Method Summary: tissue digestion, filtration, examination by scanning electron microscopy (SEM)
Samples:
(1) weighed sample of lung from within a lobe in grams (typically 2 gram)
(2) measured sample of water where the victim supposedly drowned in mL (typically 10 mL)
Method:
(1) The lung tissue was digested by heating the lung sample, 8 mL concentrated nitric acid and 2 mL of 30% hydrogen peroxide for 20 minutes in a microwave oven.
(2) The tissue fluid or environmental water sample were passed through filter membranes with a pore size of 0.45 microns.
(3) The number of diatoms were counted on the membranes by scanning electron microscopy.
number of diatoms in 10 grams of lung =
= (number of diatoms) * 10 / (weight of lung sample in grams)
number of diatoms in 10 mL of water =
= (number of diatoms) * 10 / (mL of water sample)
ratio of lung to water diatoms =
= (number of diatoms in 10 g lung tissue) / (number of diatoms in 10 mL water)
Interpretation:
• A ratio > 2 had a specificity of 100% for drowning with exclusion of postmortem immersion group.
• A ratio > 1 had a specificity of 96%. The maximum ratio for postmortem immersion was 1.26.
• A ratio > 1 had a sensitivity of 91% for drowning, meaning that 9% of drowned victims will have a lower ratio.
• The ratio in drowning victims was 0.02 to 64.7, so that a low ratio does not exclude drowning.
