Negative BACTEC 460-TB cultures. How long to incubate?

Current guidelines for all mycobacterial cultures require at least eight weeks of incubation. Thirty months' experience of 3,662 specimens with the BACTEC TB was reviewed. At the end of four weeks of incubation, 96.8% of the 343 isolates were detected and by the end of the fifth week, 98.8%. The additional three weeks' incubation cost estimates were $1,016.20 for 92 hours of labor. Of the four late yields, only one may have had any clinical significance. The authors' recommendation is that the eight weeks' incubation guidelines for all mycobacterial cultures and especially for the BACTEC TB be comprehensively reviewed to determine clinical relevance and cost effectiveness of incubation of the BACTEC TB vial beyond five weeks.