mosbys-diagnostic-and-laboratory-test-reference-12th-ed COHB

210 carboxyhemoglobin
carboxyhemoglobin (COHb, Carbon monoxide)
Type of test Blood
Normal findings
Saturation of hemoglobin
Nonsmoker: <3%
Smoker: ≤12%
Newborn: ≥12%
Possible critical values >20%
Test explanation and related physiology
This test is used to detect carbon monoxide poisoning. It
measures the amount of serum COHb, which is formed by the
combination of carbon monoxide (CO) and hemoglobin (Hb).
CO combines with Hb 200 times more readily than oxygen
(O 2
) can combine with Hb; thus fewer Hb bonds are available
to combine with O 2
. Furthermore, when CO occupies the O 2
binding sites, Hb is changed to bind the remaining O 2
more
tightly. This greater affinity of CO for Hb and this change in O 2
binding strength do not allow O 2
to pass readily from RBCs to
tissue. Less O 2
is therefore available for tissue cell respiration.
This results in hypoxemia.
CO poisoning is documented by Hb analysis for COHb. A
specimen should be drawn as soon as possible after exposure
because CO is rapidly cleared from Hb by breathing normal air.
O 2
saturation studies and oximetry are inaccurate in CO-exposed
patients because they measure all forms of oxygen-saturated Hb,
including COHb. In these circumstances, the patient’s oximetry
will be good, yet the patient will be hypoxemic.
This test can also be used to evaluate patients with complaints
of headache, irritability, nausea, vomiting, and vertigo,
who unknowingly may have been exposed to CO. Its greatest
use, however, is in patients exposed to smoke inhalation, exhaust
fumes, and fires. Other sources of CO include tobacco smoke,
petroleum and natural gas fuel fumes, automobile exhaust,
unvented natural gas heaters, and defective gas stoves. The treatment
of CO toxicity is administration of high concentrations of
O 2
to displace the COHb.
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: no.
• Blood tube commonly used: lavender or green.