NUCLEIC ACID AMPLIFICATION TEST (NAAT) M. tuberculosis complex (MTBc)

 

What is TB NAAT?  

Detecting M. tuberculosis (MTB) complex with traditional laboratory culture methods takes 1 to 8 weeks.  Nucleic Acid Amplification Test for TB is a molecular test that can detect MTB genetic material from specimens within 1 to 2 days of specimen collection.  The test is intended for use with respiratory specimens from patients showing signs and symptoms of active pulmonary TB. The WPHL has access to NAAT testing for Tuberculosis. WPHL also has access to molecular testing for rifampin resistance, a marker of multi-drug resistant TB (MDR-TB).

 

Who is at risk? 

Populations most likely to be at risk for TB infection in the region may include:          

 

·         Foreign born

·         American Indian

·         People 65 or older

·         Homeless

·         People with alcohol problems

 

Who should be tested?  

It is important to identify patients at risk for tuberculosis and who are good candidates for rapid NAA testing. The test is intended for use with respiratory specimens from patients showing signs and symptoms of active pulmonay TB.  The NAAT should not be used for patients who have taken TB medications in the last twelve months or who have taken TB medications for more than 7 days.  NAA tests can detect nucleic acids from dead as well as live organisms.  Therefore the test can remain positive for long periods in patients who are taking anti-TB medications or who have completed TB treatment. 

 

The following questions can help determine if a NAAT is required:

 

·         Is the patient hospitalized and in airborne isolation?

·         Does the patient have a chest X-ray suggestive of TB infection?

·         Does the patient have a positive TB skin test (TST) or IGRA?

·         Is the patient linked to a known case of TB? 

·         Does the patient have at least two of the following signs and symptoms?

              Persistent Cough

              Purulent/Bloody sputum

              Shortness of Breath

              Chest Pain

              Fatigue/Malaise

              Unexplained Weight Loss

              Night Sweats

              Fever/Chills

 

·         If the patient has been diagnosed with pneumonia, has he/she been treated, but is not improving?

 

How do you order a NAAT? 

Healthcare providers should collect and send sputums or respiratory specimens to rule out TB in the same manner as culture and smear specimens are currently submitted to WPHL (send 3 sputum collected at least 8 hours apart).  WPHL will test the first sputum collected with NAAT.  If the first sputum is smear negative and NAAT negative, but either one or both of the remaining sputums are smear positive, WPHL will also test this specimen.  WPHL will not test any other specimens with NAAT unless there is a special request.  All 3 sputums will also have smears and and cultures performed.  Please request on the WPHL test request form that a NAAT is to be performed.  Additionally, submit with the specimen a Supplemental NAAT form by clicking on this link.   

 

What do the results mean?  

NAATs should always be interpreted withing the context of the patient’s signs and symptoms, and should always be performed in conjunction with AFB smear and culture.

 

NAAT

AFB Smear

                            Interpretation

Positive

Positive

Presume MTB

Start TB treatment, begin isolation

Confirm findings with culture result

Positive

Negative

Likely MTB

Consider submitting another specimen for NAAT

Use clinical judgement when deciding if appropriate to begin TB treatment and place patient in isolation

Negative

Positive

Suspect nontuberculous mycobacteria

Result does not rule out MTB

Consider delaying treatment, removing patient from isolation and unless highly suspected of TB and/or lives in high risk setting

Consider submitting another specimen for NAAT

Confirm findings with culture result

Negative

Negative

Cannot exclude MTB based upon result.  Rely on clinical judgement to determine whether or not to pursue further diagnostic work up or start patient on treatment

Consider submitting another specimen for NAAT

Confirm findings with culture result

Inhibited

N/A

Amplification was inhibited, submit another specimen

 

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