Criminal prosecution is increasingly dependent on proof of the results of forensic laboratory tests. They are used, for example, to prove that a given substance contains cocaine; the prove what a driver’s blood alcohol content was; and to demonstrate that the DNA profile of some substance found at the crime scene matches that of the accused.
In Melendez-Diaz v. Massachusetts, 129 S.Ct. 2527 (2009), the United States Supreme Court resolved a question that had divided the lower courts in the wake of Crawford v. Washington, 541 U.S. 36 (2004). The Melendez-Diaz Court held by a 5-4 vote that forensic laboratory reports are testimonial for purposes of the Confrontation Clause of the Sixth Amendment. Therefore, even if the jurisdiction’s evidence law poses no obstacle to admission of the report, the Confrontation Clause prevents a prosecutor from proving the truth of an assertion made in the report merely by presenting the report. Instead, the prosecutor must present live testimony by a competent witness. Given the volume of forensic laboratory tests, and the reliance of many jurisdictions in recent years on the presentation of written reports rather than live testimony, it was plain from the outset that the potential consequences of this decision are quite significant.
Friedman, Richard D. "Potential Responses to the Melendez-Diaz Line of Cases." Crim. L. Rep. 90, no. 12 (2011): 396-402.