Fig. 1From: Effects of repeated comparative intradermal tuberculin testing on test results: a longitudinal study in TB-free red deerRed deer mean (±SD) seasonal (winter 2012 to winter 2015) skin test response in mm to the intradermal injection of a avian purified protein derivative (aPPD), b bovine PPD (bPPD) and c the mitogen phytohaemagglutinin (PHA). The dashed lines indicate the cut-off value for each PPDBack to article page