Analytical Toxicology: Biochemical and Haematological Tests

Biochemical and Haematological Tests

INTRODUCTION

Scientific methods of analysis for poisons have only recently been developed. Until the 19th century, doctors and scientists harboured faulty notions about the effect of poisons on the human body. It was believed that if a dead body was black, blue, or spotted in places, or “smelled bad”, the cause of death was a poison. Other fallacious ideas were that the heart of a poisoned person could not be destroyed by fire, and that the body of a person dying from arsenic poisoning would not decay. The first person to suggest a method for detecting poisons in tissues was the Dutch physician Hermann Boerhoave who theorised that various poisons in hot vaporous condition yielded typical odours. He placed substances suspected of containing poisons on hot coals and tested their smells.

Owing to the widespread use of arsenic as a homicidal poison in the middle ages, it is small wonder that the first mile-stones in the chemical isolation and identification of a poison in body tissues and fluids centred around arsenic. In 1775, Karl Wilhelm Scheele, the famous Swedish chemist, discovered that white arsenic (arsenic trioxide) was converted to arsenious acid by chlorine water, and the addition of metallic zinc reduced the arsenious acid to arsine gas. Gently heating the ensuing gas led to deposition of metallic arsenic on the surface of a cold vessel. In 1821, Sevillas utilised the decomposition of arsine for the detection of small quantities of arsenic in stomach contents and urine in poisoning cases. In 1836, James M Marsh, a London chemist developed the first reliable method to determine an absorbed poison (arsenic) in body tissues and fluids such as liver, kidney and blood.

We have come a long way since then to the present era of sophisticated analytical techniques which can detect even micrograms of virtually any poison in almost any kind of biological specimen. Today, an analytical (toxicology) labora-tory has become a vital adjunct to the proper management of poisoned patients. However it is to be noted that the cornerstone of the management of such patients—intensive supportivetherapy—is mostly independent of the kind of poison impli-cated, and hence routine employment of expensive analytical techniques should be avoided. The attending physician must be judicious in calling for necessary investigations, and exercise discretion in the choice of tests to be done.

The potential indications for seeking the assistance of a toxicology laboratory are as follows :

·      Prognosis—to assess the outcome of a case of poisoning.

·       Research—into toxicokinetics and mechanisms of toxicity.

·      Order—from court, or law enforcement officer.

·      Monitoring—of treatment measures and their efficacy.

·      Identification—of the nature of poison.

·      Severity—to assess the seriousness of a given case.

·      Exclusion—or confirmation of toxic exposure.

All the 7 indications mentioned can be remembered by the mnemonic PROMISE.

Mahoney and associates have categorised treatment ofa poisoning case into 4 groups with respect to toxicological evaluations:

·      Toxicity correlates very well with serum levels, and specific drug therapy can be instituted, e.g. digoxin, ethylene glycol, lithium, methanol, paracetamol, salicylates, theophylline.

·      Toxicity correlates closely with serum level, but only non-specific care is required, e.g. barbiturates, ethanol, phenytoin.

·      Toxicologic testing only serves to confirm fairly clear-cut clinical parameters suggestive of poisoning, e.g. cyanide, narcotics, organophosphates, tricyclics.

·      Toxicity correlates poorly with serum level, and only non-specific care is required, e.g. amphetamines, benzodiaz-epines, cocaine, hallucinogens, neuroleptics.

In fact, most poisoned patients can be treated successfully without any contribution from the laboratory other than routine clinical biochemistry and haematology. This is particularly true for those cases where there is no doubt about the poison involved and when the results of a quantitative analysis would not significantly affect therapy. In those cases where an analytical toxicological investigation is deemed beneficial, an orderly progression is desirable in the performance of necessary tests and their interpretation (Table 35.1).