Curr Allergy Asthma Rep (2012) 12:590–598 the opinion on medical causation if the rest of the medical community itself is not sure. While future research and studies may well validate the physician’s opinion on causa- tion, sometimes the state of medical knowledge has not yet progressed to the comfort level of the legal system. Yet, the legal system mandates a scientific disposition at this point in time. This demand may be unsettling for the physician acting as an expert witness. An Attorney’s Perspective: Confusion of the Courts in Their Use of “Differential Diagnosis” From an attorney’s perspective, the clinical treatment pro- cess of rendering a diagnosis to identify the illness causing the patient’s symptoms (a differential diagnosis) is often confused by both doctors and attorneys with the process of determining the specific scientific or medical cause of the illness diagnosed (causal assessment). Many court opinions have mixed the two methodologies by sometimes equating a causal assessment to a differential diagnosis. In other words, the courts have conflated the two methodologies when dis- cussing them in written court opinions. In reality, the courts are looking for, and want a causal assessment with, a high degree of empirical or scientific and medical support and proof, not a differential diagnosis. The goal of the physician in performing a differential diagnosis is to determine what illness is causing the symp- toms in order to treat the patient. Determination of the underlying cause or what actually produced the condition is generally not critical in most clinical interactions. How- ever, a claim requires a causal attribution. The court must have a thorough explanation of whether the doctor’s opinion on causal assessment is “ground[ed] in the methods and procedures of science,” by considering the physician’s tes- timony using a “two-pronged test of relevancy and reliabil- ity in the context of scientific evidence.” [37••]. Relevancy refers to testimony that is likely to be helpful to the court and prove a key issue in the case. Reliability refers to testimony which meets the Daubert criteria. When determining during the trial whether a defendant is liable for causing an injury (or illness), the judge or jury must find by a “preponderance of the evidence” that the defendant caused the harm, exposure, or injury to the plain- tiff. The causal attribution must be established to a degree or medical probability or certainty, clearly more than a coin toss, and more than 50 %. But Daubert motions are heard before trial and must offer the court an even higher degree of proof of causation before the physician’s opinion is admissible. Each element used by the expert must be established and generally accepted. For the scientifically-based underpinnings (as needed for “can” or general causation), the degree of certitude must follow 595 those generally required in science, or, by custom, greater than 95 % (p value of< .05). Once that foundation (e.g., that x is an established cause of y) is known with this degree of probability, the doctor then may testify to opinions that all of the causation elements were met—e.g., exposure was suffi- cient, temporal relationships were correct, and other causes were ruled out. Those elements of testimony, needed for specific causation, require, not 95 % certainty, but a prepon- derance of the evidence standard of greater than 50 %. Even attorneys have difficulty getting a clear understanding of this distinction between legal and scientific causal probabil- ities. In fact, in the interest of “catching” the witness, they may ask whether his testimony is stated to scientific or legal certainty, implying that the former is more stringent than necessary. The answer to that question is actually, “Both.” Scientific certainty is needed to solidify general causation— that such a relationship is known scientifically to be true. Legal certainty (more probable than not) follows for the remainder of the causal questions. So, in essence, a Daubert inquiry is looking for causal assessment of a high order from the physician who testifies as to what environmental condition, exposure, biological pro- cess, or other explanation led to the injury or harm that forms the basis of the lawsuit. In the context of a Daubert inquiry, it is safe to say that “legal causation” depends heavily on a reliable and scientifically established medical “causal assessment.” Ultimately, the judge acts as a “gatekeeper” in deciding to let the doctor’s opinion in or to keep it out of consideration. The legal vehicle most often used for addressing admissi- bility of the physician’s opinion on medical causation is a Daubert Motion to Exclude Evidence. An Attorney’s Perspective: Using a Hypothetical Medical and Legal Case In the brief example which follows, we shall illustrate the types of questions which an expert may face by opposing counsel. These questions will vary depending upon the expert’s specific opinions in the case. They might, for ex- ample, challenge the diagnosis, the existence of an allergy, the alleged allergenic agent, or the source of that agent. Questions will also be posed which challenge the expert’s qualifications and motivations. Consider a 60 year-old male who rented an apartment until his new condominium was completed. He has a 40 pack-years cigarette smoking history, but never had respira- tory symptoms until having lived in this apartment for 2 months. He noticed a significant water leak in the guest bedroom closet and visible mold growth limited to its ceil- ing. He went to his internist for treatment of his shortness of breath and cough, wondering if he had developed asthma from mold in the apartment.