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Published 7/1/2018
J. Mark Melhorn, MD

How Causation Is Determined in Workers’ Compensation Cases Continues to Evolve

Determining causation is a critical issue for diagnosis and treatment of medical conditions and diseases. The science of causation continues to improve with additional research; however, because the concept has different meanings and applications for various parties, its definition remains elusive.

By definition, cause is something that results in an effect. In philosophy, if A causes B, then A must always be followed by B. In science, physicians and statisticians use statistics to arrive at suggestions from observational studies that A probably caused B. Biostatistics can never establish exact cause and effect, but rather support the probability that A contributes to B. In law, causation considers two separate and distinct components: cause in fact and proximate, or legal, cause. The threshold for cause in fact, often described as “but for,” varies by jurisdiction. Proximate cause is when two events are so closely linked that the first event is deemed responsible for producing the second event, the harm.

Work-relatedness, in the context of industrial injuries, involves concepts of medical and legal causation, and the two may be mutually exclusive. For physicians who treat injured workers, understanding the differences between the two is essential.

The definition of medical causation originates from science, where evidence slowly accumulates over time. For example, criticism of published studies leads to studies that are designed better and therefore have lower risk of bias. Eventually, a conclusion is so probable it becomes generally accepted as scientific truth. The definition of legal causation, on the other hand, stems from a desire for social justice. Legal causation may occur before the truth is known; experts testify that A causes B, and opinions (conclusions) are accepted as evidence but may later be proven to be false.

Determination of work-relatedness

The courts base their decisions upon evidence that they have deemed admissible. Rules of evidence are intended to help ensure an appropriate foundation for court decisions. Yet, ensuring fairness when opposing or conflicting scientific opinions are involved is often not a simple matter. The Frye and Daubert standards established rules for admissibility of scientific evidence in legal proceedings.

Written in 1923, the Frye standard reads, “… in admitting expert testimony deduced from a well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field to which it belongs.” Viewed analytically, the Frye standard places the ultimate locus of control of admissibility not in the courts but in the scientific community.

The 1992 Daubert standard reads, “Under Daubert, we (the judges) must engage in a difficult, two-part analysis. First, we must determine nothing less than whether the experts’ testimony reflects ‘scientific knowledge,’ whether their findings are ‘derived by the scientific method,’ and whether their work product amounts to ‘good science.’ Second, we must ensure that the proposed expert testimony is ‘relevant to the task at hand,’ … i.e., that it logically advances a material aspect of the proposing party’s case.”

The National Institute for Occupational Safety and Health, American College of Occupational and Environmental Medicine, and American Medical Association’s AMA Guides to Evaluation of Disease and Injury Causation (blue book) offers six steps for determination of work-relatedness that can aid physician-scientists in providing expert testimony that is both good science and relevant.

Step 1: Determine an appropriate medical diagnosis of the condition in question.

Step 2: Once the diagnosis is confirmed, perform a literature search to find the current best science. The diagnosis may be included in the AMA Causation blue book. If it is not, it may be possible to use a similar diagnosis for the analysis. The AMA Causation blue book also provides a specific method for scoring the studies obtained during the literature search and determining a level of evidence.

Step 3: Assess evidence of exposure. In 1965, the English statistician Sir Bradford Hill proposed a set of criteria to assess epidemiologic evidence of a causal relationship between a presumed cause and an observed effect. Apply Hill’s criteria for causation to the epidemiologically identified risk/association
factors and to determine whether the individual experienced an appropriate exposure to them.

Step 4: Don’t overlook other relevant factors. For example, does the individual have nonoccupational risk factors that may outweigh occupational risk factors in the development of the diagnosis?

Step 5: Judge the validity of the science. This requires applying the appropriate Hill criteria to the individual case and the study design.

Step 6: Form conclusions; decide whether a causal association is likely to exist. Because this step requires substantial judgment, it also requires that the person(s) evaluating the body of evidence make statements of how sure they are that there is or is not a causal association. Workers’ compensation systems generally require that a physician’s opinions or conclusions regarding causation be expressed in terms of probability that a given condition has or has not arisen in the course and scope of employment (i.e., it is or is not work-related) to a reasonable degree of medical certainty; for example, a probability greater than 50 percent standard, preponderance of the evidence, or reasonable degree of medical probability. The courts usually prefer that these opinions be expressed in terms such as “more likely than not.”


The determination of causation has become the gateway to treatment and reimbursement for workers’ compensation. The science of causation is constantly evolving to improve our understanding of individual physical thresholds, associated risk factors, and individual biopsychosocial economic factors. New laws, however, place constantly changing legal thresholds for determining work-relatedness. The underlying foundation for fairness is quality science to support decisions made by the legal system to provide injured workers the appropriate treatment to decrease or restore their functional impairment.

J. Mark Melhorn, MD, is a clinical associate professor in the Department of Orthopaedics at University of Kansas School of Medicine–Wichita and the current program director for the AAOS annual continuing medical education course on workers’ compensation.