Poisons Assessment ASSESSMENT
Known toxins It is neither possible nor necessary to measure the concentration of every agent in biological fluids. Emergency measurement of the plasma concentration is essential for the poisons shown in Table 3;17 the concentration of these toxins is important to ensure appropriate clinical management. Other agents for which assays should be available on a 24-hour basis at specialist laboratories include carbamazepine, digoxin and phenobarbital. The concentration of a known toxin is usually measured in plasma or serum rather than whole blood, lead being an excep- tion. Carboxyhaemoglobin and methaemoglobin are also measured in blood e the sample should be taken in a heparin- ized blood gas syringe, taking care to exclude air bubbles. If lithium concentration is required, serum (or clotted blood)
rather than plasma (or anticoagulated blood) should be sent to the laboratory. Since many of the commercially available tubes that are used to prevent the sample from clotting already contain lithium as part of the anticoagulant, their use will lead to falsely high concentrations.
Screening for unknown toxins Plasma paracetamol concentration should be measured in all unconscious poisoned patients e multiple drug overdoses are common and paracetamol is one of the drugs most often involved, particularly in Europe. It is important to diagnose an overdose at a stage when administration of antidotes might prevent liver damage and death. In contrast, routine requests for salicylate concentrations cannot be justified. Salicylate poisoning seldom causes coma and it is unlikely that a clinically significant concentration will be present in a patient without the typical signs of salicylism. Laboratory screening for poisons in an unconscious patient is
usually requested when the cause of coma is unknown. Although identification of a toxin may reassure the clinician, this is not a good reason for the request.18 Before proceeding, the clinician should consider how the result of a screen might alter manage- ment. The pattern of drugs involved in poisoning in most devel- oped countries is such that specific treatment (e.g. antidotes, techniques to enhance elimination of the poison) is unlikely to be available, and management will therefore be supportive. Screening is labour-intensive, time-consuming and expensive,
and in most cases cannot be justified on an emergency basis because it will not alter the management of the patient. Further- more, drugs given therapeutically may confuse interpretation of the result of a qualitative drug screen. If a laboratory screen for poisons is considered essential, it is
important to discuss it with the laboratory staff in advance, to inform them of substances that might cause analytical confu- sion and to determine the nature of the samples to be assayed. Urine (20 mL) is usually the most appropriate fluid for detect- ing unknown poisons. Once identified, their concentration in blood or plasma can be determined by specific methods, if necessary.
Non-toxicological investigations
Information about the nature of poisons ingested can occasion- ally be obtained from standard haematological and biochemical investigations, and from arterial blood gas analysis (Table 4).
Haematology Generally this is of little diagnostic value, though prolongation of prothrombin time may be secondary to ingestion of anticoagu- lants or may be caused by liver necrosis in paracetamol or hepatotoxic mushroom poisoning.
Biochemistry Routine biochemistry and arterial blood gas analysis may provide useful and important information. Electrolyte, glucose, acidebase and osmolality disorders are discussed later in this chapter. In developed countries, undisclosed paracetamol over- dose is a strong diagnostic possibility in any patient who presents with, or without, jaundice associated with plasma alanine aminotransferase activity of more than 2000 IU/litre.
Non-toxicological investigations C
C
Serum sodium (e.g. hyponatraemia in ecstasy poisoning)
Serum potassium (e.g. hypokalaemia in theophylline poisoning; hyperkalaemia in digoxin poisoning, rhabdomyolysis, haemolysis)
C
Plasma creatinine (e.g. renal failure in ethylene and diethylene glycol poisoning)
C
Poisons for which emergency measurement of plasma or serum concentration is essential
C C Carboxyhaemoglobin C C C
Ethanol (when monitoring treatment in ethylene glycol and methanol poisoning) Ethylene glycol Iron
Lithium
C Methanol C
C C
Paracetamol Salicylate
Theophylline Table 3 MEDICINE 40:2 July 2012 51 C C
Blood sugar (e.g. hypoglycaemia in insulin and severe untreated paracetamol poisoning, hypoglycaemia and hyperglycaemia in salicylate poisoning)
C
Serum calcium (e.g. hypocalcaemia in ethylene glycol or paracetamol poisoning)
Serum alanine aminotransferase/aspartate aminotransferase activities (e.g. increased in paracetamol poisoning)
Serum phosphate (e.g. hypophosphataemia in paracetamol induced renal tubular damage)
C C Acidebase disturbances, including metabolic acidosis
RBC cholinesterase activity (e.g. organophosphorus insecticide and nerve agent poisoning)
C
Whole-blood methaemoglobin concentration (e.g. in nitrite poisoning)
Table 4 2011 Published by Elsevier Ltd. Africa Health 43
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