When looking at how exposures relate to disease, it is important to be sure that the outcome being investigated is the true disease, and not a condition mimicking the disease, which could lead to false conclusions about the causes of the disease of interest.
The example that David Sackett referred to in his 1979 publication to is a study investigating the relationship between oral contraceptive use and hepatitis published by Morrison and colleagues in 1977. Among a group of women attending hospital, those with hepatitis were more likely to be users of oral contraceptives than women not with hepatitis. However, the authors suggest caution in interpreting this association and suggest that:
Reporting bias also impact on our understanding of the disease and can introduce mimicry bias. To describe the potential benefits and harms of oseltamivir investigators stated that one of the outcomes of interest was pneumonia. However, detailed analysis of the clinical study reports revealed that the actual outcome was self-reported and not verified by clinical investigations and while participants may have thought they had pneumonia there was no real way of confirming this as the symptoms also mimic viral illness.
Mimicry can have an impact on diagnosis. Syphilis, for instance, can mimic a wide variety of diseases, in each of its phases. Primary syphilis presents atypically meaning only 30 to 40% of patients are diagnosed in this early phase. Another example is intussusception, which is a common abdominal emergency in early childhood, but other diseases mimicking intussusception should be kept in mind when making the differential diagnosis. The BMJ has a series easily missed dedicated to heightening awareness of conditions that are easily missed at first presentation in primary care, where undifferentiated symptoms and signs, mean an immediate diagnosis is often difficult to make.
Ensure correct diagnoses of the condition are the outcome used for studies investigating causal factors. Often this means using a gold standard diagnostic reference test to confirm the diagnosis. Readers should be wary of studies that rely on symptoms and self-reported measures to infer critical outcomes.