Differential Diagnosis

1. Asthma
What is it: paroxysmal narrowing of the bronchial airways due to inflammation of the bronchi and contraction of the bronchial smooth muscle.

Symptoms: dyspnea (unpleasant or uncomfortable breathing), cough, and wheezing, chest tightness, shortness of breath

But: Pertussis is also associated with vomiting and sputum. To confirm whether it is asthma or pertussis a definitive culture diagnosis or blood-work is done.


2. Pneumonia
What is it: inflammation of the substance of the lungs/ Inflammatory illness in the lungs. It is usually caused by a bacteria

Symptoms: presence of cough, purulent sputum , fever ,nausea, vomiting, loss of appetite, joint pains, unusually rapid breathing, chest pains, muscle aches, high fever with shaking chills , cough producing yellow sputum.

But: Pertussis is not asscociated with joint pains and shaking chills. Hence, pneumonia is ruled out.


3. Tuberculosis
What is it: caused by Mycobacterium tuberculosis and occasionally M. bovis and M. africanum.

Symptoms: cough, wheezing, lupus vulgaris(presents on head or neck with red brown nodules), choroiditis(An inflammation of the layer of the eye behind the retina, either in its entirely (multifocal choroiditis) or in patches (focal choroiditis)).

But: Lupus vulgaris and chroiditis is not associated with the symptoms of Pertussis. Hence Tuberculosis is ruled out.


4. Febrile Seizures
What is it: convulsions brought on by a fever in infants or small children.

Symptoms: upper respiratory infection, otitis media(inflammation of the middle ear), viral syndrome, and they respond with comparably higher temperatures

But: Pertussis is not associated with otitis media. Hence, Febrile seizure is ruled out.

Diagnosis:
Suggested clinically by the the characteristics of the whooping cough and a history of contact with an infected individual. It is confirmed by the isolation of the organism. Cultures or swabs of nasopharyngeal secretions result in a higher positive yield than cultures of ‘cough plates’.

Workup

These tests can be conducted to confirm the diagnosis.
Laboratory Studies

1. Blood work
   Lymphocytosis is often profound (>70% of the total WBC count), especially in children.
   The WBC count often increases to 20-40,000 or even 100,000 cells/mm2.
   In adults, especially those that had been vaccinated, lymphocytosis is rare.
2. Cultures
   A definitive culture diagnosis is not always possible.
   Results of blood culture are uniformly negative because B pertussis grows solely in the respiratory epithelium.
   An immediately plated, deep, culture of a nasopharyngeal swab sample grown in Regan-Lowe charcoal agar or fresh Bordet-Gengou is considered the criterion standard for those who present within the first 3 weeks of their cough. The results are positive in <50% (perhaps 15-40%) of cases, and results become available too late (about 1 week) to be clinically useful. The CDC recommends this test to characterize the illness.
   Direct fluorescent antibody (DFA) studies
   DFA studies are performed by using a nasopharyngeal sample.
   Although the results can be available within minutes, its use is not recommended because of both low sensitivity and low specificity.
   Results are positive in 40-80% of patients and are now used to confirm most cases.
   Specimens should be obtained within the first 3 weeks of the disease (ie, in incubation, catarrhal, or early paroxysmal stages) or the sensitivity and specificity decrease.
3. Polymerase chain reaction (PCR) testing to detect DNA
   PCR testing may reveal <10 organisms per swab sample.
   Its sensitivity may be greater than that of culturing.
   False-positive results have been a problem, with some reports of more than 50%. Although this or a positive culture is the case definition for reporting pertussis to the CDC or WHO, some are now recommending ELISA confirmation before declaring an epidemic.
4. Enzyme-linked immunosorbent assay (ELISA) is also useful. Many now consider serologic testing with ELISA to be the criterion standard.
5. Imaging Studies
   Chest radiography may show focal atelectasis and/or peribronchial cuffing.
   The CDC recommends both culture and PCR tests if a patient has a cough lasting longer than 3 weeks.
   

Link:
http://emedicine.medscape.com/article/803186-diagnosis