TABLE 1.

Interlaboratory evaluation of three methods for recovery and viability of Bacillus anthracis from spiked swabs over time

Recovery methoda	Percent recovery (no. positive/no. tested) at each time pointb
	1 day		7 days		28 days
	Lab A	Lab B	Lab A	Lab B	Lab A	Lab B
SETS	0.2 (12/12)	0.1 (12/12)	0.0006 (10/12)	0.0001 (11/12)	0 (0/12)	0 (1/12)
Sonication	0.07 (12/12)	0.05 (12/12)	0.0004 (9/12)	0.0001 (8/12)	0 (1/12)	0 (1/12)
Vortex	0.09 (12/12)	0.06 (12/12)	0.0005 (7/12)	0.0001 (9/12)	0 (1/12)	0 (0/12)

^aA total of 12 nylon-flocked swabs each were spiked with 10 μl of virulent B. anthracis cells at a concentration of 10⁷ CFU/ml, and the three methods were compared at 1, 7, and 28 days postspiking. The procedures were repeated on different days in two independent CDC laboratories (labs A and B) except on day 1, when they were repeated in the same laboratory.

^bThe mean percent recovery for each method was calculated by dividing the spiking concentration by the average concentration of viable cells recovered (CFU/ml) at each time point. When <3 out of 12 swabs were negative in culture, the percent recovery was determined to be zero due to nonreproducibility. Values in parentheses show the number of swabs that were positive for isolation of B. anthracis in culture out of the total number of swabs tested.