Diagnostic Studies : Tuberculosis

Diagnostic Studies

  • Tuberculin skin test (TST)
    • AKA: Mantoux test
    • Uses purified protein derivative (PPD) injected intradermally
    • Assess for induration in 48 – 72 hours
    • Presence of induration (not redness) at injection site indicates development of antibodies secondary to exposure to TB
    • Positive if ≥15 mm induration in low-risk individuals
    • Response decrease in immunocompromised patients
    • Reactions ≥5 mm considered positive
    • A waning immune response can cause false negative results
    • Repeating TST may boost reaction
    • Two-step testing recommended for health care workers getting repeated testing and those with decreased response to allergens
    • Two-step testing ensures future positive results accurately interpreted
  • Interferon-γ gamma release assays (IGRAs)
    • Detects  T-cells in response to Mycobacterium tuberculosis
    • Includes QuantiFERON-TB and T-SPOT.TB tests
    • Rapid results
    • Several advantages over TST but more expensive
  • Chest x-ray
    • Cannot make diagnosis solely on x-ray
    • May appear normal in a patient with TB
    • Upper lobe infiltrates, cavitary infiltrates, lymph node involvement, and pleural and/or pericardial effusion suggest TB
  • Bacteriologic studies
    • Required for diagnosis
    • Consecutive sputum samples  obtained on 3 different days
    • Stained sputum smears examined for AFB
    • Culture results can take up to 8 weeks
    • Can also examine samples from other suspected TB sites


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