Mycobacterium Fortuitum Workup

Updated: Nov 18, 2019
  • Author: Sami M Akram, MD, MHA, RDMS; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Workup

Laboratory Studies

According to American Thoracic Society criteria, [18] diagnosis of NTM lung disease requires (1) pulmonary symptoms with consistent radiographic features, (2) exclusion of other diagnoses, especially tuberculosis, and (3) appropriate microbiological findings. [18]

Microbiological findings to satisfy ATS diagnostic criteria include the following (at least one must apply):

  • Positive culture from at least 2 separate sputum samples
  • Positive culture from bronchial wash or lavage
  • 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 persistent or transient colonizer without pathogenicity. In one study, M fortuitum was present in 24% of specimens positive for pathogenic NTM, but, at 2 years, was present in only 6% of the cohort. [9]

Swab culture for acid-fast bacilli

Notifying the microbiology laboratory personnel that a NTM infection is suspected may help ensure appropriate processing of specimens. Most laboratories use liquid media (eg, BACTEC) for mycobacterial cultures.

Swab specimens are less optimal than cultures obtained via aspiration. Consider contacting laboratory personnel for proper procedures regarding adequate specimen collection to increase the yield and significance of cultures.

Interpret the result with caution because a single positive culture, especially of a superficial lesion, may represent a contaminant. M fortuitum may be a colonizer in pulmonary specimens.

Initial tests frequently include sputum smear for acid-fast bacilli (AFB) and culture for mycobacteria. Culture is the criterion standard, which can take up to 8-12 weeks for final results Smear cannot differentiate between M tuberculosis and NTM, so the diagnosis will be delayed.

Susceptibility testing

Susceptibility testing for M fortuitum should be performed for amikacin, imipenem, cefoxitin, doxycycline, fluoroquinolones, trimethoprim-sulfamethoxazole, clarithromycin, linezolid, and tobramycin. [7]  

Polymerase chain reaction

Rapid detection using polymerase chain reaction (PCR) techniques are being used with increasing frequency.

A new multiplex polymerase chain reaction (PCR) test performed on the BD MAX System was shown to be sensitive and specific for M fortuitum complex. [19] Similarly, other PCR-based assays are used to detect mycobacteria directly on respiratory samples or mycobacterial cultures.

A biochip immunoassay kit based on the 16S rRNA gene sequence is commercially available. It can identify 17 mycobacteria directly in AFB smear–positive respiratory specimens within 6 hours. [20] The concordance between biochip and mycobacterial culture was 95.4%. PCR identification of 16SrRNA gene sequence is superior to culture when multiple isolates are involved, since organisms may be growing at different speeds.

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Imaging Studies

Chest radiography

Perform chest radiography if pulmonary symptoms are present.

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 same organism with a lack of cavitary disease on chest radiography, high-resolution CT scanning is indicated.

Typical CT scan findings include bronchiectasis or diffuse small nodules; these are often not revealed by routine chest radiography.

If the chest radiographic findings are abnormal, chest CT scanning may be performed to obtain better definition of the abnormalities present. Lymphadenopathy may also be detected. This study is not necessary in every case but should be strongly considered.

CT scanning of the abdomen and pelvis

This study may be indicated to detect local abscesses or lymphadenopathy, including retroperitoneal abscesses or lymph nodes, in patients with disseminated disease, localizing signs or symptoms, or a history of injections in those locations.

Bone imaging, MRI, and nuclear imaging

These studies may be helpful in detecting osteomyelitis or joint disease if suspected, especially in patients with a history of penetrating trauma. [21]

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Other Tests

Erythrocyte sedimentation rate (ESR), C-reactive protein, and other inflammatory markers may be useful if mycobacterial disease is suspected. However, these tests are nonspecific, and their precise role in aiding diagnosis and follow-up care is not well-defined.

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Procedures

Lung procedures

Perform bronchoscopy with bronchial washes and/or bronchoalveolar lavage, ideally in conjunction with transbronchial biopsy, for AFB smear, culture, and histology. Because the diagnosis is usually uncertain at this stage, bacterial and fungal cultures are often also sent.

Open or thoracoscopic lung biopsy may be considered if suspicion is high but diagnostic criteria have not been met. Send specimens for fungal and AFB cultures, as well as histology.

A biopsy specimen culture positive for M fortuitum is considered diagnostic. A positive AFB smear result correlates with an increased number of organisms and further supports the diagnosis.

The presence of either AFB or granulomas in a lung biopsy specimen or a transbronchial biopsy specimen, along with even a single positive culture result of sputum or bronchial wash (even in low numbers), is considered diagnostic.

Skin tests

Perform a biopsy for localized or disseminated skin lesions. Send specimens for mycobacterial and fungal cultures, as well as histology.

Purified protein derivative (PPD) testing with NTM-specific antigens is nonspecific and generally not indicated. These tests are not commercially available.

Aspiration biopsy

Perform an aspiration biopsy of a localized abscess for culture.

Perform a fine-needle aspiration biopsy or a surgical excision of lymph nodes for histology and culture.

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Histologic Findings

Histologic findings may reveal acute inflammation, microabscesses, granulomatous inflammation, or granulomas (with or without caseation). These findings may be mixed. Special stains for AFB may reveal organisms.

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Staging

The disease may be limited or disseminated (two noncontiguous organs) or in the blood (mycobacteremia).

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