Using Reduced Inoculum Densities of Mycobacterium tuberculosis in MGIT Pyrazinamide Susceptibility Testing to Prevent False-Resistant Results and Improve Accuracy: A Multicenter Evaluation

Table 2

Compiled PZA results for 14 susceptible and 3 resistant isolates tested in duplicate by 9 laboratories.

Outcome	Predicted total	Number (%) by inoculation method
		BD	A	C
True-susceptible	252	186 (61.2)	226 (76.4)	230 (79.2)
True-resistant	54	52 (17.1)	42 (14.2)	42 (14.5)
False-resistant	0	64 (21.1)	17 (5.7)	8 (2.8)
False-susceptible	0	2 (0.7)	11 (3.7)	9 (3.1)
Total valid tests	306	304	296	289
Timed Out	0	2 (0.7)	10 (3.3)	17 (5.6)
Total tests	306	306	306	306

Results from three isolates categorized as “inconclusive” not included. Results for one laboratory excluded due to <70% reproducibility. 18 of the false-susceptible results occurred in isolate 17. The remaining 4 occurred in isolate 14, all in the same laboratory. Denominators used to calculate outcome percentage. Test result not achieved within the 21-day instrument protocol. 19 of these events occurred in one strain (isolates 11 and 23). Denominator used to calculate Timed-Out percentages.