Effects of pre-culture holding time and temperature on interferon-γ responses in whole blood cultures from Mycobacterium bovis-infected cattle

The Bovigam™ assay is approved for use within the United States as a complementary tuberculosis test. Prior to whole blood culture and the ensuing ELISA to detect interferon-(IFN)-γ, samples are subjected to various holding time/temperature combinations due, in part, to practical constraints associated with shipment of samples to approved laboratories. To evaluate these effects, 5-month-old Holstein calves (n = 7) received 103 cfu Mycobacterium bovis by aerosol. Heparinized blood was collected 2 months after challenge and held at 4 or 22 °C for 0, 8 or 24 h prior to culture with mycobacterial antigens or pokeweed mitogen (PWM). Responses of samples held for 8 or 24 h were comparable and lower than responses of cultures prepared immediately after collection, regardless of holding temperature. Differences in responses of samples held at 4 °C versus 22 °C were also minimal. A subset of samples was held for 2 h at 37 °C at the beginning of the holding period. This subset of samples had diminished responses to all stimulants and increased holding times (i.e., 24 h versus 8 h) negatively impacted the response. Pre-processing conditions, particularly delays in set-up and initial high sample temperatures, reduces IFN-γ responses of cells from infected cattle increasing the risk of false negatives in this assay of regulatory importance.