January 31, 2010

What does PPD/BCG/TB mean?

Clinical scenario:
In the Hawaii HOME project, we had a series of patients who were being screened for TB with PPDs.  Three of them had a history of (+)PPDs.  Two of them claimed to have a history of (+) chest x-ray without follow-up of medications.  One of them came in with fever and cough.

  • TB: Tuberculosis
    • Caused by Mycobacterium tuberculosis
    • Tubercles are warty, cheesy lesions
    • Respiratory infection characterized by:
      • Cough
      • Hemoptysis (bloody sputum)
      • Fever/Chills/Night sweats
      • Weight loss
    • DIFFICULT: to catch, to culture, to get rid of
  • BCG: Bacillus Calmette-Guerin
    • Calmette (French bacteriologist) and Guerin (assistant) cultured a cow-version of the TB bacteria and created a TB vaccine
    • Similar theory to the smallpox vaccine
      • Jenner made a vaccine from cowpox
    • Causes a false-positive PPD skin test
  • PPD: purified protein derivative aka Mantoux test
    • Sterilized tuberculin glycerol extract from the tubercle bacillus
    • Injected subcutaneously (right under skin)
    • Read 2-3 days later (delayed type IV hypersensitivity reaction)
      • (+) if induration (hardness) forms >5/10/15mm depending on situation
      • (-) if no reaction is present
  • Chest x-ray
    • Test of choice in non-active TB for determining clinical status
    • (+) indicates that lesions suspicious for TB are observable
    • (-) indicates that the patient is at risk for secondary TB activation in the future, but currently does not have active TB assuming the patient is asymptomatic

Why is this important?
Swaziland has the highest prevalence of TB in Africa (1,198:100,000), shortly followed by South Africa (948:100,000). In the Pacific Islands, Cambodia, Kiribati and the Philippines have high rates as well (495, 365 and 290:100,000 respectively.) In Southeast Asia, DPR Korea, Timor-Leste and Bhutan have high rates; also places of conflict.
Many immigrants pass through Hawaii to the mainland US and the immigrant homeless population has a significantly higher risk of exposure to TB and subsequent infection.

Active TB Standard recommended regimen:
2 months of RIPE: rifampicin, isoniazid, pyrazinamide, ethambutol
4 months of RI: rifampicin, isoniazid

Latent TB infection:
6 months of isoniazid, or 3 months of RI
-Advise patients about side effects of isoniazid:
GI upset (loss of appetite, nausea, vomiting, stomach pains)
Weakness, peripheral neuropathy (prevented by vitamin B6 intake)
Liver damage

-Do Cr and LFTs monthly for monitoring

Resolution of clinical scenario:
The two patients with (+)PPDs and (+)chest x-rays were questioned further.
The one with fever and cough was confused and through a translator, thought that the word "positive" meant "good."  In the context of the tests, both were negative.
The other patient had a card with a record locator number.  The main office for TB screening was contacted for confirmation: the chest x-ray was reportedly normal.  This patient had unintentional weight loss of 35 lb over the past year (since becoming homeless.)
The patient was warned about the possibility of "reactivation TB" and was advised to start isoniazid.  The patient agreed to prophylactic treatment and routine testing.

Take home lesson:
Always question your patients about what they mean!  Positive in terms of disease is may not mean the same thing to a patient as it does to a clinician.

WHO estimates of TB incidence by country, 2007
NICE 2006 guidelines: Clinical diagnosis and management of TB and measures for its prevention and control by the UK's National Institute for Health and Clinical Excellence

No comments:

Post a Comment