Non-tuberculous mycobacterial disease

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Non-tuberculous mycobacteria are environmental organisms outside the M. tuberculosis complex and M. leprae. Isolation may represent contamination, transient airway carriage or true pulmonary disease. Common UK syndromes include Mycobacterium avium complex in nodular-bronchiectatic or cavitary disease, M. kansasii in a tuberculosis-like cavitary phenotype, M. xenopi and M. malmoense in structurally abnormal lungs, and rapidly growing M. abscessus complex, particularly in bronchiectasis or cystic fibrosis. Species identity matters because pathogenicity, expected response and susceptibility interpretation differ markedly.

The diagnostic threshold deliberately requires concordance. Symptoms such as chronic cough, sputum, fatigue, weight loss or haemoptysis must accompany nodules, tree-in-bud change, multifocal bronchiectasis or cavities, with repeated culture evidence or a positive bronchoscopic or tissue sample. Even after criteria are met, treatment is not automatic. Clinicians should estimate disease trajectory, symptom burden, organism, resistance, reversibility of host risks, treatment toxicity and the patient's goals. Observation is an active management strategy with predefined review and escalation triggers, not abandonment.

NTM is not usually transmitted person to person in routine care, but local cystic-fibrosis infection-control rules may be stricter and suspected TB remains a separate hazard. Notify or seek public-health advice only when the identified organism or clinical situation falls within current jurisdictional requirements; do not label NTM as generally notifiable. Laboratory processing is slow, and cultures may become positive after discharge, so ownership of results and communication with the patient need to be explicit.

Key points

A positive culture is not synonymous with disease: combine compatible symptoms, characteristic imaging and reproducible microbiology, while excluding tuberculosis and convincing alternatives.
Bronchiectasis, COPD, previous tuberculosis, cystic fibrosis, thoracic skeletal abnormality, low body mass and immune compromise increase susceptibility, but NTM can also occur without an obvious host defect.
Send at least two sputum samples collected on separate days for mycobacterial culture; ask for species or subspecies identification and appropriate susceptibility testing.
High-resolution CT distinguishes nodular-bronchiectatic disease from fibrocavitary or disseminated patterns and provides a baseline for multidisciplinary decisions.
Stable, mildly symptomatic disease may be observed with structured surveillance; cavitation, smear positivity, systemic illness or radiological progression shifts the balance towards treatment.
Never use macrolide monotherapy for MAC pulmonary disease: acquired macrolide resistance can remove the most important active oral drug.
Treatment is prolonged, toxic and species-specific, usually continuing for at least 12 months after culture conversion; specialist oversight and shared decisions are essential.
Until Mycobacterium tuberculosis has been reasonably excluded, use the local infection-control pathway for potentially infectious pulmonary TB rather than assuming NTM.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Environmental mycobacteria

Non-tuberculous species are acquired from water, soil and aerosols rather than usually spreading person to person in ordinary clinical settings.

Structural lung disease

Bronchiectasis, COPD, cystic fibrosis, prior tuberculosis and fibrotic lung disease impair clearance and create niches for persistent infection.

Host immune vulnerability

Immune suppression and selected defects in cellular immune pathways increase pulmonary or disseminated disease risk, with the final risk shaped by exposure and individual susceptibility.

Susceptible phenotype

Older lean women with nodular bronchiectatic disease and people with reflux or impaired mucociliary clearance may develop disease without overt systemic immune deficiency.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Environmental inoculation
Aerosolised organisms reach distal airways repeatedly, where normal clearance usually prevents established disease, which links the underlying lesion to the observed respiratory dysfunction.
2
Persistent airway infection
In susceptible lung, organisms survive within macrophages and biofilm, provoking chronic granulomatous and neutrophilic inflammation, with effects that increase as the pathological process progresses.
3
Structural progression
Inflammation worsens bronchiectasis, nodules and mucus retention or causes cavitary parenchymal destruction, and the downstream physiological effect determines clinical severity.
4
Self-reinforcing susceptibility
Increasing airway damage further reduces clearance, allowing higher microbial burden and continued radiological and physiological decline.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Nodular-bronchiectatic phenotype

Persistent productive cough, recurrent exacerbations and fatigue accompany cylindrical bronchiectasis, small nodules and tree-in-bud opacities, often in the middle lobe or lingula. Symptoms may be subtle despite extensive radiology.

Fibrocavitary phenotypeRed flag

Upper-lobe cavities, pleural thickening, weight loss, night sweats and haemoptysis can closely mimic pulmonary tuberculosis or malignancy. COPD, smoking-related lung damage and previous TB are common backgrounds.

Rapidly progressive or disseminated diseaseRed flag

Hypoxaemia, major haemoptysis, sepsis physiology, rapid radiological spread or extrapulmonary features in profound immune compromise require acute stabilisation, urgent respiratory-infection input and assessment for disseminated infection.

Host and treatment risks

Look for bronchiectasis, reflux or aspiration, impaired airway clearance, cystic fibrosis, immunosuppressive medicines and inhaled corticosteroid exposure. Record hearing, vision, renal, hepatic and cardiac risks before discussing prolonged therapy.

A result that may not be disease

A single low-burden isolate from a poor-quality sputum sample, without compatible symptoms or CT abnormalities, has low diagnostic weight. M. gordonae is often less pathogenic than M. kansasii, but interpretation is always clinical and species-aware.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Serial sputum mycobacterial microscopy and cultureFirst step
Why
Obtain at least two expectorated samples on separate days, ideally before NTM-active antibiotics, and request acid-fast microscopy, culture and species-level identification.
Interpretation and limitations
Repeated growth of the same species strengthens causality; smear positivity suggests high organism burden but neither distinguishes TB from NTM nor establishes disease alone.
02
High-resolution CT chest
Why
Define bronchiectasis, centrilobular nodules, tree-in-bud change, consolidation, cavities and distribution; compare with prior studies for progression.
Interpretation and limitations
Nodular-bronchiectatic and fibrocavitary patterns are supportive but not specific. A new mass, focal obstruction or pulmonary embolic pattern requires an alternative pathway.
03
Bronchial wash or bronchoalveolar lavage
Why
Use when sputum cannot be produced or serial sputum remains negative despite convincing disease, balancing procedural risk and local infection-control arrangements.
Interpretation and limitations
A positive bronchoscopic specimen can satisfy microbiological criteria in the right syndrome, but contamination and colonisation still require clinicoradiological correlation.
04
Species identification and susceptibility or resistance markers
Why
Ask the reference laboratory for clinically appropriate testing, including macrolide and amikacin susceptibility for MAC and inducible macrolide resistance assessment in M. abscessus complex.
Interpretation and limitations
Results guide regimen design but are not a stand-alone prescription; genotype-phenotype discordance and species-specific breakpoints need specialist microbiology interpretation.
05
Baseline safety assessment
Why
Check FBC, renal and liver profile, ECG where QT-active drugs are considered, visual acuity and colour vision before ethambutol, and audiology before prolonged aminoglycoside exposure.
Interpretation and limitations
Abnormalities may change drug choice or monitoring intensity. Pregnancy, frailty, interactions and renal impairment require individual specialist review.
06
Search for alternatives and host drivers
Why
Consider TB NAAT and culture, HIV testing with consent, immunoglobulins, Aspergillus work-up, sputum bacterial culture and assessment of aspiration or airway clearance according to phenotype.
Interpretation and limitations
A co-pathogen or structural driver may explain symptoms, coexist with NTM or offer a safer initial treatment target.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Transient isolation or colonisation

A single positive non-sterile respiratory sample without compatible symptoms and imaging does not establish progressive NTM pulmonary disease.

Pulmonary tuberculosis

Epidemiology, nucleic-acid testing and species identification distinguish M tuberculosis, which carries different infection-control and public-health duties.

Chronic pulmonary aspergillosis

Pre-existing cavities with pleural thickening, fungal markers or an aspergilloma may mimic or coexist with cavitary NTM disease.

Bronchiectasis exacerbation

Acute bacterial deterioration can explain increased sputum despite stable NTM burden; serial cultures and imaging clarify trajectory.

Lung cancer

Focal mass, enlarging cavity or constitutional decline may require tissue because infection and malignancy can coexist.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01DiagnoseFrom isolate to pulmonary diseaseFirst stepAn NTM is reported from a respiratory specimen.

1Review symptoms, specimen quality, species identity, smear burden, exposures, immune state and previous imaging; urgently retain TB precautions if M. tuberculosis complex remains plausible.
2Arrange high-resolution CT and further sputum cultures on separate days; use bronchoscopy only when it will materially resolve uncertainty and the patient can tolerate it.
3Discuss concordant clinical, radiological and microbiological evidence in an NTM-experienced multidisciplinary setting, document whether criteria are met and name important alternatives.
4Explain uncertainty to the patient and ensure ownership of pending reference-laboratory results.

02ObserveStructured surveillance without immediate antibioticsDiagnostic criteria are met or nearly met, but disease is mild and stable and treatment harm may exceed near-term benefit.

1Agree goals and optimise airway clearance, nutrition, smoking cessation, reflux or aspiration management and bronchiectasis care.
2Record baseline symptoms, weight, sputum status, lung function and CT, then schedule clinical and sputum reviews at a frequency matched to risk.
3EscalationEscalate for new cavitation, smear positivity, repeated cultures with increasing burden, haemoptysis, weight loss, functional decline or objective radiological progression.
4Revisit the decision with the patient rather than allowing surveillance to continue by inertia.

03TreatSpecies-led multidrug therapyProgressive, cavitary, smear-positive or substantially symptomatic NTM pulmonary disease, or another specialist-agreed indication.

1Confirm species or subspecies, resistance information, baseline toxicities, interacting medicines and the patient's priorities before selecting a regimen.
2For macrolide-susceptible MAC, use a macrolide plus ethambutol and rifampicin-based regimen; daily treatment and an initial aminoglycoside may be considered for severe or cavitary disease, while selected non-severe disease may use intermittent therapy under specialist guidance.
3Use species-specific expert regimens for M. kansasii, M. xenopi, M. malmoense or M. abscessus; never extrapolate a MAC regimen uncritically.
4Monitor cultures and toxicity throughout and usually continue at least 12 months beyond culture conversion, verifying the current BTS or local definition and plan.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Core active drug for macrolide-susceptible MAC and some other species-led regimens.

Azithromycin or clarithromycin as part of a MAC regimen

Specialist-selected daily or three-times-weekly schedule according to severity, species, interactions and current guidance; always combined with effective companion drugs.

Never use alone for MAC. Check QT risk, hearing, hepatic effects and interactions; rifampicin can reduce clarithromycin exposure. Verify susceptibility and current BNF or specialist guidance.

Companion drug that helps protect the macrolide and contributes activity in MAC and several other NTM regimens.

Ethambutol

Weight-based specialist schedule, daily or intermittent as part of a multidrug regimen; confirm current BNF and NTM protocol.

Optic neuropathy can be irreversible. Record acuity and colour vision, counsel prompt reporting of visual change, and adjust or avoid according to renal function and specialist advice.

Backbone medicine in MAC, M. kansasii, M. xenopi and M. malmoense regimens when susceptible and tolerated.

Rifampicin

Specialist-selected daily or intermittent oral schedule as a companion drug; verify weight, formulation and current BNF.

Powerful enzyme inducer with major interactions, including contraception, anticoagulants, antiretrovirals and immunosuppressants. Hepatotoxicity, cytopenia and orange secretions require counselling and monitoring.

Additional early activity in severe, cavitary, smear-positive or resistant disease and selected M. abscessus regimens.

Amikacin intravenous or nebulised

Individualised weight- and renal-function-based specialist regimen with serum-level monitoring when systemic; duration depends on species, severity and response.

Ototoxicity, vestibular injury, nephrotoxicity and neuromuscular blockade. Arrange audiology and renal monitoring; inhaled administration does not remove systemic or airway risk.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Progressive cavitary destruction

Untreated active disease can enlarge cavities, reduce lung function and cause chronic respiratory failure, with severity determined by its extent and the patient's underlying reserve.

Haemoptysis

Inflamed bronchiectatic airways and cavity-associated vessels may bleed, occasionally threatening the airway, creating an additional need for recognition and targeted treatment.

Disseminated infection

Severe cellular immune deficiency permits bloodstream spread to lymph nodes, skin, bone and other organs, adding morbidity beyond the initial pulmonary disorder.

Treatment toxicity

Prolonged multidrug regimens can cause hepatic, ocular, auditory, renal and interaction-related harm requiring species-led monitoring, particularly when baseline cardiopulmonary reserve is limited.

Antimicrobial resistance

Inadequate regimens, poor absorption or adherence and high organism burden can select resistance and sharply restrict treatment options.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Track symptoms, weight, exercise tolerance, exacerbations and haemoptysis; deterioration should prompt earlier review rather than waiting for a routine appointment.
Obtain sputum mycobacterial cultures at the specialist-agreed interval, commonly every 4 to 12 weeks during treatment, and continue post-treatment surveillance for relapse or reinfection.
Repeat renal, hepatic and haematological tests according to the regimen; intensify after dose changes, intercurrent illness or addition of interacting medicines.
Perform targeted vision, colour discrimination, hearing, vestibular and ECG monitoring for ethambutol, aminoglycosides and QT-active combinations.
Reconcile every new prescription against rifamycin and macrolide interactions, including over-the-counter and herbal products.
Compare lung function and imaging at clinically meaningful intervals; avoid frequent CT without a management question, but do not rely on symptoms alone.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Culture conversion is a defined event

Specialist services define conversion using serial negative cultures, not a single negative sample. The date drives intended duration, so record it transparently.

Reinfection is possible

A later isolate may be a new strain or species rather than relapse. Preserve species and molecular information where available before labelling treatment failure.

Airway care changes the substrate

Good sputum clearance, nutrition and management of bronchiectasis or aspiration may reduce symptoms and bacterial co-infection even when NTM antibiotics are deferred.

Macrolides can mask the diagnosis

Long-term macrolide monotherapy for bronchiectasis can select resistant MAC if unrecognised NTM is present; screen according to bronchiectasis guidance before prophylactic use.

The hardest decision may be whether to treat

A diagnostic label does not erase uncertainty about benefit. Shared decision-making should state likely duration, monitoring burden, probability of culture conversion and the possibility of recurrence.

11Common pitfallsFrequent interpretation and management errors.

01
Treating one positive sputum result without compatible CT or repeat microbiology.
02
Calling an acid-fast smear 'tuberculosis' or 'NTM' before molecular or culture identification.
03
Starting macrolide monotherapy and selecting resistance before an NTM work-up is complete.
04
Using a single generic regimen for every NTM species or ignoring inducible macrolide resistance in M. abscessus complex.
05
Failing to monitor vision, hearing, renal function, liver tests and interactions throughout a long treatment course.
06
Describing watchful waiting without an owner, interval or objective escalation criteria.