Tuberculosis other diagnostic studies

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mashal Awais, M.D.[2]; Alejandro Lemor, M.D. [3]

Overview

Other diagnostic studies that could be performed in a patient with tuberculosis are the Xpert MTB/RIF test, Adenosine Deaminase Test, and Nucleic Acid Amplification Test(NAAT).

Other Diagnostic Studies

Xpert MTB/RIF Test

• The Xpert MTB/RIF test is a molecular test that detects the DNA of the Mycobacterium tuberculosis complex (MTBC) and genetic mutations associated with resistance to rifampin (RMP) in unprocessed sputum and concentrated sputum sediments
• WHO recommends the Xpert MTB/RIF test for the initial diagnosis of MDR-TB or HIV-TB co-infection.
• The advantages of this rapid TB test are the following:

• Detects M. tuberculosis and rifampicin drug resistance simultaneously.
• Results are available in < 2 hours so the patient can be treated the same day of the test.
• The bio-safety requirements and training are minimal.
• It can be stored in non-conventional laboratories.

Adenosine Deaminase

It is usually an auxiliary test if tuberculosis is suspected in the patient.

• ADA is used for diagnosing tuberculosis in endemic countries where TB diagnostic procedures are expensive.
• ADA isoenzymes are more accurate. For both pleural TB and TB meningitis , ADA has a high degree of sensitivity.

Nucleic Acid Amplification Tests (NAAT) Adapted from CDC

• This is a heterogeneous group of tests that use polymerase chain reaction (PCR) to detect a mycobacterial nucleic acid.
• These tests vary in which nucleic acid sequence they detect and vary in their accuracy.
• The two most common commercially available tests are the amplified Mycobacterium tuberculosis direct test (MTD, Gen-Probe) and Amplicor (Roche Diagnostics).
• The CDC recommends that NAA testing should be performed on a respiratory specimen from each patient with signs and symptoms of active pulmonary TB disease for whom a diagnosis of TB is being considered (i.e., TB suspect), but has not been established.
• NAA testing does not replace the need for AFB smear and culture. All current guidelines and recommendations for culture-based testing should remain in effect, especially recommended turn around times for culture and DST.
• A single positive NAA test result can support the diagnosis of TB in a patient for whom there is a reasonable index of suspicion. This result should trigger reporting to public health officials, initiation of treatment if not already started, and intensified efforts to obtain an isolate for drug susceptibility testing.
• In a patient with little suspicion of having active TB, a single positive NAA test result should be viewed with suspicion (i.e., a possible false-positive result) and interpreted in the same way as a single culture-positive result, i.e., by correlating the results with other diagnostic findings.
• A single negative NAA test result should never be used as a definitive test to exclude TB, especially in suspects with a moderate to high clinical suspicion of TB. Rather, the negative NAA test result should be used as additional information to aid in making clinical decisions to expedite a work-up for an alternative diagnosis or to prevent unnecessary use of TB treatment in suspects with a low clinical suspicion.
• The FDA-approved NAA tests for TB have slightly less sensitivity than culture-isolation methods, and the 15% to 20% of U.S. TB cases that are reported with negative culture results may also have negative NAA test results. Thus, a negative NAA test result does not exclude the diagnosis of TB.
• Further research is needed before specific recommendations can be made on the use of NAA testing in the diagnosis of TB in children who cannot produce sputum and in the diagnosis of extrapulmonary TB, although there is much anecdotal evidence of the utility of such testing in individual cases.