Chemical Analysis - Medicolegal Aspects of Poisoning

CHEMICAL ANALYSIS

In every case of death due to poisoning, an attempt must be made to demonstrate the presence of poison by standardised analytical methods. For this purpose, the pathologist conducting the autopsy must collect certain of the viscera and body fluids, and despatch them through the police to the nearest forensic science laboratory. 

While submitting the samples for analysis it must be ensured that the correct quantity has been preserved in appropriate preservative in suitable, sealed containers. The materials to be collected are mentioned in Table 4.3, while the recommended preservatives are mentioned in Table 4.4.

Sample Collection and Preservation

The stomach is cut between double ligatures at the cardiac and pyloric ends, and transferred to a clean tray. It is slit open along the greater curvature and the contents examined. The stomach is then placed along with its contents in a clean, wide-mouthed glass bottle of 1 litre capacity. It is preferable to preserve the contents separately in a different container. Similarly, the first part of the jejunum is identified and a length of about 30 cm is cut between double ligatures at either end, and transferred to a tray. It is then slit open and placed along with its contents into the same container as the stomach. However, the contents can be preserved separately.

The liver is removed from the body in the usual manner, and about 500 grams portion is cut and preserved in another container of 1 litre capacity. It is desirable to include the gall bladder, since some drugs are concentrated in the bile, such as paracetamol, barbiturates, and opiates. The liver should always be sliced into pieces before placing it in the container, so that the preservative can exert its action more thoroughly.

The kidneys are dissected out of the body and one half of each is sliced and placed in the same container as the liver. Preserving one kidney alone may not be advisable, since it may happen to be dysfunctional.

It is important that the blood collected for analysis should never be withdrawn directly from the heart or scooped out of the thoracic or abdominal cavities, since some poisons such as alcohol and barbiturates can diffuse passively after death from the stomach or intestines into adjacent organs or cavities leading to erroneous results. Blood should always be collected into a clean syringe from a peripheral vein such as those in the neck or the limbs. The femoral vein is most suitable for withdrawing a blood sample and can easily be approached percutaneously. A 30 or 50 ml syringe with a wide-bore needle must be used. The vein is located in the inguinal canal midway between the ante-rior superior iliac spine and the pubic tubercle, just medial to the femoral artery. The needle should be inserted perpendicularly. Do not “milk” the limb while collecting the blood sample since this can produce significant alterations in drug concentrations in the expressed blood. About 10 to 20 ml is withdrawn and transferred to a clean vial or bottle which should be capped tightly and sealed with molten wax to minimise evaporation of volatile poisons.

In putrefying bodies, it may be difficult to obtain blood from peripheral veins, besides the fact that the results of analysis may be vitiated by postmortem generation of alcohol as a result of decomposition. In such cases, it may be preferable to use cerebrospinal fluid or vitreous humor for analysis. Some investigators suggest that if alcohol is detected in blood but not in urine and vitreous humour, it should be assumed that postmortem synthesis of alcohol has occurred.

Urine can be collected by puncturing the bladder with a needle and syringe, and aspirating about 30 to 50 ml. However, if there is very little urine it may be necessary to make a small incision on the anterior bladder wall and scoop out the urine with a spoon, or aspirate it with a syringe or pipette. Care should be taken to ensure that the urine obtained is not contaminated with blood. The collected sample should be transferred into a described for the blood sample.

In some cases, additional viscera or body fluids may have to be preserved depending on the nature of the poison (Table 4.3). Cerebrospinal fluid is difficult to obtain by lumbar puncture at autopsy. It is easier to collect it by cisternal puncture. With the neck flexed, palpate the atlanto-occipital membrane in the midline and using a needle and syringe, gently introduce the needle through the skin at that point, directing the needle towards the bridge of the nose. As the atlanto-occipital membrane is punctured at a depth of approximately 2 cm, loss of resistance will be felt, at which point the CSF can easily be aspirated. As an alternative, CSF can be aspirated anteriorly after evisceration by introducing a needle into the spinal theca via the spinal foramina between the 1st and 2nd lumbar vertebrae. If lung is to be preserved for analysis (in inhaled poisons and solvent abuse), it is preferable to place it inside a nylon bag and then heat-seal the bag.

If all the specimens collected for chemical analysis can be despatched to the laboratory immediately and analysis can be done within 24 hours, no preservative need to be added to any of the specimens. However this is almost never possible in reality, and so either the samples should be sent in an insulated ice box with sufficient crushed ice, or suitable preservative must be added to all the containers before despatching (Table 4.4). It is important to label all the containers with details as to the postmortem number, crime number (if any), name of the deceased (if known), police station to which the case belongs, the date of collection, the exact nature and quantity of the specimen, the nature of preservative, and the name, designation, and signature of the pathologist. A sample of the preservative used should be sent separately so that it can be analysed as to its purity and presence of contaminants, if any. All containers should be properly packed and sealed with sealing wax.

It is important to remember that chemical analysis reports are not always infallible, and the medical officer would do well to consider all other aspects including clinical notes, eye witness accounts, and his own observations at autopsy, before pronouncing the cause of death.