To test or not to test: Effective use of toxicology laboratories (Proceedings)


To test or not to test: Effective use of toxicology laboratories (Proceedings)

Aug 01, 2008

The establishment of a toxicologic diagnosis requires the same approach as that for any non-toxicologic diagnosis: obtaining a comprehensive history, establishing a minimum database, collection of appropriate samples for analysis and correct interpretation of laboratory results. In suspected intoxications, three scenarios are possible: an asymptomatic or symptomatic animal has been exposed to a known toxicant, an asymptomatic or symptomatic animal has been exposed to an unidentified toxicant or an animal is showing clinical signs due to an unknown cause for which toxicants should be included in a differential diagnosis. In many situations, a client calls asking whether an asymptomatic animal should be examined following a known ingestion of a chemical. In such situations, establishment of an exposure dose, if possible, and comparison of the exposure dose with specific toxicity information about a chemical is a critical first step in deciding subsequent advice (e.g., observing the animal at home or bringing the animal to the hospital for possible decontamination).

Case History

A proper history is an essential first ingredient for a successful toxicologic diagnosis. A good clinical history may provide specific clues or may only point the veterinarian and the diagnostician in a general direction. A good clinical history includes animal signalment, a thorough description of clinical signs, if observed, husbandry practices, environmental conditions and possible chemical exposures. Notation of pre-existing health problems is also important since underlying disease processes can contribute to increased sensitivity of an animal to a chemical. Important historical information includes the following: breed, sex, age, weight, current medical treatments, including any dietary supplements or herbs, number of animals in the group, number of animals affected, number potentially exposed, number exhibiting clinical signs, type, duration and severity of signs, possible time of exposure to identified chemicals, degree of exposure (dose) if known or, if worst case estimates can be made, and chemical formulations if possible [1, 2, 3, 4]. If an animal has been exposed or potentially exposed to an identified product, and a decision is made that the exposure is clinically significant, it is always recommended that the product be brought to the clinic for inspection if it can be handled in a safe manner. Examination of product labels can assist in the exposure assessment and provide a phone number if additional information about the product is needed.

It is important that the veterinarian not be misled by the perceptions of the owner. In many cases, owners are convinced that their animal has been "poisoned." This statement can lead many veterinarians to consider only a toxicologic etiology in lieu of other infectious or non-infectious causes of illness. Alternatively, the veterinarian should be careful in suggesting to a client that his or her animal appears to have been poisoned, when inadequate evidence exists to support such a conclusion.

Suspected cases of intoxication are often emotional and potentially involve litigation, especially when affected animals are rare and/or valuable. Because of the potential for litigation, it is incumbent upon the veterinarian to document all procedures and consider following a chain-of-custody process for all samples. Proper chain-of-custody involves creating a paper trail for each sample that documents in detail how a sample was stored, shipped, tested and discarded.

In many toxicoses, the only clinical sign is death. A thorough postmortem examination is essential in such circumstances. This may help eliminate non-toxicologic etiologies or perhaps narrow the list of possible toxicants. It should be kept in mind that many toxic agents may cause non-specific lesions or no lesions at all. Often, when a postmortem examination is done in the clinic, tissue samples are collected for either histologic or toxicologic examination, but not both. Two sets of tissue samples from animals with suspected toxicoses should be routinely saved. One set should be preserved in 10 percent buffered formalin for histologic evaluation and another set frozen for possible toxicologic analysis. A common and often unforgiving mistake is failure to submit brain, spinal cord, or peripheral nervous tissue when signs referable to the central or peripheral nervous system are present. A prudent and cost-effective procedure in unexplained deaths is to submit a full set of tissues for histologic examination following gross examination and to keep a second set frozen for later toxicologic analysis pending the histologic findings. It is always easier to dispose of unneeded frozen tissues than to collect tissues from an animal that has already been buried or otherwise discarded.