What is the test for a defendant to be found with deliberate indifference to a prisoner's medical needs?

prisoner's civil rights have been abridged, "the indifference to his medical needs must be substantial. Mere 'indifference,' 'negligence,' or 'medical malpractice' will not support this cause of action." Broughton v. Cutter Laboratories, 622 F.2d 458, 460 (9th Cir. 1980) (citing Estelle, 429 U.S. at 105-06). A defendant acts with deliberate indifference when he knowingly fails to respond to a serious medical need, thereby inflicting harm on the plaintiff. Farmer, 511 U.S. 825 at 837-42. Even gross negligence is insufficient to establish deliberate indifference to serious medical needs. See Wood v. Housewright, 900 F.2d 1332, 1334 (9th Cir. 1990).