Type of test Blood, sputum, fluid analysis
Normal findings
Test explanation and related physiology
Blood tests such as interferon gamma release assays (IGRA, e.g., QuantiFERON-TB or T-Spot.TB), nucleic acid amplification test (NAAT), and serologic TB testing are used to diagnose active or latent TB infection. The gold standard for making the diagnosis of active TB is the TB culture. However, it takes 2 to 6 weeks to obtain results. Identifying acid-fast bacilli in a smear (AFB smear) (p. 906) of the body fluid (usually sputum) is a rapid method of identifying TB in 24 hours. Unfortunately, AFB is not very sensitive or specific. The IGRA is a whole-blood test used in diagnosing Mycobacterium tuberculosis infection. The NAAT is a rapid and accurate test of sputum and is used as corroborative information in the diagnosis of TB. Serology testing on blood is also a rapid test used to identify active TB disease infection.
The determination as to whether the patient has active or latent TB still requires additional testing (including chest radiography, sputum smear, and culture). IGRA is a diagnostic aid that measures a component of cell-mediated immune reactivity to M. tuberculosis similar to the tuberculin skin testing (TST) (p. 904). IGRA can be performed on patients with prior bacille Calmette-Guérin (BCG) vaccination without causing a hyper sensitivity response. Similar to TST, false negatives can occur in anergic patients.
IGRA and NAAT are used in the same patient population as TST. These include contact investigations, evaluation of recent immigrants (from some countries), HIV, dialysis, malnourished patients, and sequential-testing surveillance programs for infection control, such as those for health-care workers.
NAAT is designed to identify TB complex DNA in a body fluid (bronchoalveolar lavage, bronchial washing, sputum, stool, pleural or abdominal fluid, tissue, or urine sample). This test pro vides a rapid result in 24 hours. Not only does this test provide early diagnosis and allows treatment, it can also indicate resistance to rifampicin, a drug commonly used to treat TB. Similar to the above-described rapid tests, NAAT cannot indicate active infection from a previously treated TB infection.
TB antibody serology is designed to identify IgG antibodies to TB mycobacteria in patients with active TB infections. This blood test can be used in previously vaccinated BCG patients. It is particularly useful in evaluating the effectiveness of anti TB therapy and documenting a response to therapy. Similar to IGRA, serology results may not be positive in immunocompromised patients, making use in HIV-infected patients less helpful.
Interfering factors
• Heterophile (e.g., human antimouse) antibodies in serum or plasma of certain individuals are known to cause interference with immunoassays.
• A false-negative IGRA result can be because of the stage of infection (i.e., specimen obtained before the development of cellular immune response), comorbid conditions that affect immune function, or other individual immunologic factors.
Procedure and patient care
Before
* Explain the procedure to the patient or the family.
During
• Collect 1 mL of whole blood in each of three laboratory- specified collection tubes. The accuracy of the IGRA depends on the proper collection and incubation of the blood specimen. Blood should fill the tube as close to the 1-mL mark as possible. Under- or overfilling the tubes outside the 0.8- to 1.2-mL range may lead to erroneous results.
• Immediately after collection, each specimen tube must be shaken vigorously by shaking the tube up and down 10 times to ensure that the entire inner surface of the tube has been coated with blood. This distributes the stimulating antigens, allowing optimal processing and presentation of the antigens to T-cells, which causes release of interferon-γ.
• For NAAT testing, 1 to 3 mL of sputum or body fluid is required. This should be refrigerated in a screw cap sterile container.
After
• Apply pressure or a pressure dressing to the venipuncture site. If the patient’s results are positive, educate the patient on the necessary follow-up studies, such as chest radiography and sputum cultures.
Abnormal finding
- TB infection
