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 (O₂ ) can combine with Hb; thus fewer Hb bonds are available to combine with O₂ . Furthermore, when CO occupies the O₂ binding sites, Hb is changed to bind the remaining O₂ more tightly. This greater affinity of CO for Hb and this change in O₂ binding strength do not allow O₂ to pass readily from RBCs to tissue. Less O₂ is therefore available for tissue cell respiration. This results in hypoxemia.

Carbon monoxide 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. O₂ 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 com plaints 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.

Procedure and patient care

 • See inside front cover for Routine Blood Testing.

 • Fasting: no

• Blood tube commonly used: lavender or green

• Obtain the patient’s history for possible sources of CO.

• Assess the patient for signs and symptoms of mild CO toxicity (e.g., headache, weakness, dizziness, malaise, dyspnea) and moderate to severe CO toxicity (e.g., severe headache, bright red mucous membranes, cherry-red blood). Maintain patient safety precautions if confusion is present.

 • Treat the patient as indicated by the physician. Usually the patient receives high concentrations of O2 to displace the COHb.

* Encourage respirations to allow the patient to clear CO from the Hb.

 Abnormal findings

 Carbon monoxide poisoning