Results from 8 yrs of susceptibility testing of clinical Mycobacterium tuberculosis isolates in Denmark.

Increased rates of multidrug-resistant (MDR) tuberculosis (TB) has been reported from countries close to Denmark. This study evaluated the incidence of drug resistance in Denmark in order to determine the magnitude of the problem. Susceptibility testing was performed in isolates from 85.4% of all notified patients during 1991-1998. Epidemiological information was retrieved from the mandatory notification forms. Total drug resistance remained largely constant, although a minor increase was observed in 1997-1998. Monoresistance was observed in 7.3%, of the isolates. Among 3.6% polyresistant isolates, resistance to isoniazid and streptomycin accounted for 2.8%, whereas MDR accounted for 0.5%. The MDR strains displayed different restriction fragment length polymorphism (RFLP) patterns, and no matches were identified in the international MDR database. Drug resistance in untreated Danes and foreigners were 5.9% and 14.6%, respectively. Among Danes and foreigners with previous TB, 6.2% and 22.7% had drug resistance, respectively. Increased drug-resistance was found among untreated Danes aged 25-54 yrs mainly due to a single isoniazid and streptomycin-resistant RFLP-cluster. Among all patients with isoniazid and streptomycin-resistance, 77.0% had clustered strains. In conclusion, although drug resistance among untreated Danes was close to the rate estimated in good national programmes, close monitoring is needed in future years, as active transmission of isoniazid- and streptomycin-resistant Mycobacterium tuberculosis was demonstrated.