Principles of Toxicologic Diagnosis

Toxicologic diagnosis is based on four principal types of evidence: an accurate and thorough history, evaluation of clinical signs and physical examination, prudent use of clinical pathology laboratory tests and/or gross/histologic evaluation of lesions, and appropriate use of specific toxicologic tests. Each of these is important, and a diagnosis is almost never obtained from only one of these four pillars of diagnosis. This chapter will focus on appropriate laboratory testing.

Choosing appropriate specimens and laboratory tests almost always requires attention to a thorough history of actual or potential exposures. This requires effective questioning of owners or caretakers, some knowledge of the animal environment and care, and a reasonable account of the time course of a potential toxicosis. Important historic findings include changes in location or in food source, recent chemical applications (e.g., spraying for insects, fertilizing a lawn, changing radiator coolant), administration of medications, whether the animal is free to roam, and distance to public or commercial areas.

In most cases of acute poisoning, a combination of history and clinical findings must be used to initiate life support and effective detoxification of the animal. One can rarely wait for results of a laboratory chemical test. Prompt clinical laboratory testing (complete blood count [CBC], serum clinical chemistry, blood gas analysis, electrolytes, and urinalysis) can quickly detect disturbances in major metabolic pathways or specific organ damage. Such information can also be correlated with known laboratory effects from specific toxicants.

To assist with clinical observations and help determine organ systems affected, Tables 17-1 through 17-6 show probable toxicoses associated with different organ systems and clinical signs (e.g., seizures, hepatic failure, vomiting); however, very few toxicoses have pathognomonic signs. While many signs of poisoning (e.g., vomiting, seizures) are also caused by infectious, metabolic, and endocrine diseases, knowledge of high-risk toxicology differentials is the first step in working through the diagnostic process. A thorough history and physical examination should be completed to determine potential exposure to toxicants. Telephone instructions should include having the owners collect any potential baits, vomitus, or other suspect materials for possible chemical analysis.

TABLE 17-1 COMMON TOXICANTS OF THE NERVOUS SYSTEM BY MAJOR EFFECT(S)

TABLE 17-2 TOXICANTS OF THE DIGESTIVE SYSTEM

TABLE 17-3 TOXICANTS of the hepatobiliary system

TABLE 17-4 TOXICANTS of the cardiovascular system

TABLE 17-5 TOXICANTS OF THE BLOOD AND BONE MARROW

TABLE 17-6 TOXICANTS of THE KIDNEYS

Clinical Laboratory Tests

After clinical data are obtained from history and physical examination, laboratory testing is used to identify and characterize pathophysiologic effects typical of specific toxicants (e.g., basophilic stippling and nucleated red blood cells [RBCs] without reticulocytosis suggest lead toxicosis; high serum osmolality, hypocalcemia, and an increased anion gap are consistent with ethylene glycol [EG] toxicosis). Table 17-7 summarizes clinical laboratory tests that may be altered by specific toxicoses. Several clinical laboratory parameters may be altered by some toxicants, although others may cause no significant changes.

TABLE 17-7 CLINICAL LABORATORY TESTS THAT MAY BE ALTERED BY TOXICOSES

A major use of the toxicology laboratory is to determine the presence and possibly the amount of a specific toxicant. Two levels of toxicant testing are suggested by various authorities. The first is clinical evaluation (as in the previous paragraph). These include blood gases, electrolytes, glucose, blood urea nitrogen (BUN), creatinine, hepatic function tests, anion gap, osmolality, and renal function. The second level is specific drug or toxicant assays.⁶ After clinical laboratory evaluation, clinicians may utilize some form of “screening” test for rapid turnaround to suggest potential toxicant groups. Depending on results of screening procedures, a specific confirmatory instrumental or immunologic procedure may be chosen to support the initial diagnosis. These are often most important when there is potential product liability or a need for forensic evidence (as in suspected malicious poisoning). Quantification of a positive test may be important for interpretation of some toxicoses, but not for others. For example, any amount of strychnine in stomach contents is considered diagnostic of a toxicosis while other toxicant diagnoses depend on quantitation (e.g., lead) above a threshold to establish toxicosis. Quantification of toxicants is greatly influenced by passage of time and the half-life of that specific toxicant. Thus if specific confirmatory tests are anticipated, early collection of vomitus, feces, blood, and urine may be the key to a good chemical laboratory confirmation.