Slice isolation is a key feature of the network slicing technique and refers to protecting slices from negative impact of fault, attack or workload increase in other slices. Dynamic resource slicing policies, although provide efficient multiplexing and resource utilization, may lead to situation when a traffic surge in one slice hinders performance of other slices. The level of performance isolation cannot be specified and evaluated without defining an adequate metric. This paper addresses network slice degradation probability as a metric for defining performance isolation of slice. We use teletraffic theory to derive an analytical expression for the degradation probability in a single slice. © 2022, Springer Nature Switzerland AG.