190_tuberculin test _ fact and myth
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Tuberculin Test
Fact & Myth
Magdy M Khalil
Prof. Chest Medicine, Ain Shams University
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History
Tuberculin is an extract of tubercle bacilli
Robert Koch 1890: TB remedy Clemen von Pirquet 1890s: TB immunity
Seibert 1930s: PPD
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Mantoux test
ID, 0.1ml of PPD, 5TU
Reading after 48-72 h
Induration
Positive: 10 mm
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Five years ago
32 ys old lady , pregnant 3rd trimester
R. cervical lymphadenopathy TB skin test: positive
Anti TB
Do you recommend TB test in this case?Q:
Anti TB for 3 months with good response
Delivered and stopped medications
Resumed anti TB medications 6 months later
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Two weeks ago
R. thyroid swelling Ultrasound: Mass with breakdowns
TB skin test: positive
Referral to chest clinic for anti TB
Q: Do you recommend TB test in this case?
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Natural history
Organism: Myc. tuberculosis
Entry
Respiratory tract
Alimentary tract
Skin
Immune response Cell-mediated immunity
Delayedtype hypersensitivity
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InfectionLatent TB
No symptoms
No spread of infection Radiology ?
Microbiology: -ve
TB skin test: Positive (usually)
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DiseaseTB
Symptoms: +ve
May spread infection Radiology: abnormal
Microbiology: positive
TB skin test: Positive (usually)
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TBHealedOld May have radiographic findings
No symptoms or symptoms related to
fibrosis
TB skin test: Positive (Usually)
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Facts Searching for diseases?
Positive TB skin test indicates infection;
not necessarily disease
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Facts Screening for infection?
TB skin test is not absolutely
sensitive or specific for TB
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Screening for TB infection?
Contacts
Anti TNF-alfa
Immigration
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Highly +ve TB skin test?
Disease: No prognostic significance
Infection:
>25 mm ? Risk for active disease
Strong antigenic stimuliMore efficient response
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TB skin test and BCG?
Distinguishing between a TB skin testreaction caused by infection with M.tuberculosis and a reaction caused by
BCG is difficult However:
Larger induration
Contact with TBinterval between vaccination and testing
the greater is likelihood that the reaction
represents infection with M.tuberculosis
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TB blood test?IFN- release assays (IGRAs)
QuantiFeron-TB Gold or T Spot-TB
Infectionspecificity (latent TB vs prior BCG)
Disease? sensitivity in detecting active TB
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Back to the lady!
Would you have recommended TB test?