What is the test for bad faith by the police to preserve potentially useful evidence?

In Youngblood, supra, 488 U.S. 51, the high court considered whether due process required law enforcement "to preserve evidentiary material that might be useful to a criminal defendant." (Id. at. p. 52, italics added.) The court held that "unless a criminal defendant can show bad faith on the part of the police, failure to preserve potentially useful evidence does not constitute a denial of due process of law." (Id. at p. 58, italics added; Illinois v. Fisher (2004) 540 U.S. 544, 549 [Youngblood's bad faith requirement is based on the "distinction between 'material exculpatory' evidence and 'potentially useful' evidence"].) Evidence is "potentially useful" when "it could have been subjected

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to tests, the results of which might have exonerated the defendant." (Youngblood, supra, at p. 57.)

Youngblood explained that "requiring a defendant to show bad faith on the part of the police both limits the extent of the police's obligation to preserve evidence to reasonable bounds and confines it to that class of cases where the interests of justice most clearly require it, i.e., those cases in which the police themselves by their conduct indicate that the evidence could form a basis for exonerating the defendant." (Youngblood, supra, 488 U.S. at p. 57.) Bad faith on the part of authorities involves "'malice'" or a "'design to seek an unconscionable advantage over the defendant.'" (People v. Coles (2005) 134 Cal.App.4th 1049, 1055.) Youngblood clarified, moreover, that "the police do not have a constitutional duty to perform any particular tests." (Youngblood, supra, at p. 59.)