Mycobacterium Chelonae Workup

Updated: Nov 14, 2017
  • Author: Alfred Scott Lea, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Workup

Laboratory Studies

According to American Thoracic Society (ATS) criteria, diagnosis of Mycobacterium chelonae lung disease requires the following 3 elements [25] :

  • Pulmonary symptoms with consistent radiographic features (see Imaging Studies)

  • Exclusion of other diagnoses (especially tuberculosis and other nontuberculous mycobacteria [NTM])

  • Appropriate microbiological findings (which are listed immediately below)

Primary testing methodology

Primary testing methodology is sputum smear for acid-fast bacilli (AFB) and culture for mycobacteria.

Microbiological findings to satisfy ATS diagnostic criteria include the following (at least one needed for accurate diagnosis):

  • Positive culture from 2 separate sputum samples

  • One positive culture from bronchial wash or lavage

  • A biopsy specimen with appropriate histopathologic features and a positive culture from an associated bronchial wash or biopsy culture

Induced sputum samples may be substituted for expectorated sputum samples, but data establishing the effectiveness of this technique are lacking.

A single positive isolate may represent a contaminant or a persistent or transient colonizer without pathogenicity. [35, 36]

Swab culture for acid-fast bacillus

Swab specimens are less optimal than cultures obtained via aspiration. Notifying microbiology laboratory personnel to communicate clinical suspicion and concerns results in the proper procedures regarding adequate specimen processing to increase the yield and significance of cultures.

Interpret microbiological results with caution because a single positive culture, especially of a superficial lesion or sputum, may represent a contaminant or colonization. In one study from Spain, 13 of 24 isolates of M chelonae were deemed of questionable clinical significance. [17]

Species identification of the NTM as well as susceptibility studies performed by a competent reference laboratory are important steps when evaluating all forms of NTM, since their clinical presentation and manifestations, as well as their response to therapy, can be quite different.

Additional testing

If M chelonae,Mycobacterium abscessus or any rapidly growing mycobacteria (RGM)infection is discovered, additional testing for underlying disease may be needed. Speciation points to the clinical sequelae that can be anticipated.

An HIV test may be warranted, especially if disseminated disease with any NTM is diagnosed without an obvious underlying condition.

Sweat chloride and/or genetic screening for cystic fibrosis may be warranted if lung infection is found not only in a relatively young patient, but also in older patients with evidence of bronchiectasis.

A purified protein derivative (PPD) of tuberculin test should be considered to assist in ruling out tuberculosis. Similarly, interferon-gamma release assays (IGRA) may help identify a true Mycobacterium tuberculosis infection.

Susceptibility testing

Susceptibility testing should be performed on all isolates to guide treatment. General susceptibility results reported in the ATS guidelines are as follows [25] :

  • Tobramycin - 100%

  • Clarithromycin - 100%

  • Linezolid - 50-90%

  • Imipenem - 60%

  • Amikacin - 50%

  • Doxycycline - 25%

  • Ciprofloxacin - 20%

In vitro susceptibility testing may not correlate with in vivo results and vice versa.

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Imaging Studies

Chest radiography

Perform chest radiography if pulmonary symptoms are present. Typical findings are bilateral patchy nodular or cavitary opacities (15% are cavitary) with an upper lobe predominance.

Normal chest radiographic findings with a single positive culture suggest that the organism is a contaminant or a transient colonizer and is not clinically significant. However, in the presence of chronic persistent pulmonary symptoms or repeatedly positive culture results, additional testing may be necessary.

Chest CT scanning

If the patient has significant respiratory symptoms or repeatedly positive cultures for the M chelonae with a lack of cavitary disease on chest radiography, high-resolution CT scanning is likely indicated.

Typical CT scan findings include bronchiectasis, “tree-in-bud” appearance, or diffuse small nodules, which may not be apparent on routine chest radiography.

If the chest radiographic findings are abnormal, chest CT scanning may be performed to obtain better definition of the abnormalities present. Small cavities, nodules, and lymphadenopathy may be detected.

CT scanning of the abdomen and pelvis

CT scanning of the abdomen and pelvis may be indicated to detect intra-abdominal or retroperitoneal abscesses. It may also be used to evaluate localizing signs or symptoms of soft-tissue infection, especially in patients with a recent history of abdominal wall injections.

Bone imaging, MRI, and nuclear imaging

These studies may be helpful in detecting suspected osteomyelitis or joint disease, especially in patients with a history of penetrating trauma.

MRI can be helpful in evaluating the anatomy sinus tracts and fistulae and whether underlying abscess formation, foreign material, or marrow edema (osteomyelitis) is present.

Gallium scanning may be useful to screen for supradiaphragmatic lymphadenopathy (intrathoracic or axillary).

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Other Tests

Erythrocyte sedimentation rate or C-reactive protein assessments may be useful to differentiate colonizer and pathogen, but these are nonspecific tests and the results must be carefully evaluated within the clinical context of the patient.

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Procedures

Pulmonary procedures

Bronchoscopy is frequently indicated. Tests to be sent include bronchial washes for AFB culture and transbronchial biopsy samples sent for culture and histopathology. Bronchoalveolar lavage (BAL) may also be useful. Fungal cultures are typically sent as well, given an uncertain differential diagnosis.

Open or Video-assisted thorascopic (VATS) lung biopsy may be considered if suspicion is high but diagnostic criteria have not been met. Specimens for fungal and AFB cultures should be sent along with histopathology.

A positive culture from a sterile surgical biopsy is considered diagnostic for M chelonae.

Lung biopsy histology documenting the presence of AFB or granulomas along with positive sputum or BAL culture is considered diagnostic.

Skin tests

Skin testing with nontuberculous mycobacterial specific antigens is nonspecific and generally not indicated. These tests are not commercially available.

A positive standard PPD test result may raise suspicion for an unsuspected M tuberculosis infection.

A biopsy for localized or disseminated skin lesions should be performed. Specimens for AFB and fungal cultures should be sent, as well as histopathology. The microbiology laboratory needs to be alerted of the concern for atypical organisms since the organism can be overlooked as a pathogen.

Abscess drainage

Fluid from a localized abscess should be obtained for culture; this method is preferred over swab culture of a draining abscess, sinus tract, or fistula.

Fine-needle aspiration and biopsy should be attempted on lymphadenitis for histopathology, cytology, and culture.

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Histologic Findings

Histologic findings may reveal acute inflammation, microabscesses, granulomas (with or without caseation), and/or diffuse granulomatous inflammation. Special tissue stains for AFB may or may not reveal organisms.

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Staging

There is no formal staging classification for M chelonae. Isolates represent either contamination, colonization, or true infection. When present, infections are either localized or disseminated.

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