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

Abstract

Methods: 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. Results: 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. Conclusions: 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.