Emedinews:Insights on Medicolegal Issues:Failure to detect poison in chemical analysis by forensic lab

In some cases no trace of poison is found on analysis although from other circumstances it is quite certain that poison was the cause of illness or death. The possible explanations of a negative finding are:

• The poison may have been eliminated due to vomiting, stomach wash or diarrhea
• The whole of the poison may have disappeared from the lungs by evaporation or oxidation
• The poison after absorption may have been detoxified, conjugated and eliminated from the system
• Some drugs are rapidly metabolized, making extraction difficult.
• Some biological toxins and venoms, which are protein in nature cannot be separated from body tissues.
• Some organic poisons especially alkaloids may, by oxidation during life, or due to faulty preservation, or a long interval of time, or from decomposition of the body, deteriorate and cannot be detected.
• If the poison act slowly and death is delayed following production of irreversible organic changes, the poison may be completely metabolized and excreted.
• Many drugs may be present in very small amounts and these may require considerable amount of viscera for their identification.
• When the wrong or insufficient material has been sent for analysis.

Emedinews:Insights on Medicolegal issues:Very old, very young or in patients in poor health harmed more by poison

• The effect of exposure depends partly on how long the contact lasts and how much poison gets into the body, and partly on how much poison the body can get rid of during this time. Exposure may happen only once or many times.
• Acute exposure is a single contact that lasts for seconds, minutes or hours, or several exposures over about a day or less.
• Chronic exposure is contact that lasts for many days, months or years. It may be continuous or broken by periods when there is no contact. Exposure that happens only at work, for example, is not continuous. Chronic exposure to small amounts of poison may not cause any signs or symptoms of poisoning at first.
• It may be many days or months before there is enough chemical inside the body to cause poisoning. For example, a person may use pesticide every day.
• Each day the person is exposed to only a small amount of pesticide, but the amount of pesticide in the body gradually builds up, until eventually, after many days, it adds up to a poisonous dose. Only then does the person begin to feel unwell.
• WHO states that a poison is any substance that causes harm if it gets into the body. Harm can be mild for example, headache or nausea or severe for example, fits or very high fever, and severely poisoned people may die.

(Contributed by Dr Sudhir Gupta)