Mycobacterium tuberculosis - TB

Clinical Background

Mycobacterium tuberculosis (TB), as well as other nontuberculosis mycobacteria (NTM), can be infectious agents in humans.

Mycobacterium tuberculosis

Epidemiology

  • Incidence and prevalence
    • 4.4/100,000 reported in the U.S. (CDC 2007)
    • 8-9 million new cases annually worldwide
      • 3-4 million cases are infectious pulmonary disease (smear positive)
  • Age – incidence increases with age; peak is 35-44 years in the U.S.
  • Sex
    • <25 years – M:F, equal
    • >25 years – M>F, 1.5-2:1
  • Ethnicity – in U.S., more frequent in African Americans and foreign-born individuals

Organism

  • Mycobacterium is a genus of the order Actinomycetales (Mycobacteriaceae family)
  • Mycobacterium tuberculosis – rod-shaped aerobic bacterium with acid-fast staining properties
  • M. tuberculosis complex (MTB) – transmission via inhaled droplet nuclei
    • Includes M. tuberculosis, M. bovis, M. africanum, M. microti and others

Risk Factors

  • Risk of infection (greatest risk)
    • Homeless, shelter dwelling
    • Incarceration
    • Alcoholism, intravenous drug abuse
    • Military (foreign deployment)
    • Foreign born
    • Immunocompromised (HIV, transplant drugs, chemotherapy, chronic steroid treatment)
  • Risk of developing active disease once infected  
    • Comorbidity present (eg, HIV, immunosuppression, chronic renal failure, diabetes mellitus)
    • Recent prior tuberculosis infection (<1 year)
    • Malnutrition, alcoholism
    • Fibrotic lesions present on chest x-ray

Clinical Presentation

  • Primary disease
    • Ranges from mild, self-limited illness to severe disseminated disease
  • Post-primary disease
    • Reactivation of latent infection
    • Moderate to severe, progressive and often fatal
  • Extrapulmonary TB
    • Gastrointestinal disease
    • Genitourinary
    • Lymphadenitis
    • Meningitis, central nervous system tuberculoma
    • Miliary
    • Pericarditis
    • Pleural disease
    • Skeletal (Pott disease)
  • TB in patients with HIV
    • Extrapulmonary disease is common
    • Atypical chest x-ray findings
    • Delays in diagnosis and treatment are common
    • Increased risk of latent disease reactivation or rapid progression of newly acquired infection

Nontuberculous (atypical) mycobacteria (NTM)

Epidemiology

  • Age – usually isolated
  • Transmission routes vary – cutaneous, inhalation, or parenteral routes
    • Organisms are widely distributed in water and soil and by animals; person-to-person transmission has not been reported

Organisms

  • Free-living organisms; more than 125 species of NTM
  • Nontuberculous organisms
    • M. avium complex, M. kansasii, M. fortuitum, M. abscessus, M. chelonae, M. marinum, M. haemophilum
  • Rare new organisms
    • M.nebraskense, M. parmense, M. saskat-chewanense,  M. arupense, M. caprae, M. colombiense, M. florentium, M. montefiorense
  • Pathogenicity and clinical significance vary with species and with host; asymptomatic infections are common

Clinical Presentation

  • Cutaneous – M. fortuitum, M. abscessus, M. chelonae, M. marinum, M. haemophilum
    • Nodular or ulcerating chronic lesions that fail to respond to standard antimicrobial therapy
    • Lymphadenitis - M. avium complex, especially in children
  • Pulmonary – M. avium complex, M. kansasii, M. abscessus
    • Chronic cough usually present
    • Immunocompromised hosts
    • Increased in elderly and patients with underlying pulmonary disease
    • Bronchiectatic, nodular or cavitary disease
  • Disseminated
    • Weight loss, fever, fatigue, lymphadenopathy, hepatosplenomegaly, gastrointestinal complaints
    • Usually immunocompromised patients (advanced HIV, transplant) 
  • Other (in-dwelling catheters, skeletal)

Diagnosis

Mycobacterium tuberculosis (TB)

Indications for Testing

  • Cough, fever, weight loss in at-risk patients

Laboratory Testing

  • Acid-fast bacilli (AFB) smear, culture and nucleic acid amplification testing
    • Culture provides definitive diagnosis
  • Susceptibility testing is mandatory
    • Purified protein derivative (PPD) is old standard of diagnosis
  • Latent TB – interferon alpha release assays (QuantiFERON®)
    • Not affected by previous vaccination with bacillus Calmette-Guérin (BCG)
  • Adenosine deaminase (ADA)
    • Adjunctive test – use in evaluating suspected extrapulmonary TB involving body cavities or cerebrospinal fluid
    • Reported sensitivity and specificity vary
      • Meta-analysis derived summary receiver operator curves from pleural fluid: sensitivity 92%; specificity 92%
    • Not specific for TB
    • AFB culture should always be performed concurrently on fluid

Imaging Studies

  • Chest x-ray confirmation is required (may have been performed prior to TB testing)

Nontuberculous (atypical) mycobacteria (NTM)

Indications for Testing

  • Clinical setting suspicious for disease

Laboratory Testing

  • Concentrated AFB smear
    • Traditional standard screening method (sputum)
    • Not as sensitive as culture (requires ~104 bacilli/mL sputum for detection)
    • Not specific for MTB complex
  • Mycobacterial culture
    • Most sensitive test for detection of mycobacteria (gold standard)
    • Slow (2-8 weeks)
    • Should be performed on specimen from any site suspected for mycobacteria infection
    • Positive culture from site of suspected disease required to make a diagnosis of disseminated disease
  • Nucleic acid amplified testing
    • Emerging recommended method for rapid, sensitive and specific detection of MTB complex
    • Less sensitive than culture

Imaging Studies

  • Chest x-ray – cavities, frank parenchymal infiltrates (upper lobes more common), bronchiectasis, pulmonary nodules
    • Does not distinguish between TB and NTM
  • CT – may give better global picture of disease present

Differential Diagnosis

Screening

TB

  • Targeted tuberculin testing for latent TB infection for foreign-born persons entering the U.S. within the past 5 years

Monitoring

Sputum culture

  • Follow up treatment with repeat cultures of sputum
  • 2-month culture may be predictive of relapse, if positive

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
AFB Culture (Includes AFB Stain 0060151) 0060152
Method: Standard reference procedures for stain and culture.  Identification of AFB is ordered and billed separately.  DNA probes are available for M. tuberculosis complex and M. avium-intracellulare complex as indicated.  DNA sequencing and other molecular techniques are used for identification. For drug susceptibilities, refer to Antimicrobial Susceptibility - AFB Mycobacteria (0060217).
 

Gold standard, most sensitive test for diagnosing mycobacteria

Specimen from any suspected site, including sputum, CSF, tissue, urine and other body fluid or gastric aspirate

Mycobacteria are slow growing organisms; culture requires several weeks

DNA probes are available for MTB complex and M. avium-intracellulare complex as indicated

Other species require DNA sequencing or different molecular techniques for identification

For drug susceptibilities, refer to Antimicrobial Susceptibility - AFB Mycobacteria test
Mycobacterium tuberculosis Amplified Direct Detection 0060095
Method: Transcription Mediated Amplification (Gen-Probe®)

Order as adjunct to AFB culture in untreated patients with smear-positive and smear-negative respiratory samples

Rapid, sensitive and specific detection of MTB complex from respiratory specimens

AFB culture should always be ordered with this test

Low level false-positive results can occur in specimens with high concentration of mycobacteria other than M. tuberculosis

Negative result does not exclude M. tuberculosis infection

  
QuantiFERON®-TB Gold In Tube 0051729
Method: Cell Culture/Enzyme-Linked Immunosorbent Assay

Detect latent disease among persons at increased risk for TB

Specific for M. tuberculosis, not NTM or BCG vaccine

Do not use alone to diagnose or exclude TB or to assess possible latent disease; result interpretation requires a combination of epidemiological, historical, medical, and diagnostic findings

  
QuantiFERON®-TB Gold In Tube & Mycobacterium tuberculosis Antibody, IgG 2001627
Method: Cell Culture/Enzyme-Linked Immunosorbent Assay

Detect latent disease among persons at increased risk for TB

Components include QuantiFERON®-TB Gold in Tube and Mycobacterium tuberculosis Antibody, IgG by ELISA

    
Mycobacterium tuberculosis Complex Speciation 0060771
Method: Polymerase Chain Reaction

Identify the species of MTB complex to species level by comparing genomic deletion patterns; includes testing for M. tuberculosis, M. bovis, M. bovis BCG-vaccine strain, M. africanum, M. microti,  M. caprae

NTM may have indeterminate patterns of the genomic deletions used as targets in this assay

Changes in DNA sequence at the primer annealing sites may affect genomic deletion pattern determination and speciation

  
Mycobacterium tuberculosis Antibody, IgG by ELISA 0051698
Method: Enzyme-Linked Immunosorbent Assay

Rapid serologic assay; order when clinical symptoms and potential exposure indicate MTB infection

Detect IgG antibodies against MTB in patients with active infection

Monitor the success of therapeutic intervention

Differentiate active and latent cases of TB

Can also be used in previously BCG vaccinated individuals because antigens utilized are not present in M. bovis

Not intended for use as a screening assay

Results should be interpreted in conjunction with PPD or QuantiFERON® testing as well as the complete history of the patient

Not validated for use on patients with HIV/TB co-infection and is not recommended for this population

  
AFB Culture (Includes AFB Stain 0060151) with Reflex to Mycobacterium tuberculosis Amplified Direct Detection (0060095) 0060738
Method: Standard reference procedures for stain and culture. Identification tests of AFB are ordered and billed separately. DNA probes are available for M. tuberculosis complex and M. avium-intracellulare complex as indicated. DNA sequencing and other molecular techniques are used for identification. For drug susceptibilities, refer to Antimicrobial Susceptibility - AFB Mycobacteria (0060217). 

Single-order test incorporating culture and stain with reflex to amplified MTB complex testing when smear positive

Available for respiratory specimens only

Low-level false-positive results can occur in specimens with high concentration of mycobacteria other than M. tuberculosis

Negative result does not exclude M. tuberculosis

  
Blood Culture, AFB 0060060
Method: BD Continuous monitoring system.  Standard reference procedures for identification and/or DNA Sequencing

Order for AFB culture of blood or bone marrow specimen

    
Mycobacterium tuberculosis Amplified Detection , CSF 0060063
Method: Transcription Mediated Amplification (Gen-Probe®)

Amplified test offers rapid and specific detection of MTB complex from CSF

AFB culture should always be ordered concurrently

Test does not differentiate among members of the MTB complex

Low level false-positive results can occur in specimens with high concentration of mycobacteria other than M. tuberculosis (rare in CSF)

Limited sensitivity

Negative result does not exclude the possibility of isolating MTB during culture

  
AFB Identification with Reflex to Susceptibility 0060997
Method: Standard reference procedures for identification and susceptibility testing of AFB. DNA probes available for M. tuberculosis complex and M. avium-intracellulare complex. DNA sequencing and other molecular techniques are used for other mycobacteria species.

Identify AFB isolated in pure culture with reflex to susceptibility testing, if indicated

Susceptibility testing may not be performed on all isolates (varies by species)

  
Antimicrobial Susceptibility - AFB/Mycobacteria 0060217
Method: Varies with organism identification

Availability:

  • M. tuberculosis primary and secondary panel
  • M. avium-intracellulare complex c.
  • Laboratory should be notified when the presence of Mycobacterium genavense is suspected because this organism will not grow on media routinely used for mycobacterial isolation
  • Rapid-growing mycobacteria
  • Other slow-growing NTM
    
Additional Tests Available
 
 
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
AFB Stain Only 0060151
Method: Auramine O Stain

Monitor respiratory specimens on previously diagnosed patients

Should not be ordered without culture in previously undiagnosed patients
AFB Identification 0060999
Method: Standard reference procedures for identification of AFB. DNA probes available for M. tuberculosis complex and M. avium-intracellulare complex. DNA sequencing and other molecular techniques are used for other mycobacteria species.

Identification of acid-fast organisms (AFB) isolated in pure culture
Blood Culture, AFB & Fungal 0060024
Method: BD Continuous monitoring system.  Standard reference procedures for identification and/or DNA Sequencing
Adenosine Deaminase, Body Fluid 0098115
Method: Spectrophotometry
Chylomicron Screen, Body Fluid 0098457
Method: Electrophoresis