The detection of mycobacteria in blood cultures using the Bactec system: 6 weeks versus 12 weeks of incubation? routine terminal Ziehl—Neelsen?

Abstract

MethodsIn this prospective study, all blood cultures submitted for mycobacteria detection, taken mainly from HIV-infected patients, were incubated for 12 weeks. The clinical impact of a late positive blood culture result was assessed retrospectively.ResultsFrom a total of 750 blood cultures, 68 had a growth index (GI) >10 due to the presence of mycobacteria. Of 545 negative blood cultures with a GI <10 within 12 weeks examined by Ziehl—Neelsen, one bottle revealed acid-fast bacilli further identified as Mycobacterium genavense by PCR-restriction fragment length polymorphism analysis of the hsp65 gene. For six of 39 patients with positive blood cultures, the delay to positivity was > 6 weeks (one M. tuberculosis, three M. genavense, two M. avium intracellulare complex). The prolonged incubation and the systematic terminal Ziehl—Neelsen increased the recovery of M. genavense from 5% to 14.5%. However, for only three patients did the late microbiological result lead to the introduction of antimycobacterial therapy.ConclusionsNeither a prolonged incubation longer than 6 weeks nor a terminal Ziehl—Neelsen-stained smear of the negative blood cultures at 12 weeks seem to be clinically justified. In this prospective study, all blood cultures submitted for mycobacteria detection, taken mainly from HIV-infected patients, were incubated for 12 weeks. The clinical impact of a late positive blood culture result was assessed retrospectively. From a total of 750 blood cultures, 68 had a growth index (GI) >10 due to the presence of mycobacteria. Of 545 negative blood cultures with a GI <10 within 12 weeks examined by Ziehl—Neelsen, one bottle revealed acid-fast bacilli further identified as Mycobacterium genavense by PCR-restriction fragment length polymorphism analysis of the hsp65 gene. For six of 39 patients with positive blood cultures, the delay to positivity was > 6 weeks (one M. tuberculosis, three M. genavense, two M. avium intracellulare complex). The prolonged incubation and the systematic terminal Ziehl—Neelsen increased the recovery of M. genavense from 5% to 14.5%. However, for only three patients did the late microbiological result lead to the introduction of antimycobacterial therapy. Neither a prolonged incubation longer than 6 weeks nor a terminal Ziehl—Neelsen-stained smear of the negative blood cultures at 12 weeks seem to be clinically justified.