These factors are adjusted for in the other methods of defining platelet refractoriness. HLA alloimmunization is the commonest immune cause of platelet refractoriness. Platelet transfusion refractoriness can be defined in several different ways. Sand with too low refractoriness will melt and fuse to the casting. The refractoriness of the ventricular myocyte increases at lower heart rates. Agents in this class also cause decreased conductivity and increased refractoriness. The cause of refractoriness may be either immune or non-immune. This condition is known as refractoriness to platelet transfusion. Among immune-related refractoriness, antibodies against HLA antigens are the primary cause. The refractoriness under load can be determined by using the ISO 1893 standard. At 16 hours post-transfusion a PPR < 10% is considered evidence of platelet refractoriness. At 1 hour post-transfusion, a PPR < 20% is considered evidence of platelet refractoriness. Class Ib agents shorten the action potential duration and reduce refractoriness. A significant problem for multitransfused patients is refractoriness to platelet transfusions. Platelet refractoriness can be due to immune causes or non-immune causes.