Croup is a clinical diagnosis. Generally laboratory tests are unnecessary. The white count is typically normal or with a slight lymphocytosis. Blood cultures or nasopharyngeal washings are not clinically useful, since knowing the exact etiologic agent does not change management.
Radiographs are primarily used to help rule out other causes of stridor in atypical or prolonged cases. A lateral neck film and posteroanterior (PA) chest radiograph should be obtained as appropriate. The aryepiglottic area is normal. The lateral neck radiograph may demonstrate slightly ill-defined tracheal air shadows, narrowing on inspiration greater than that on expiration, and slight distention of the hypopharynx. Typically fixed subglottic obstructions—such as papillomas, foreign bodies, hemangiomas, and subglottic stenosis—cause narrowing of the airway that does not change with the phase of respiration. Many of the latter noncroup causes also cause asymmetry in appearance, whereas croup causes symmetrical changes in the air column. The PA chest x-ray is most useful to rule out radiopaque foreign body. In cases of croup, the normally squared shoulders of the subglottic tracheal air shadow will appear more like a "steeple," "pencil tip," "nail," or "hourglass."
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