The conservatism of the RTNDT temperature indexing parameter and the ASME KIR-reference curve with respect to crack arrest toughness, has been evaluated. Based on an analysis of the original ASME KIa data, it was established that inherently, the ASME KIR-reference curve corresponds to an overall 5% lower bound curve with respect to crack arrest. It was shown that the scatter of crack arrest toughness is essentially material independent and has a standard deviation (S.D.) of 18% and the temperature dependence of KIa has the same form as predicted by the master curve for crack initiation toughness. The ‘built in’ offset between the mean 100 MPa√m crack arrest temperature, TKIa, and RTNDT is 38°C (TKIa=RTNDT+38°C) and the experimental relation between TKIa and NDT is, TKIa=NDT+28°C. The KIR-reference curve using NDT as reference temperature will be conservative with respect to the general 5% lower bound KIa(5%)-curve, with a 75% confidence. The use of RTNDT, instead of NDT, will generally increase the degree of conservatism, both for non-irradiated as well as irradiated materials, close to a 95% confidence level. This trend is pronounced for materials with Charpy-V upper shelf energies below 100 J. It is shown that the KIR-curve effectively constitutes a deterministic lower bound curve for crack arrest The findings are valid both for nuclear pressure vessel plates, forgings and welds.