Anti-HCV immunoblot indeterminate results in blood donors: non-specific reactivity or past exposure to HCV? 
The significance of anti-HCV immunoblot (IB) indeterminate results can be difficult to determine.  We analysed results for blood donors tested on the MP Diagnostics HCV Blot 3.0 IB assay to determine whether indeterminate results representing past exposure to HCV could be distinguished from those due to non-specific reactivity.  Results for all donors tested by IB during the study period (July 2010 to December 2013) were included in this study.  Of 131 donors tested by IB, 34 (26.0%) were negative, 38 (29.0%) were indeterminate, and 59 (45.0%) were positive.  There was no significant difference in IB band reactivity strength between indeterminate and positive donors.  The PRISM HCV chemiluminescent immunoassay (ChLIA) sample to cut-off (s/co) ratio distribution for the indeterminate donors was significantly higher than for those with biological false reactivity (P = 0·037), but significantly lower than for donors who were IB positive/HCV RNA negative (P < 0·001) or IB not tested/HCV RNA positive (P < 0·001).  Of donors available for follow-up, 53.1% of the indeterminate group disclosed a putative risk factor for HCV infection compared to 39.4% (P < 0·001) for the IB-negative group, 76.6% (P = 0·065) for the IB-positive group and 83.4% (P < 0·001) for the HCV RNA-positive group.  The results of this study indicate that PRISM ChLIA s/co ratios >2·00 with IB indeterminate results predict exposure to HCV, particularly in the presence of putative risk factors for HCV infection.  These findings may be applied to optimizing counselling of donors with indeterminate HCV results.