FIG. 1.

Effects of preculture time on IFN-γ production. (A) IFN-γ responses to SEB were reduced (P < 0.0001) after 24 h versus 8 h of blood storage. Data are presented as OD values for naturally infected animals (n = 34), each tested at multiple sampling points. (B) Blood was obtained from cattle within a TB-positive herd. IFN-γ responses to PWM were reduced (P < 0.0001) after 25 to 31 h (n = 98) compared to the levels observed after 8 to 17 h (n = 100) of blood storage. The horizontal lines indicate the median OD values. (C) Blood samples were obtained from skin test-positive cattle with bovine TB confirmed by culture and/or pathology (n = 107) and from TB-free animals from areas where TB is not endemic (n = 146). Bars indicate percent sensitivity and specificity values for samples obtained within 8 h of blood sampling and after 24 h of storage. IFN-γ responses were determined using the standard criterion for test interpretation (i.e., PPD B OD₄₅₀ − PPD A OD₄₅₀ ≥ 0.1). *, increased (P < 0.05) specificity with samples stored for 24 h.