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ISO 27368:2008 — Analysis of Blood for Asphyxiant Toxicants — Carbon Monoxide and Hydrogen Cyanide
Forensic Toxicology Methods for Fire Casualty Blood Analysis
1. Standard Scope and Forensic Context
ISO 27368:2008 specifies analytical methods for determining carbon monoxide (as carboxyhaemoglobin, COHb) and hydrogen cyanide (as cyanide ion, CN⁻) in blood samples from fire casualties. These two gases are the primary toxic combustion products responsible for smoke inhalation fatalities. The standard covers both ante-mortem and post-mortem blood analysis, providing validated methods for forensic toxicology and fire investigation.
For forensic toxicologists: COHb levels > 50% are typically lethal, while CN⁻ levels > 1 μg/mL indicate significant cyanide exposure. The combination of both toxicants often produces synergistic toxicity — lower individual levels can be fatal when both are present.
2. Analytical Methods
For COHb determination, the standard recommends spectrophotometric methods (visible range, dual-wavelength) and gas chromatography with thermal conductivity detection (GC-TCD). For CN⁻ determination, microdiffusion followed by spectrophotometric detection is the primary method, with ion chromatography as an alternative. Each method includes detailed specifications for reagents, calibration standards, quality control, and interference management.
| Analyte | Primary Method | Alternative Method | Sample Volume | Detection Limit |
|---|---|---|---|---|
| COHb | Spectrophotometry (540/555 nm) | GC-TCD | 0.1-0.5 mL | ~1% COHb |
| CN⁻ | Microdiffusion + spectrophotometry | Ion chromatography | 0.5-2 mL | ~0.1 μg/mL |
3. Sample Handling and Quality Assurance
Blood samples must be collected in appropriate anticoagulant tubes (EDTA or heparin), stored at 2-8 °C, and analyzed within defined timeframes to minimize analyte loss. COHb is relatively stable (days at 4 °C), while CN⁻ degrades more rapidly due to microbial action and chemical reactions with blood components. The standard requires use of certified reference materials and participation in proficiency testing programs.
Engineering insight: The standard’s emphasis on sample storage conditions reflects the practical challenge in fire investigation — samples are often collected at the scene by non-laboratory personnel. Pre-assembled collection kits with preservatives and clear instructions significantly improve analytical reliability.
4. Interpretation of Results in Fire Scenarios
COHb levels indicate the extent of smoke exposure and the efficiency of combustion (incomplete combustion produces more CO). CN⁻ levels reflect the presence of nitrogen-containing materials in the fire load (polyurethane foam, wool, silk, acrylonitrile polymers). The ratio of COHb to CN⁻ can provide insights into the fire environment — high COHb/low CN⁻ suggests smoldering combustion of cellulosic materials, while both elevated suggests rapid fire with synthetic materials.
Important analytical consideration: Cyanide can continue to be produced post-mortem from decomposition of blood components and can also be lost through volatilization. Prompt analysis (within 24-48 hours) with proper sample preservation is essential for reliable CN⁻ quantification.
Carbon monoxide is also produced endogenously at low levels (0.5-1.5% COHb in non-smokers) from heme metabolism. Levels below 3% COHb in non-smokers or 5% in smokers should be interpreted cautiously when attributing fire-related toxicity.
5. Frequently Asked Questions
Q: Can these methods be used for living fire victims?
A: Yes, the methods are validated for ante-mortem samples. However, treatment interventions (hyperbaric oxygen, cyanide antidotes) alter measured levels, so sampling timing is critical.
A: Yes, the methods are validated for ante-mortem samples. However, treatment interventions (hyperbaric oxygen, cyanide antidotes) alter measured levels, so sampling timing is critical.
Q: What is the stability of hydrogen cyanide in stored blood samples?
A: CN⁻ stability depends on storage temperature and preservatives. At 4 °C in fluoride-oxalate tubes, CN⁻ is stable for approximately 7 days. At room temperature, significant losses occur within 24 hours.
A: CN⁻ stability depends on storage temperature and preservatives. At 4 °C in fluoride-oxalate tubes, CN⁻ is stable for approximately 7 days. At room temperature, significant losses occur within 24 hours.
Q: Are there interferences from other fire gases?
A: Hydrogen sulfide (H₂S) can interfere with CN⁻ analysis by microdiffusion. The standard includes procedures to identify and correct for such interferences.
A: Hydrogen sulfide (H₂S) can interfere with CN⁻ analysis by microdiffusion. The standard includes procedures to identify and correct for such interferences.
Q: What sample volume is typically required?
A: For complete analysis (COHb + CN⁻ in duplicate), approximately 3-5 mL of whole blood is recommended. Pediatric cases may use reduced volumes with validated modifications.
A: For complete analysis (COHb + CN⁻ in duplicate), approximately 3-5 mL of whole blood is recommended. Pediatric cases may use reduced volumes with validated modifications.
