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The role of MAC in CF
Patients with moderately severe CF pulmonary disease have longstanding chest symptoms, widespread CXR abnormalities, and frequent exacerbations making differentiation from MAC pulmonary infection difficult. MAC has long been recognized as an important pathogen in certain high-risk groups like HIV positive patients, elder smokers, pneumoconiosis, and pre-existing bronchiectasis. In the last decade the role of MAC in CF has become clearer.

Prior to 1990 the incidence of MAC in CF was thought to be low and its clinical impact was thought to be minimal. The earlier studies were limited by being retrospective, often based on less than 3 sputum samples, and did not use standard accurate microbiological methods. Recently many well-conducted multicenter studies have elucidated the prevalence and clinical impact of MAC in CF patients. The ATS has also published criteria for the diagnosis and guidelines for treatment of NTM disease.

Epidemiology of MAC in CF
A recent well conducted cross sectional multicenter study of 986 CF patients found a 13% prevalence of NTM.MAC was the most common species, accounting for 72% of all NTM. Based on molecular typing of the NTM, it seems that horizontal or nosocomial spread is unlikely. Clinical presentation of MAC disease: Patients most commonly present with variable and non-specific symptoms of cough, sputum production, loss of weight and fatigue. Disseminated MAC disease is very rare in CF.

The radiological manifestations of MAC
Chest radiographic findings are very similar to post primary TB.These include heterogeneous linear and nodular opacities of the apical and posterior segments of the upper lobes, small thin walled cavities especially in the upper lobes. MAC can also show non-classic CXR features like cylindrical bronchiectasis and multiple centrilobular nodules. It must be emphasized that plain chest radiographs have limited specificity and sensitivity for diagnosing MAC in CF.

Chest HRCT has the advantages of exact anatomical localization and spatial resolution. The coexistence of bronchiectasis, air space consolidation and pulmonary nodules on HRCT is strongly predictive of progression of MAC in CF. Microbiological diagnosis should have at least 3 sputum samples, cultured in rapid growth media and species identification should use rapid DNA probes and high-pressure liquid chromatography. Routine susceptibility testing for macrolide sensitivity is not recommended for macrolide naïve patients. In cases of treatment failure susceptibility testing should be considered.

Treatment of MAC in CF should be done by experts in respirology and infectious disease because of the complex drug regimens with a high potential for toxicity as well as interaction with CF therapy. Most treatment regimes include a macrolide, ethambutol and a rifamycin, with or without an aminoglycoside. It is essential to regularly monitor patients for drug toxicity, as well as clinical, microbiological and radiological response to therapy.

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