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
