USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What is a characteristic presentation of Carbon Monoxide Poisoning on blood analysis?

1. Increased total O2 content

2. Decreased RBC count

3. Increased RBC stack formation

4. Decreased %saturation of Hemoglobin

The correct answer is: Decreased %saturation of Hemoglobin

In carbon monoxide poisoning, the primary issue is that carbon monoxide (CO) binds to hemoglobin with a much higher affinity than oxygen, forming carboxyhemoglobin. This binding decreases the amount of hemoglobin available to carry oxygen, leading to a reduced percentage saturation of hemoglobin with oxygen. Thus, the characteristic blood analysis finding in carbon monoxide poisoning is indeed a decreased percentage saturation of hemoglobin, indicating a compromised ability to transport oxygen throughout the body. The other options do not accurately reflect the typical changes seen in patients with carbon monoxide poisoning. The total oxygen content in the blood may not significantly increase, as the CO-bound hemoglobin does not deliver oxygen effectively. The red blood cell (RBC) count may remain normal unless there is an additional contributing factor. Similarly, while changes such as increased RBC stack formation may indicate other pathological states, they are not specifically characteristic of carbon monoxide poisoning.