Nucleic Acid Amplification (NAA) Testing

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Brief Description

Nucleic Acid Amplification (NAA) testing is a rapid screening test for Mycobacterium  tuberculosis complex (MTBC).  It is performed on respiratory specimens from patients with symptoms of pulmonary tuberculosis and results in a presumptive diagnosis in 48 hours of specimen collection compared to more than 7-10 days for culture.

Laboratory Unit

Tuberculosis

Methodology

Ribonucleic Acid (RNA) target-amplified genetic probe; GenProbe MTD®

Specimen Collection

Sputum:
  • An early morning, deep cough specimen collected on three (3) consecutive days is best for initial diagnosis of tuberculosis
  • If sputa are collected within the same 24-hour period, a minimum of eight (8) hours between specimens are required
  • Minimum acceptable specimen volume is two (2) mL
  • Ship specimens each day collected to facilitate rapid diagnosis; do not hold until three (3) specimens are collected
  • Refrigerate until shipped
Bronchial Washing:
  • Preferred minimum acceptable volume is two (2) mL
  • Refrigerate until shipped

Storage/Transport

Utilize the SPHL Category B TB kit for shipment

Acceptable Specimen
Type(s)

Sputum and Bronchial Washing

Only clinical respiratory specimens are accepted for NAA testing.  Processed specimens will not be tested.

Test Request Form(s)

Tuberculosis Test Request Form

Possible Results

Positive for M. tuberculosis complex rRNA.

Negative for M. tuberculosis complex rRNA.

Equivocal

Reference Range

N/A

Unacceptable Conditions

Patients who have not received antitubercular therapy, less than 7 days of such therapy, or have not received such therapy in the last 12 months may be evaluated with the NAA test.

Bloody specimens cannot be tested.

Processed specimens will not be tested.

Interfering Substances

N/A

Result Reported

Results are reported within 48 hours of receipt or when test is requested.

Fees

$48 charge – Private healthcare submitters

CPT Codes

N/A

LOINC/SNOMED Codes

48174-7

Additional Information

Guidelines for Testing: The Centers for Disease Control and Prevention (CDC) recommends NAA testing be performed on at least one respiratory specimen from each patient with signs and symptoms of pulmonary tuberculosis. There should be a high level of suspicion the patient has tuberculosis when requesting NAA testing. This high level of suspicion is more than just a positive Acid-fast bacillus (AFB) smear from sputa.

Ordering NAA Test: Use the TB laboratory request form to order “Smear & Culture” and “NAA testing” located in the clinical specimen area of the form. Be certain to answer all questions asked under NAA testing to avoid delaying processing of the specimen.

Limitations: The NAA test result is presumptive; culture is the gold standard for laboratory diagnosis of tuberculosis. The positive predictive value of the U.S. Food and Drug Administration (FDA)-approved NAA test is > 95% in AFB smear-positive cases. When the clinical suspicion is low, positive predictive value of the NAA test falls to < 50%.