Which of the following tests is NOT commonly used to diagnose asthma?

The complete blood count (CBC) is not commonly used to diagnose asthma because it mainly provides information about the overall health of a patient, including red blood cells, white blood cells, and platelets. While it can indicate the presence of an infection or other underlying health issues, it does not provide specific insights into lung function or airway responsiveness, which are critical for diagnosing asthma.

In contrast, pulmonary function tests (PFTs) are a cornerstone in asthma diagnosis, as they measure lung function and airflow obstruction. These tests help determine if the patient's airways are constricted and can show the reversibility of airflow obstruction after bronchodilator administration, which is a hallmark of asthma.

Similarly, arterial blood gas (ABG) tests assess the gases in the blood, including oxygen and carbon dioxide levels, allowing for an evaluation of respiratory function and the effectiveness of gas exchange. An X-ray, while not definitive for asthma, can be useful to rule out other conditions that may mimic asthma symptoms, such as pneumonia or lung tumors.

Thus, the CBC's lack of direct relevance to assessing lung function or airway hyperreactivity makes it the least suitable among these options for diagnosing asthma.