Clinical relevance of differentiation between Mycobacterium avium and M. intracellulare in M. avium complex pulmonary disease in Croatia

<b>Introduction:</b> Clinical differences between M. avium and M. intracellulare haven’t been fully studied. <b>Aims and Objectives:</b> to asses the relevance of M. avium complex (MAC) members; record treatment outcomes and long term survival of MAC pulmonary disease (MAC-PD) patients. <b>Methods:</b> retrospective study on all Croatian residents with MAC respiratory isolates from 2006 to 2016 with follow up to 2018. ATS/IDSA guidelines were used to establish MAC-PD and evaluate treatment adequacy. <b>Results:</b> Medical records were available for 93/149 (62.4%) isolates. MAC-PD was diagnosed in 36/49 (73.4%) of M. avium and 24/44 (54.5%) of M. intracellulare isolates. Fibrocavitary form and smear positivity were more common with M. intracellulare than M. avium - 33.3 vs. 22.2% and 61 vs. 27.6%, respectively. Treatment was started in 23/36 (63.9%) of M. avium and 21/23 (91.3%) of M. intracellulare. Adequate regimen was administered in 26.1% (6/23) of M. avium and 55% (11/20) of M.intracellulare patients, and microbiological cure was achieved in 83.3% and 72.7% of cases, respectively. One-year mortality of M. avium patients was 19.4% compared to 8.3% of M. intracellulare. 5-year all-cause mortality amounted to 36% and 30% of M. avium and M. intracellulare cases, respectively (p=0.876). <b>Conclusions:</b> M. avium showed a higher percentage of isolates meeting the MAC-PD criteria. Still, M.intracellulare disease was more severe in its presentation, which partly explains the higher rate and adequacy of treatment. Although one-year mortality of M. avium cases was doubled compared to M. intracellulare, there were no differences in long term survival between the two groups.