Laboratory:
A) In a cyanotic patient, a methemoglobin level should be
obtained to confirm the diagnosis. Methemoglobin levels
will be artificially low if blood is not analyzed within
a few hours.
B) Pulse oximetry may give a false reading in the 80% to low
90% range. An arterial blood gas test will reveal a
falsely normal calculated oxygen saturation despite low
measured pulse oximetry. If oxygen saturation is
measured, it will be low relative to the pO2. This
saturation gap suggests methemoglobinemia.
C) An ECG should be obtained to screen for myocardial
ischemia. Cardiac biomarkers should be obtained if
evidence of ischemia is present on ECG.
D) A CBC with microscopy should be performed to evaluate for
hemolysis.
E) Blood with more than 15% methemoglobinemia will appear
chocolate brown and may be an early bedside indication of
methemoglobinemia.
F) If chronic cyanosis (not due to pulmonary disease) is
present, especially if a family history of cyanosis is
suggested, hemoglobin electrophoresis, methemoglobin
reductase level, and/or G-6-PD activity may be indicated.