What initial diagnostic test may be considered for breath-holding spells?

Breath-holding spells, which are involuntary, reflexive episodes often seen in young children, can sometimes be associated with underlying health issues, such as anemia. Therefore, performing a complete blood count (CBC) to check for anemia is a prudent initial diagnostic test. Anemia can contribute to these spells by causing episodes of pallor or fainting, thus addressing possible blood deficiencies can help in understanding whether anemia is a contributing factor in the child's episodes.

Other diagnostic tests listed, such as MRI scans, chest X-rays, or allergy testing, do not directly address the common causes or associations with breath-holding spells. An MRI would be excessive as it is not typically warranted in cases like this unless there are other neurologic concerns. Chest X-rays focus on respiratory issues and would not provide insight into the blood or neurological factors that could be involved in breath-holding spells. Allergy testing would also not relate directly to this condition, as breath-holding spells are primarily a reflex action rather than an allergic response. Thus, the CBC is the most relevant initial test in this context.