In historical toxicology papers, “mg%” commonly means milligrams per 100 mL, not the percentage of ethanol lost. Chang and colleagues reported blood ethanol losses often falling around 20–40 mg% after years of storage. Reading that as a 20–40% loss confuses an absolute concentration change with a relative change.
What the original study measured
The 1984 paper examined whole-blood specimens collected for driving investigations, stored at room temperature in evacuated tubes with approximately 0.36% weight/volume sodium fluoride. It was not a study of ethanol formation in decomposed bodies.
One series included 37 previously unopened second tubes tested after three years. Original concentrations ranged from 84 to 348 mg/100 mL. Losses ranged from 3 to 37 mg/100 mL, averaging 19 mg/100 mL.
A second series included 57 unopened specimens stored for 6.75 years. Initial concentrations ranged from 12 to 352 mg/100 mL. Losses ranged from 3 to 72 mg/100 mL, averaging 33 mg/100 mL. In 32 cases, previously opened tubes were also examined; their losses ranged from 34 to 93 mg/100 mL, averaging 61 mg/100 mL.
Absolute and relative changes answer different questions
A decrease of 20 mg/100 mL from an initial 100 mg/100 mL is a 20% reduction. The same absolute decrease from 200 mg/100 mL is a 10% reduction. These are illustrative calculations, not additional study observations.
Likewise, an average loss of 33 mg/100 mL is equivalent to 0.033 g/dL; it is not an average 33% loss. To calculate a relative loss, the original concentration must be the denominator for the same specimen. A group mean absolute loss cannot be relabeled as a group mean relative loss.
Method changes qualify the historical comparison
The three-year series used the same headspace GC instrument for initial and repeat analyses. The 6.75-year comparison crossed two instrument configurations, internal standards and dilution procedures. Both used flame-ionization detection.
The reported coefficients of variation were 4.6% for the older configuration and 2.0% for the newer one. These are measures of analytical variability, not storage-loss percentages. The paper does not provide a complete modern validation dataset for detection limits, quantification limits and every matrix interference.
The typical range was not a guarantee
The authors described many losses as moderately predictable around 20–40 mg/100 mL and suggested a role for oxygen availability and oxidation. Their actual ranges included substantially smaller and larger changes, particularly in opened tubes. They did not directly measure the oxidative mechanism in each sample or validate a universally applicable add-back.
The specimens came from different cases, with differing starting concentrations and container histories. The observed direction of loss does not establish that all forensic blood must behave the same way. The study did not determine anatomical collection sites or postmortem intervals because it concerned specimens collected during life.
Applying old data to a present report
When citing a historical storage paper, transcribe the units in full and identify whether the number is a concentration, an absolute difference, a relative percentage or an analytical CV. Then compare the actual matrix, preservative, container, opening history and temperature.
This paper supports attention to ethanol loss during long storage. It does not establish a person’s earlier impairment or supply a correction for autopsy blood. A separate experiment showing both increases and decreases under warm putrefactive conditions is discussed in ethanol change during blood putrefaction.
Primary source
Chang RB, Smith WA, Walkin E, Reynolds PC. The Stability of Ethyl Alcohol in Forensic Blood Specimens. Journal of Analytical Toxicology. 1984;8:66–67. doi:10.1093/jat/8.2.66.
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