Investigations

1st investigations to order

blood gas analysis

Test
Result
Test

Use venous or arterial blood gas analysis to measure carboxyhaemoglobin levels.[17][19]​​[22]​​ This is the gold standard test.

  • An initial carboxyhaemoglobin level >30% indicates severe poisoning; however, significant poisoning cannot be excluded at lower concentrations.[1][10]​ ​

  • Toxic effects of carbon monoxide poisoning appear at carboxyhaemoglobin levels of 15% to 20%.[22]​​

  • Non-smokers will have a baseline carboxyhaemoglobin of 1% to 3%.[1][10]​​[22]​​​

  • People who smoke will tend to have a baseline carboxyhaemoglobin of 5% to 10%.[1][10]​​​ This may reach 15% in those who smoke heavily.[22]​ Acute carbon monoxide poisoning can lead to a carboxyhaemoglobin level of 50% or more in people who smoke.[32]​

  • Be aware that carboxyhaemoglobin levels do not correlate well with clinical outcome.[1][19][17]

Determine the length of time since the patient left the carbon monoxide environment.[19]

  • Carboxyhaemoglobin levels may decrease with time and treatment and may not reflect the true severity of the exposure.[15]

  • If the patient has been breathing room air for several hours, carboxyhaemoglobin levels may be unreliable.[19]

Practical tip

Note that some patients may have a physiologically high carboxyhaemoglobin level due to endogenous production as a result of heme metabolism (e.g., haemolytic anaemia or haemolysis in patients with sickle cell disease).[1]

Measure lactate.[22]

Result

carboxyhaemoglobin level increased above the baseline; >30% may indicate severe poisoning but significant poisoning cannot be excluded at lower concentrations[10]

lactate may be elevated in severe carbon monoxide poisoning[22]

12-lead ECG

Test
Result
Test

Perform a 12-lead ECG to check for sinus tachycardia.​[1][19]

  • Check cardiac rhythm, QRS duration, QT interval, and for features of cardiac ischaemia or infarction.[1][10][22]​​

  • Carbon monoxide poisoning may be associated with angina and cardiac ischaemia and, less commonly, arrhythmias and myocardial infarction.​[1][22]

Result

may show sinus tachycardia; may show arrhythmia; may show ECG changes consistent with angina, cardiac ischaemia, and, less commonly, arrhythmias and myocardial infarction​[1][22]

cardiac monitoring

Test
Result
Test

Check for arrhythmias or other cardiovascular effects.[1]

Result

may show non-specific cardiac rhythm abnormalities: tachycardia, arrhythmias, ischaemia, infarction[1][10]

glucose

Test
Result
Test

Check blood glucose.[10][19]​​ Check for hypoglycaemia in an obtunded patient.

Result

may be normal or elevated[25]

full blood count

Test
Result
Test

Always request a full blood count; check WBC count.

Result

may show leukocytosis

urea and electrolytes

Test
Result
Test

Take blood for urea and electrolytes.[10]

Result

may be deranged

creatinine

Test
Result
Test

Acute kidney injury is a feature of carbon monoxide poisoning.[10]​ See our topic Acute kidney injury. 

Result

may be elevated

troponin

Test
Result
Test

Test cardiac biomarkers.[19]

Elevated troponin-I level may indicate cardiomyopathy and predict poor outcome.[34]

Result

may be elevated

creatine kinase

Test
Result
Test

Provides evidence of skeletal muscle damage.

Creatine kinase level greater than 5 times the normal level, or more than 1000 IU/L indicates rhabdomyolysis. See our topic Rhabdomyolysis.

Result

may be elevated

Investigations to consider

CT head

Test
Result
Test

Request a CT scan of the head when patients present with severe carbon monoxide poisoning (especially loss of consciousness or cardiopulmonary signs and symptoms), or acute neurological symptoms, such as confusion, headache, blurred vision, and/or seizure.[9][19]​ This is to rule out cerebral infarction that is directly due to carbon monoxide poisoning, or differential diagnoses.[9][19]

Result

may show cerebral infarction due to hypoxia or ischaemia

may identify alternative intracranial pathology

magnetic resonance spectroscopy

Test
Result
Test

Consider magnetic resonance spectroscopy and discuss with the neuroradiology consultant. This may indicate sequelae from carbon monoxide poisoning.[19][36][37]​​

Result

may demonstrate cerebral oedema, cerebral atrophy, basal ganglia injury, or cortical demyelination in people with severe poisoning

chest x-ray

Test
Result
Test

Consider for severe carbon monoxide poisoning, especially for patients with loss of consciousness or cardiopulmonary signs and symptoms.[19]​ Be aware that patients can develop pulmonary complications (such as pulmonary oedema) at a later stage.

Result

may show atelectasis, pulmonary oedema

liver function tests

Test
Result
Test

Consider in patients with severe poisoning. Derangement may indicate liver ischaemia.

Result

may be deranged

pregnancy test

Test
Result
Test

Order a pregnancy test for all women of childbearing age with suspected carbon monoxide poisoning.[17][19]​​​ In practice, this is particularly important for moderate or severe poisoning.

  • Exposure to carbon monoxide during pregnancy can cause harm to the fetus.[1]

Result

positive in pregnancy

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