The islands from tearing modes driven unstable and sustained by the helically perturbed neoclassical bootstrap current often provide the practical limit to long-pulse, high confinement tokamak operation. The destabilization of such ‘‘metastable’’ plasmas depends on a ‘‘seed’’ island exceeding a threshold. A database from similar regimes [high confinement H-mode with periodic edge localized modes (ELMs) and periodic central sawteeth] was compiled from the tokamaks ASDEX Upgrade (AUG) [Plasma Phys. Controlled Fusion 41 , 767 (1999)], DIII-D [Nucl. Fusion 38 , 987 (1998)], and JET (Joint European Torus) [Plasma Phys. Controlled Fusion 41 , B1 (1999)]. A comparison is made of the measured critical beta for onset of the m / n = 3/2 mode (m and n being the poloidal and toroidal Fourier harmonics, respectively) to a model in terms of dimensionless parameters for the seed and threshold islands. This modeling is then used for extrapolation to a reactor-grade tokamak design such as ITER/FDR (International Thermonuclear Experimental Reactor/Final Design Report) [Nucl. Fusion 39 , 2137 (1999)]; this indicates that the seed island from sawteeth could be too small to sufficiently disturb the metastable plasma and excite the m / n = 3/2 neoclassical tearing mode.