Why a THC Concentration Must Identify Whole Blood, Plasma or Serum
A THC concentration needs a specimen label before it can be interpreted. Whole blood, plasma and serum are different laboratory materials. A number reported for one should not be silently substituted for a number reported for another, even when the units match.
The matrix is part of the measurement
Whole blood contains blood cells and liquid components. Plasma and serum are liquid fractions obtained through different preparation processes. Wong and colleagues' 2014 review discussed higher THC concentrations in plasma than in whole blood. Its approximate population-level comparison illustrates why the specimen matters; it does not supply a case-specific conversion factor for every person or collection time.
A threshold comparison must use matching definitions
Before comparing a laboratory result with a published research threshold, establish the analyte, matrix and units used on both sides. Parent THC and the metabolite THC-COOH are also different measurements. A specimen mismatch cannot be repaired merely by observing that both reports say nanograms per milliliter. If a conversion is proposed, its scientific basis and uncertainty need to accompany it.
Correct specimen matching does not settle impairment
The review also discussed rapid changes in blood THC after use and delays before specimen collection. Identifying the matrix is necessary for a coherent comparison, but it does not reconstruct concentration at an earlier event or establish the person's functional performance. An accurately measured concentration remains exposure evidence requiring interpretation in its timing and clinical or behavioral context.
Interpreting the evidence
This analysis uses the historical review's pharmacokinetic discussion. Its descriptions of jurisdictions and laws are not presented as current legal rules. The practical scientific lesson is to retain the original specimen definition and avoid undocumented conversions.
Related reading: what concentration and functional tests can establish.
Research reference
Kristin Wong; Joanne E Brady; Guohua Li (2014). Establishing legal limits for driving under the influence of marijuana. Injury Epidemiology. doi:10.1186/s40621-014-0026-z. Evidence basis: review/commentary abstract and selected explanatory passages.
