Sodium Fluoride / Potassium Oxalate (Grey-Top Tube)
Definition
The standard blood collection tube for forensic ethanol analysis. Sodium fluoride inhibits microbial fermentation (which would produce artefactual ethanol post-collection); potassium oxalate is the anticoagulant to prevent clotting.
- Tube colour code
- Grey top
- Preservative
- Sodium fluoride
- Anticoagulant
- Potassium oxalate
- Primary forensic use
- Blood ethanol (GC-headspace) analysis
Common questions
What error would result from using a plain clotted (no-additive) tube for a blood alcohol sample?+
Without sodium fluoride, microorganisms in the blood can continue metabolising glucose and can ferment it into ethanol after the sample is drawn, artificially inflating the measured blood alcohol level well after collection.
Why is an anticoagulant necessary in addition to the fluoride preservative?+
Without an anticoagulant such as potassium oxalate, the blood would clot in the tube, which interferes with accurate pipetting and headspace sampling on the gas chromatograph and can make the specimen unsuitable for automated analysis.
Does the grey-top tube preserve other drugs the same way it preserves ethanol?+
Not necessarily. Sodium fluoride is specifically effective at inhibiting the fermentation pathway relevant to ethanol; other drug classes have their own stability considerations, and toxicology protocols may specify different tubes or storage conditions for a broader drug panel.
Related terms
- 2100:1 Partition Ratio
- The conventional blood/breath partition assumption used by breath alcohol analysers worldwide: 2100 mL of venous blood contains the same mass of ethanol...
- Beta Elimination Rate (Beta)
- The rate at which BAC falls during the post-absorptive phase of alcohol metabolism, expressed as mg/100 mL/hour. Population mean approximately 15 mg/100...
- Blood Alcohol Concentration (BAC)
- Mass of ethanol per unit volume of whole blood, expressed as mg per 100 mL (equivalent to mg/dL or mg%), or as...
- DB-ALC1 / DB-ALC2
- A matched pair of Agilent fused-silica GC columns used for dual-column orthogonal confirmation of blood ethanol. DB-ALC1 uses a polyethylene glycol stationary...
- Internal Standard (IS)
- A compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sample at a known...
- NIST SRM 1828
- A series of NIST-certified reference material aqueous ethanol standards (0.02 to 0.40 per cent w/v) used to calibrate forensic blood ethanol analytical...
- Partition Coefficient (K)
- The ratio of analyte concentration in the liquid phase to concentration in the gas phase at equilibrium. For ethanol in blood at...
- Static Headspace
- A sample preparation technique in which a sealed vial is heated until volatile analytes reach thermodynamic equilibrium between the liquid matrix and...
- Widmark Factor (Rho / R)
- A dimensionless pharmacokinetic constant that represents the distribution of ethanol across body water. Averages approximately 0.68 for adult males and 0.55 for...