POSSIBILITIES AND LIMITATIONS OF CHEMICAL EXCLUSION SCREENING FOR SOME COMMON POISONS

Consideration of exogenous chemicals as etiologic agents of illness is frequently only an appended, parenthetical thought, or even entirely omitted. This is partly due to the assumption that laboratory examinations for “poisons” are impracticably complex and time consuming, except for forensic purposes. This may be true for exhaustive, systematic toxicologic analyses. However, most toxicologic situations can be narrowed down to relatively few possibilities by cogently evaluating the environment, history, signs, and symptoms. At this point, two kinds of approaches to toxicologic laboratory examinations are possible. One classical toxicologic approach is to go through specific isolation, identification, and quantitation procedures for the chemicals under consideration to obtain accurate values for definitively identified substances. This approach may not be as time consuming as it sounds, but it does often require the highly specialized skills, experience, and apparatus usually available only to the practicing forensic toxicologist. The second approach is that of toxicologic exclusion screening by test procedures in which negative results definitely rule out any contributions of the suspected agents to the clinical situation. Positive results may require further confirmatory testing as well as careful evaluation. For many, if not most, common poisons such simple and rapid exclusion tests are available and their performance does not require great skill, experience, or specialized equipment. Unfortunately, such methods-their sensitivities and selectivities as well as their toxicologic interpretabilities-are widely scattered through the literature and generally only practicing forensic toxicologists have collected and evaluated them for their own use. However, these specialists are no further away than the nearest telephone. For this presentation, nine chemicals have been chosen which are likely to be of concern to veterinarians. Of these, thallium, elemental phosphorus, arsenic, mercury, fluoride, and borate are usually encountered as economic poisons. Selenium and oxalate usually become problems by their natural presence in high concentrations in certain plants eaten by animals. Nitrate as a natural contaminant of water or as an ingredient of fertilizer is of concern because it may be converted to the methemoglobin forming nitrite in the gastrointestinal tract of animals.