What is the test for a defendant to deny a prisoner's right to access to the courts?

Prisoners have a constitutional right under the First Amendment of access to the courts. Lewis v. Casey, 518 U.S. 343, 346 (1996). To plead a viable access to the courts claim, a prisoner must allege facts showing that the defendant's misconduct impeded the prisoner's ability to pursue a nonfrivolous legal claim that seeks to vindicate basic constitutional rights. Id. at 353-55. In other words, there must be some "actual injury" caused by defendant's actions "such as the inability to meet a filing deadline or to present a claim." Id. at 348.