TABLE 2.

Checker board titration of SPS concentration and inoculum size for direct tube coagulase testing

Final dilution of SPS (0.5%)	Grade of plasma clota for S. aureus suspensionb
	2 × 109 CFU/literc (105 organisms)d			2 × 108 CFU/liter (104 organisms)			2 × 107 CFU/liter (103 organisms)			2 × 106 CFU/liter (102 organisms)			2 × 105 CFU/liter (10 organisms)			0 (saline)
	2 h	4 h	O/N	2 h	4 h	O/N	2 h	4 h	O/N	2 h	4 h	O/N	2 h	4 h	O/N	2 h	4 h	O/N
1:2c	0a	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
1:8	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
1:32	1	1	1	1	2	1	0	1	1	0	0	1	0	0	0	0	0	0
1:64	3	3	3	4	4	4	2	3	3	0	2	3	0	0	3	0	0	0
1:128	4	4	4	4	4	4	4	4	3	0	4	4	0	0	4	0	0	0
0 (saline)	4	4	3	4	4	4	4	4	4	0	4	4	0	0	4	0	0	0

^aClot size was graded by estimating the volume of coagulated plasma in the tube as a proportion. Thus, grades 1 to 4 represent plasma clots occupying 25%, 50%, 75%, and 100% of the plasma solution, respectively. Absence of a plasma clot was scored as 0% (grade 0). Grades were recorded after 2-h, 4-h, and overnight (O/N) incubations.

^bStrain used in the duplicate assays was a clinical isolate from phase 2.

^cEach tube was inoculated with 50 μl of bacterial suspension (10⁵ to 10 organisms) and 50 μl SPS dilution (final dilution, 1:2 to 1:128 of 0.05% SPS).

^dActual inoculum.