Croup is diagnosed primarily by its symptoms, not by lab tests or imaging. A doctor can usually identify croup based on the combination of a distinctive barking cough, a harsh breathing sound called stridor, and a hoarse voice, often preceded by cold-like symptoms. Most children with croup never need an X-ray or blood work.
The condition typically affects children between 6 months and 3 years old, though it can appear as early as 3 months and as late as 15 years. It peaks in fall and early winter but occurs year-round.
What Doctors Listen and Look For
The hallmark of croup is a cough that sounds like a seal barking. It’s distinctive enough that many parents recognize it before they even call the doctor, and many pediatricians can identify it over the phone. The cough results from swelling in and just below the voice box, which narrows the airway and produces that characteristic sound.
Beyond the cough, doctors listen for stridor, a high-pitched, raspy sound your child makes when breathing in. Stridor that only happens when a child is upset or crying suggests a milder case. Stridor that’s audible while the child is resting at rest points to something more serious. Doctors also check for retractions, which are visible tugging motions in the skin around the chest and neck as your child works harder to pull air through a narrowed airway. They’ll look at your child’s overall color, energy level, and how well air is moving into the lungs.
Croup symptoms almost always worsen at night, so your child may seem fine during the day and suddenly develop that barking cough after bedtime. This pattern itself is a diagnostic clue.
How Severity Is Scored
Once croup is identified, doctors assess how serious it is using a standardized scoring system called the Westley Croup Score. It evaluates five things: level of consciousness, skin color changes (particularly bluish discoloration around the lips), the presence and timing of stridor, how well air is entering the lungs, and how much the chest retracts with each breath. Each component gets a numerical score, and the total determines severity.
- Mild (score 0 to 2): Occasional barking cough, no stridor at rest, no visible retractions. Most children with croup fall into this category.
- Moderate (score 3 to 7): Stridor at rest, visible retractions in the chest and neck, but the child is still alert and has normal skin color.
- Severe (score 8 to 11): Frequent barking cough, prominent stridor on both inhaling and exhaling, marked retractions, and the child may appear noticeably lethargic.
A child approaching respiratory failure can actually become quieter, not louder. The cough may fade because the child is too exhausted to cough forcefully, and stridor may become harder to hear because so little air is moving. Lethargy, decreased consciousness, or a dusky or bluish skin tone without supplemental oxygen are signs that a child needs intensive care immediately.
When Imaging Is Used
X-rays are not part of a routine croup diagnosis. However, if a doctor is uncertain whether the problem is croup or something else, a neck X-ray can help. The classic finding on a frontal neck X-ray is called the “steeple sign,” a narrowing of the airway below the voice box that makes the air column look like a church steeple instead of having its normal rounded shape.
For a child in severe distress, even a single side-view X-ray may be all that’s practical. The goal is to rule out other causes of breathing difficulty, not to confirm croup itself.
Why Blood Tests and Swabs Are Rarely Needed
Lab work is seldom necessary for diagnosing croup. Viral cultures and rapid antigen tests can identify the specific virus causing the infection (most often parainfluenza), but knowing which virus it is doesn’t change treatment. Blood tests like inflammatory markers similarly add little to the picture when the symptoms are straightforward. These tests are reserved for unusual presentations or when a doctor suspects a bacterial complication.
Conditions That Can Look Like Croup
Part of diagnosing croup is ruling out more dangerous conditions that cause similar symptoms, particularly in children who look unusually sick or aren’t responding to treatment.
Epiglottitis is the most important one to distinguish from croup. Children with epiglottitis develop symptoms rapidly, often within 12 to 24 hours, and typically have a high fever, drooling, and a muffled “hot potato” voice. They tend to sit leaning forward in a tripod position and resist lying down. Unlike croup, coughing is uncommon with epiglottitis, and the child generally looks much more toxic and anxious. Epiglottitis has become rare since the introduction of the Hib vaccine, but it still occurs.
Bacterial tracheitis is another serious mimic. Children with this condition have a high fever and a toxic appearance that doesn’t improve with standard croup treatments. It requires antibiotics and sometimes intensive care.
Doctors also consider the possibility that a child has inhaled a small object, especially in toddlers with sudden onset of breathing difficulty and no preceding cold symptoms. A foreign body in the airway can produce stridor and breathing distress that mimics croup but won’t follow the typical pattern of worsening at night and improving during the day.
Red Flags That Change the Assessment
Certain signs shift a doctor’s thinking from routine croup to a potentially life-threatening situation. A toxic appearance, drooling, and difficulty swallowing are important red flags that suggest something more serious than viral croup. Within croup itself, signs of impending respiratory failure include lethargy or decreased consciousness, dusky or bluish skin without supplemental oxygen, and paradoxically quieter breathing sounds as the child tires. Children with these features who don’t respond to initial treatment are referred for intensive care or advanced airway management.