In 2005, the CDC revised the Fukuda criteria in an attempt to identify patients with more precision. My research team believes the new criteria are still too vague and could potentially include people with primary depression. This ambiguity over definitions has made it difficult for researchers to pinpoint a biological cause. When investigators compare very different samples, it is difficult, if not impossible, to replicate findings from one lab to another. And when consistent biological findings do not emerge, investigators might inappropriately conclude that CFS is only a psychiatric problem.