Chapter 6 Establishing a Minimum Database in Small Animal Poisonings
Successful diagnosis of a particular poisoning is based on a complete history, a thorough physical examination, and physical parameters and laboratory results obtained through establishment of a minimum database. Identification of the presence of a specific toxin allows the clinician to rapidly initiate a specific treatment that may lead to a successful outcome.
Thousands of different molecules have the capacity to poison living organisms. Unfortunately, less than 5% of all potential toxins have antidotes or effective physiological antagonists. Nevertheless the public has unrealistic expectations about antidotes and “poison screening” tests available to veterinarians to identify toxicants. Veterinary toxicology laboratories are currently able to detect and quantitate 200 or 300 or so true poisons. Because of the limitations of (1) expense, (2) time necessary to obtain the samples, run the tests, and obtain results, (3) sample volumes, and (4) the sheer staggering number of potentially poisonous molecules in existence, toxicology screening tests are not commonly pursued in veterinary medicine. However, since only about 15 or 20 types of substances account for almost 90% of all small animal veterinary poisonings, if the clinician has a high index of suspicion of a particular toxin, certain screening tests can be very rewarding and should not be overlooked.
Screening tests are maximally effective only if the clinician has good reason (based on the history and clinical signs) to suspect a particular poison and requests a specific laboratory procedure. Just as specific antidotes should not be administered without a definitive diagnosis, the use of toxicology tests as “fishing expeditions” must be avoided if they are to be successful or meaningful. In cases with potential medicolegal consequences, the use of toxicology tests to confirm or deny the presence of a particular poison and its concentration may be recommended. Identification of a poison in toxic levels in appropriate tissues and organs of an animal that is thought to be poisoned is often the only way to confirm a diagnosis. What steps must be taken to obtain samples to provide such information?
Following a complete history, a thorough physical examination and assessment of any clinical signs that may be present must be undertaken to arrive at a diagnosis of poisoning. Veterinarians must be thoroughly familiar with the vital signs of the species that they care for so that they can instantly recognize any abnormal signs. Vital signs are the first real physical data the veterinarian receives. Respiratory rate and nature, heart rate and rhythm, pulse rate and rhythm, core body temperature, capillary refill time, mucous membrane color, and any apparent dehydration all reveal clues about an animal’s condition. Electrocardiography, pulse oximetry, and Doppler blood pressure monitoring are all readily available methods of documenting an animal’s immediate condition more accurately. If the animal is in a life-threatening state, emergency methods must be initiated long before a diagnosis of poisoning can be obtained or therapy started. It must be remembered that a final diagnosis is not necessary to stabilize or correct a life-threatening condition. Once the animal is stable, the specific tests and steps necessary to achieve a minimum toxicological database can be started.
Physical examination and assessment of clinical signs are invaluable and cannot be separated from either the history or laboratory results in the attempt to obtain a definitive diagnosis of poisoning. At the same time, it should be noted that clinical signs for many conditions are stereotyped and similar, and that cells, tissues, and organs can respond in only a limited number of ways that can look much the same, regardless of the cause (e.g., traumatic, toxicological, infectious, metabolic, or neoplastic). For this reason, poisoning is rarely diagnosed through the physical examination or clinical signs alone. Finally, basing a diagnosis solely on the clinical signs is unreliable because the veterinarian may be seeing only one phase of the disease and may have missed important earlier phases that help to reveal the identity, course, and chronology of the intoxication.
In developing a minimum toxicological database, the small animal clinician must keep in mind what diagnostic tests laboratories are capable of running, which toxins can be detected and which ones cannot be detected, sample types and minimum sample volumes, the turn-around times involved in obtaining results, and the costs. Laboratory results remain a supportive adjunct to a complete history, a thorough physical evaluation of the animal, and recognition of specific clinical signs of certain poisons. To maximize the efficacy of laboratory results, veterinarians must have knowledge of the proper use of each test, a basic grasp of specific laboratory procedures, and an understanding of how to obtain and handle specimens properly. This information can be readily obtained by consulting directly with laboratory personnel. The best guide to the diagnosis, treatment, and selection of the most appropriate laboratory tests is the clinical condition of the animal in question. In addition, the small animal clinician can also check with local human hospitals to see what routine toxicology tests are available.
For every suspected life-threatening poisoning, routine laboratory studies should include a complete blood count, determination of serum electrolytes, glucose, blood urea nitrogen, creatinine, and calcium. A urinalysis should be routinely obtained. Prothrombin time, activated clotting time, or a coagulation panel can be included to identify abnormal coagulation. Pulse oximetry and electrocardiography are performed to assess both hemoglobin saturation (SpO2) and cardiac irregularities. Liver enzyme tests are usually performed to monitor damage caused by poisons that are directly toxic to or degraded by the liver. Arterial blood gas evaluation is included when available to evaluate respiratory status and acid-base abnormalities. The anion gap should be determined to assist in the diagnosis and management of certain types of poisoning. Finally, radiographs should be taken of both the chest and the abdomen to help identify pulmonary edema, aspiration pneumonia, and any radiopaque toxins in the gastrointestinal tract. Vomitus must be examined as an indicator of recent oral ingestion of any type of poison. Feces should also be collected. Veterinarians can assist the laboratory by including a list of which toxins are suspected.
Clinicians must remember to “treat the patient, not the poison.” There is no single accurate, rapid, and inexpensive method (“poison screen”) that can detect all toxins. Just as specific antidotes should not be used without a narrow index of suspicion based on a matching history, confirming physical examination and clinical signs, and supportive laboratory results, veterinarians must never delay supportive therapy while awaiting a confirmatory laboratory test or positive toxin level in a critically ill animal. The following are the individual components of the minimum toxicological database.
This test reveals hemoconcentration, anemia (e.g., possible zinc ingestion), aplastic anemia, platelet deficiency, potential basophilic stippling, and the morphology of thrombocytes, erythrocytes, and leukocytes. Blood is also useful for detecting most elements and metals, some pesticides, cholinesterase activity, ethylene glycol, iron, ethanol, methanol, and many drugs (e.g., acetaminophen, salicylates, theophylline, digoxin).
Metabolic acidosis results either from the increased production or decreased excretion of nonvolatile acids, or from the loss of body alkali. The serum anion gap helps to distinguish between these two types of metabolic acidosis. Most toxins produce an increased anion gap by the accumulation of organic acids. The anion gap is measured by subtracting the measured anions (Cl− and HCO3−) from the measured cations (Na+ and K+). The normal anion gap is 12 to 25 mEq/L. Accumulation of unmeasured anions (e.g., sulfates, phosphates, protein, and organic acids) results in an increased or abnormal anion gap. Most often a high anion gap is associated with metabolic acidosis and is caused by accumulation of organic acids, such as lactate or formate. Toxins that can cause an elevated anion gap and metabolic acidosis include alcohol, methanol, toluene, ethylene glycol, paraldehyde, iron, salicylates, and any toxin that causes lactic acid build-up. Ethylene glycol should be suspected whenever an animal presents with a high anion gap of unknown cause.