling, or data-entry errors can make COAs appear different. Chain-of-custody lapses or testing the wrong sample will also produce misleading changes.

7) Cross-contamination and lab cleanliness

Trace contamination in the lab or during manufacturing (shared equipment, insufficient cleaning) can introduce low-level cannabinoid signals that fluctuate between tests.

8) Regulatory and standards updates

Regulators and standards bodies periodically change guidance on reporting, LOQs, and allowable limits. A lab updating its methods to comply with new standards can cause results to change even for identical samples.

Interpreting changes in a THC-free CBD context

  • Small THC findings near or below the LOQ are common with hemp-derived products because natural plant material can contain trace THC. Such results may toggle between “ND” and a trace value depending on method sensitivity and sample.
  • When a COA shows a new low-level THC detection for a product previously reported as THC-free, check the COA date, method, LOQ, and whether the test was performed on the same batch or the finished product.

Practical steps to investigate changing results

  • Compare COAs side-by-side: look at sample ID, batch number, test date, method used, LOQ/LOD, units, and lab accreditation.
  • Ask for raw data or chromatograms: these show retention times and spectra and can help verify peaks and identifications.
  • Confirm sample identity and chain of custody to rule out mislabeling or cross-sample testing.
  • Request a retest on the same sealed product sample or an independent sample from the same sealed batch.
  • Check storage conditions and shelf life: see whether degradation or conversion could explain a change.
  • Verify units and basis (percent vs mg/g, dry weight vs finished product) before comparing values.
  • If concerned about drug testing, consult an occupational-health professional or legal advisor with the COA in hand.

Pitfalls and common misinterpretations

  • Misreading “ND” as zero: “Not detected” is not the same as zero concentration; it means below the labs LOD.
  • Ignoring LOQ/LOD: small numeric differences near LOQ may be statistically indistinguishable due to measurement uncertainty.
  • Comparing different sample types or bases: e.g., comparing a COA for raw biomass with one for a finished oil without accounting for extraction factors.
  • Assuming all labs are equivalent: different labs have different methods, validation, and quality control; accreditation and method details matter.

FAQs

1. Why did a product go from “THC not detected” to a tiny detected amount?

Possible reasons include a more sensitive method or lower LOQ, testing a different batch or sample spot, slight cross-contamination, or natural variability in the hemp feedstock. Review the COAs for LOQ/LOD and batch information.

2. Can different labs give different results for the same sample?

Yes. Differences in sample handling, extraction, analytical method, instrumentation, and calibration can create variation. Accredited labs with validated methods tend to produce more reproducible results.

3. What is the difference between LOD and LOQ and why does it matter?

LOD is the smallest amount that can be detected reliably; LOQ is the smallest amount that can be quantified with acceptable precision. Results below LOQ are less reliable numerically and are often reported as “

4. How long should COAs be considered reliable?

COAs accurately describe the tested sample and test date. For ongoing quality, COAs should be tied to specific batch numbers and production dates. Shelf life, storage, and time since testing can change the product composition.

5. If I see a low-level THC on a COA, should I worry about drug tests?

Low-level THC detections near the LOQ may or may not result in a positive drug test, depending on exposure, dosing, test type, and cutoff values. This page does not provide legal or medical advice; consult an occupational-health specialist or legal advisor for specific concerns.

6. What information should I request from a lab when results change?

Ask for the full COA with LOQ/LOD and units, chromatograms or raw spectral data, method descriptions, the sample chain of custody, and whether the lab is accredited and the method validated.

When to contact the manufacturer or lab

Contact them when results change unexpectedly, especially if a previously certified THC-free product now shows quantifiable THC, or if a test appears inconsistent with batch records. Provide COAs, lot numbers, and purchase/sample details. A cooperative manufacturer or accredited lab should be willing to investigate and, if needed, retest.

Short checklist before you draw conclusions

  • Confirm same batch and sample type.
  • Check LOQ/LOD and measurement uncertainty.
  • Verify method and lab accreditation.
  • Request raw data if results affect safety, legality, or employment.

Compliance disclaimer

This page is informational and not medical, legal, or regulatory advice. Laws and workplace policies vary by jurisdiction and employer. For decisions about health, legal risk, or employment consequences, consult qualified professionals and local authorities.

THC-Free CBD Hub offers resources and product information about THC-free options; for batch-specific questions, reach out to the product manufacturer or an accredited testing laboratory.

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