Investigations
1st investigations to order
serum paracetamol concentration
Test
Escalate to critical care if the serum paracetamol concentration is very high (>700 mg/L) and is associated with coma and elevated lactate level.
Use serum paracetamol concentration to risk-stratify the likelihood of liver injury and to determine whether treatment with acetylcysteine is needed.[3]
Acute single overdose <8 hours since ingestion
Wait until at least 4 hours after paracetamol ingestion before taking the sample in patients who have ingested paracetamol as an acute overdose (i.e., all doses taken within 1 hour).[3]
Consider repeating the serum paracetamol concentration at 6 hours after ingestion if the result at 4 hours is below the threshold for treatment with acetylcysteine and the patient has taken concomitant medications that decrease gastric emptying (e.g., opioids) as these can falsely decrease the result.[38]
Staggered overdose or therapeutic excess
Take the sample at least 4 hours after the last dose of paracetamol was ingested in patients who have ingested all paracetamol tablets over more than 1 hour but less than 24 hours.[3]
Consider repeating the serum paracetamol concentration at 6 hours after ingestion if the result at 4 hours is below the threshold for treatment with acetylcysteine and the patient has taken concomitant medications that decrease gastric emptying (e.g., opioids) as these can falsely decrease the result.[38]
Acute single overdose >8 hours since ingestion
Take blood immediately for urgent measurement of serum paracetamol concentration (before acetylcysteine is given if possible) in patients who have ingested paracetamol as an acute overdose (i.e., all doses taken within 1 hour).[3]
Risk stratification
Assess the risk of liver injury using the treatment nomogram for your region to determine whether acetylcysteine is needed.[3][27] MHRA: treatment nomogram for paracetamol overdose Opens in new window
Result
may be positive
liver function tests
Test
Suspect acute liver injury if alanine aminotransferase (ALT) is above the upper limit of normal.[3]
Result
may be elevated
prothrombin time and INR
Test
Measure the prothrombin time and INR.
Discuss urgently with a senior colleague if prothrombin time >30 seconds or INR is >1.3.
A higher cut-off of >100 seconds is used to identify patients who may require liver transplantation. However, it is common in practice to escalate patients to a senior colleague at this lower cut-off of 30 seconds.
Result
prothrombin time may be prolonged; INR may be increased
blood glucose
Test
Discuss the patient urgently with a senior colleague if hypoglycaemia is present as this may indicate acute liver injury.[28]
Result
<3.3 mmol/L [60 mg/dL]
urea, creatinine, and electrolytes
Test
Discuss the patient urgently with a senior colleague if serum creatinine is >300 micromoles/L, as this is an indication for urgent liver transplant.[3]
Creatinine will be raised in acute kidney injury. This may occur as part of acute liver injury (hepatorenal syndrome) or, rarely, in the absence of acute liver injury.[3]
Re-check creatinine in 8 to 12 hours if it has risen significantly.[3]
Result
creatinine may be acutely elevated in acute kidney injury; creatinine >300 micromoles/L is an indication for urgent liver transplant
venous or arterial blood gas
Test
Take a venous blood gas initially. If this shows significant abnormalities, it is common practice to take an arterial blood gas.
A blood gas may show lactic acidosis in two scenarios:
Early after a large paracetamol ingestion[43]
Lactic acidosis is commonly severe and associated with coma. Most patients do not develop liver damage if treated with acetylcysteine.[43]
Late after development of liver failure[43]
Elevated lactate in these patients strongly predicts high mortality.[43]
Discuss with a senior colleague urgently if serum lactate >3.5 mmol/L on admission or >3.0 mmol/L post-paracetamol ingestion or after fluid resuscitation as this is an indication for urgent liver transplant.[3]
Result
may show lactic acidosis
Investigations to consider
salicylate level
Test
Order if you suspect concurrent salicylate poisoning.[44] See Salicylate poisoning.
Result
may be positive
urinalysis
Test
Haematuria or proteinuria present at least 24 hours after paracetamol ingestion may indicate acute kidney injury.[3]
Result
may show haematuria or proteinuria
urine drug screen
Test
Order a urine drug screen in the following groups:
Vulnerable people
Involve a senior colleague and follow your local safeguarding protocol if there is a positive result as this may indicate abuse
Patients with suspected mental illness
Drug intoxication may mimic some mental illnesses[45]
People who display signs of drug misuse.
Result
may be positive
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