Chickenpox is usually diagnosed by taking a patient’s complete medical history and performing a physical examination. In most cases, no other testing is needed. However, in rare cases other tests may be used to help diagnose chickenpox or a related condition. These include:

• Tzanck smear. A blister is opened and the base scraped with a scalpel blade for microscopic analysis on a glass slide. A Tzanck smear may be performed to confirm or rule out the presence a herpes simplex virus (HSV) which may have similar symptoms.
  
  
• White blood cell count (WBC). Tests the overall number of white blood cells (cells that fight disease) in the body. A WBC showing an abnormally high level of white blood cells (leukocytosis) may indicate the body is fighting infection.
  
  
• Viral culture. May be performed to detect the presence of the virus that causes chickenpox (varicella-zoster) but is usually unnecessary. Bacterial culture of blisters may also be done if signs of additional infections are present (superinfection).
  
  
• Imaging studies (e.g., x-ray). May be used if a secondary complication is suspected (e.g., pneumonia).