Whooping cough (pertussis) and croup are distinct respiratory illnesses that affect breathing, particularly in children. Pertussis is a highly contagious infection caused by the bacterium Bordetella pertussis, known for its severe, uncontrollable coughing fits. Croup is typically a viral infection characterized by swelling in the upper airway that leads to a harsh, seal-like “barking” cough. Differentiating these conditions is essential for proper treatment and infection control, and diagnosis relies on the timely collection of specific biological materials.
Diagnostic Specimen Collection for Whooping Cough
The diagnosis of whooping cough depends on collecting a specimen from the patient’s upper respiratory tract where the bacteria reside. The preferred specimen for detecting Bordetella pertussis is a nasopharyngeal swab or aspirate, collected from the nasopharynx (the upper part of the throat behind the nose).
The collection process involves gently inserting a flexible, sterile swab deep into the nasal cavity until it reaches the posterior nasopharynx. The swab is rotated to gather mucus and epithelial cells before being withdrawn. Alternatively, a nasopharyngeal aspirate involves flushing a saline solution into the nose and then suctioning it out. This aspirate method is especially effective for infants and young children, as it often yields a higher concentration of the bacteria.
Timing is a determining factor for the specimen’s reliability because the bacterial load decreases quickly over the course of the infection. Collection is most effective during the initial catarrhal phase, typically the first one to three weeks after the cough begins. Once the patient enters the paroxysmal phase or has started antibiotic treatment, the ability to isolate the bacterium drops significantly. For the most accurate results, the specimen should be collected before the third week of coughing and prior to any antimicrobial therapy.
Croup Diagnosis: Clinical Assessment vs. Specimen Testing
Unlike whooping cough, the diagnosis of croup rarely requires laboratory testing. Croup is overwhelmingly a clinical diagnosis, meaning a healthcare provider identifies the condition based on the patient’s history and physical examination. The classic symptom of a barking cough, sometimes accompanied by stridor (a high-pitched, noisy breathing sound), is usually sufficient for diagnosis.
Routine specimen collection is not recommended because the majority of croup cases are caused by common, self-limiting viruses and testing does not change the course of treatment. Treatment focuses on managing airway inflammation, often with steroids, regardless of the exact viral cause. Furthermore, obtaining a swab can cause distress in a child, which may worsen airway obstruction and is therefore avoided.
A specimen is only collected in specific circumstances, such as when the diagnosis is unclear, the illness is unusually severe, or the patient requires hospitalization. Testing may be performed to rule out other respiratory illnesses that share similar symptoms, such as influenza, respiratory syncytial virus (RSV), or COVID-19. The specimen collected is a nasopharyngeal swab or aspirate, used for viral identification rather than bacterial culture.
Analyzing the Respiratory Specimens
Once the nasopharyngeal specimen is collected, different laboratory techniques identify the causative agent. For suspected whooping cough, the two main methods are Polymerase Chain Reaction (PCR) and bacterial culture. PCR testing is the preferred method because it is highly sensitive, detecting minute amounts of the bacterium’s genetic material (DNA).
PCR provides rapid results and remains reliable for up to four weeks after the cough onset, even if the bacteria are no longer viable. Bacterial culture has lower sensitivity and takes longer—sometimes up to two weeks—but is considered the gold standard for specificity. Culture is important for public health surveillance as it allows the laboratory to isolate the living Bordetella pertussis organism for strain identification and antibiotic susceptibility testing.
When a specimen is collected for suspected viral croup, the laboratory primarily uses molecular techniques, such as multiplex PCR panels. These tests quickly screen the specimen for multiple potential viral pathogens, including parainfluenza virus, the most common cause of croup. Rapid antigen detection tests may also be used for certain viruses like influenza, providing a quick result, though these tests are less sensitive than PCR.