Investigations
1st investigations to order
plasma salicylate concentration
Test
Take blood for plasma salicylate concentration at least 2 hours (symptomatic patients) or 4 hours (asymptomatic patients) after ingestion, because it may take several hours for peak plasma concentrations to occur with enteric-coated preparations.[2]
Repeat every 2 hours in all symptomatic patients or those with initial plasma salicylate concentrations of ≥200 mg/L (≥1.4 mmol/L) until concentrations are falling and any clinical features have improved.[2]
Ongoing absorption may cause the salicylate concentration to continue rising.[7]
Concentrations of enteric-coated preparations may take up to 12 to 18 hours after ingestion to peak.[2]
However, be aware that salicylate levels taken after 12 hours may underestimate the degree of toxicity owing to tissue binding.[8]
Practical tip
Do not measure salicylate concentrations in patients who are conscious, who report that they have not taken salicylate-containing preparations, and who have no features suggesting salicylate toxicity.[2]
Result
may be positive or negative
ABG
Test
In symptomatic patients, request an ABG at presentation. In practice, continue monitoring acid-base status:
Every 2 hours until the patient’s symptoms have improved, or salicylate levels have peaked and are showing a downward trend[2][7]
With any significant deterioration.
Metabolic acidosis is associated with increased mortality.[2]
In practice, calculate the serum anion gap using the formula [(Na + K) - (Cl + HCO3)]. When using this formula, note the normal range is 8 to 16 mEq/L.
Do not be misled by the absence of a raised serum anion gap if you suspect salicylate poisoning on clinical grounds.
Result
a mixed respiratory alkalosis and metabolic acidosis with normal or high arterial pH (normal or reduced hydrogen ion concentration); serum anion gap is usually increased in severe cases
serum electrolyte panel, urea and creatinine
Test
Check in symptomatic patients on presentation.[2][7]
May be helpful in identifying chronic kidney disease related to chronic salicylate poisoning; however, in practice, a single elevated creatinine concentration does not indicate chronic poisoning (creatinine results repeatedly raised beyond the acute presentation [over several days] are required).
Check calcium, magnesium, and phosphate if the patient has convulsions.[2]
Result
potassium may be low; bicarbonate may be low; hypocalcaemia and/or hypomagnesaemia may also be present; may show renal insufficiency
serum prothrombin time (PT), activated PTT, INR
Test
Check for disseminated intravascular coagulation (DIC) in patients who have any symptoms on presentation. The National Poisons Information Service in the UK only recommends checking INR.[2] In practice, INR alone will not detect all instances of DIC.
Result
INR/PT may be raised
FBC
capillary blood glucose
ECG
Test
Perform a 12-lead ECG in all patients who require assessment. Check cardiac rhythm, QRS duration, and QT interval.[2]
Repeat 12-lead ECGs especially in symptomatic patients or in those who have ingested sustained-release preparations.[2] In practice, do this every 4 to 6 hours.
If available, monitor cardiac function continuously.
Practical tip
Think about the possibility of co-ingestion of other cardiotoxic drugs such as tricyclic antidepressants if ventricular dysrhythmia, monomorphic ventricular tachycardia, and torsades de pointes occur.
Result
tachycardia is common; may show prolonged QRS or QT interval; ventricular dysrhythmias may occur; monomorphic ventricular tachycardia and torsades de pointes may be present; asystole may occur
Investigations to consider
other toxicology tests
Test
In practice, conduct a broader work-up for suspected poisoning in patients who have intentionally self-harmed or attempted suicide.
Measure serum paracetamol levels in all cases of suspected or confirmed salicylate poisoning; patients and carers may err when reporting analgesic exposures.
Order serum ethanol levels, levels of other toxicants that cause metabolic acidosis (e.g., methanol, ethylene glycol), and urine screens for drugs of misuse if clinically indicated, where facilities are available.
In practice, results of urine screens may not be available soon enough to influence emergency management, but they may be important in identifying child abuse.
Result
may show increased concentrations of co-ingestants
CXR
Test
If there are clinical signs of heart failure, it may be helpful to differentiate between pre-existing cardiac failure and acute pulmonary oedema that may be present as a symptom of toxicity.
In practice, differentiating between pre-existing heart failure or acute pulmonary oedema is difficult, particularly in older people who may have pre-existing heart failure and develop acute pulmonary oedema due to salicylate poisoning. Take into account the whole clinical picture.
A CXR may also identify aspiration pneumonia related to the vomiting and depressed level of consciousness that can be associated with salicylate poisoning.
Result
pulmonary oedema may be present; localised consolidation, often in the right lower lobe, may indicate aspiration
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