Uncertainty in estimating blood ethanol concentration by analysis of vitreous humour

Aim-To determine the concentrations of ethanol in femoral venous blood (FVB ) and vitreous humour (VH) obtained during forensic necropsies. The ratios of ethanol concentrations in VH and FVB, the reference interval, and the as sociated confidence limits were calculated to provide information about the uncertainty in estimating FVB ethanol concentrations indirectly from that measured in VH. Methods-Ethanol concentrations were determined in specimens of FVB and VH o btained from 706 forensic necropsies. The specimens were analysed in duplic ate by headspace gas chromatography (HS-GC), with a precision (coefficient of variation) of 1.5% at a mean ethanol concentration of 500 mg/litre. The limit of detection of ethanol in body fluids by HS-GC in routine casework w as 100 mg/litre. Results-In 34 instances, ethanol was present in VH at a mean concentration of 154 mg/litre, whereas the FVB ethanol concentration was reported as nega tive (< 100 mg/litre). These cases were excluded from the statistical analy sis. The concentration of ethanol in FVB was higher than in VH in 93 instan ces, with a mean difference of 160 mg/litre (range 0 to 900). The mean conc entration of ethanol in FVB (n = 672) was 1340 mg/litre (SD, 990) compared with 1580 mg/litre (SD, 1190) in VH. The arithmetic mean VH/ FVB ratio of e thanol was 1.19 (SD, 0.285) and the 95% range was 0.63 to 1.75. The mean an d SD of the differences (log VH log FVB) was 0.063 (SD, 0.109), which gives 95% limits of agreement (LOA) from -0.149 to 0.276. Transforming back to t he original scale of measurement gives a geometric mean VH/FVB ratio of 1.1 6 and 95% LOA from 0.71 to 1.89. These parametric estimates are in good agr eement, with a median VH/FVB ratio of 1.18 and 2.5th and 97.5th centiles of 0.63 and 1.92. Conclusions-The ethanol distribution ratios (VH/FVB) show wide variation an d this calls for caution when results of analysing VH at necropsy are used to estimate the concentration in FVB. Dividing the ethanol concentration in VH by 2.0 would provide a very conservative estimate of the ethanol conten t in FVB, being less than the true value, with a high degree of confidence.