What is the next best step in diagnosing carbon monoxide poisoning after clinical presentation?

The next best step in diagnosing carbon monoxide poisoning after assessing the clinical presentation is measuring carboxyhemoglobin levels. Carbon monoxide binds to hemoglobin to form carboxyhemoglobin, and its levels in the blood help quantify exposure and severity of poisoning. Elevated levels indicate that carbon monoxide is present and confirm the diagnosis.

While chest X-rays and blood gas analyses can provide useful information in various scenarios, they do not directly measure carbon monoxide exposure or its effects. A chest X-ray might show lung pathology unrelated to carbon monoxide poisoning, and blood gas analysis might demonstrate metabolic acidosis or hypoxia, but neither will specifically confirm carbon monoxide binding. A complete blood count, although informative regarding overall health and potential other conditions, does not help in diagnosing carbon monoxide poisoning. Therefore, measuring carboxyhemoglobin levels is the most direct and informative diagnostic step following clinical presentation.