Blood culture is done to diagnose:

Interceptive orthodontics aims to intercept and correct a developing malocclusion at an early stage, before it becomes fully established. This window corresponds to the mixed dentition period (roughly 6–12 years), when both primary and permanent teeth are present. During this period, problems such as crossbites, crowding, habits (thumb sucking), and arch-length discrepancies can be corrected more easily because the jaws are still growing and teeth are still erupting, making treatment simpler and more effective than after the permanent dentition is fully established.

Reference: Proffit WR, Fields HW, Sarver DM. Contemporary Orthodontics, 6th ed. Chapter on Early Treatment (Interceptive Orthodontics).