The diagnosis of a variety of diseases has been effectively determined through clinical and experimental data. Research studies provide valuable information on the pathogenesis of many conditions and are often the primary source of information regarding new diseases or conditions. Case reports and case series are first level research studies, offering the most initial insights on a particular health issue through the personal experience of one or more people with a disease or condition. The following article describes the purpose of case reports and case series, and how they provide clinical and experimental data.
Table of Contents
1. Case reports and case series describe the experience of one or more people with a disease.
2. Case reports and case series are often the first data alerting to a new disease or condition.
3. Case reports and case series have specific limitations:
Case reports and case series represent the most basic type of study design, in which researchers describe the experience of a single person (case report) or a group of people (case series). Typically, case reports and case series describe individuals who develop a particular new disease or condition. Case reports and case series can provide compelling reading because they present a detailed account of the clinical experience of individual study subjects. In contrast, studies that evaluate large numbers of individuals typically summarize the data using statistical measures, such as means and proportions.
Example 3.1. A case series describes 15 young women who develop breast cancer; 9 of these women report at least once weekly ingestion of foods packaged with the estrogenic chemical bisphenol A (BPA). Urine testing confirms the presence of BPA among all nine case women.
It is tempting to surmise from these data that BPA might be causally related to breast cancer. However, case reports/case series have important limitations that preclude inference of a causal relationship.
First, case reports/case series lack denominator data that are necessary to calculate the rate of disease. The denominator refers to the population from which the diseased subjects arose. For example, to calculate the incidence proportion or incidence rate of breast cancer among women exposed to BPA, the total number of women who were exposed to BPA or the total number of person-years at risk is needed.
Disease rates are needed for comparison with historically reported disease rates, or with rates from a selected comparison group. Unfortunately, obtaining the necessary denominator data may not be easy. In this example, additional data sources are needed to determine the total number of BPA-exposed women from whom the breast cancer cases arose. The case series data alone cannot be used to calculate the rate of breast cancer because they do not include the total number of women who were exposed to BPA.
A second problem with case report/case series report data is the lack of a comparison group. The 60% prevalence of BPA exposure among women with breast cancer seems unusually high, but what is prevalence of BPA exposure among women without breast cancer? This comparison is critical for addressing the hypothesis that BPA might be a cause of breast cancer.
A third limitation of case reports/case series is that these studies often describe highly select individuals who may not represent the general population. For example, it is possible that the 15 breast cancer cases originated from a single hospital in a community with high levels of air pollution or other potential carcinogens. Under these conditions, a fair estimate of breast cancer incidence among non-BPA exposed women from the same community would be required to make an inference that BPA causes breast cancer.
A fourth limitation of case reports/case series is sampling variation. This concept will be explored in detail later in this book. The basic idea is that there is tremendous natural variation in disease development in humans. The fact that 9 of 15 women with breast cancer reported BPA exposure is interesting; however, this number may be very different in the next case series of 15 women with breast cancer simply due to chance. A precise estimate of the rate of a disease, independent from chance, can be obtained only by increasing the number of diseased subjects.
Recall the list of factors that are used to judge whether a factor may be a cause of disease:
1. Randomized evidence
2. Strength of association
3. Temporal relationship between exposure and outcome
4. Dose-response association
5. Biological plausibility
In general, case reports/case series rely almost exclusively on biological plausibility to make their case for causation. For the BPA and breast cancer case series, there is no randomized evidence, no measure of the strength of association between BPA and breast cancer, no reported dose–response association, and no evidence that BPA exposure preceded the development of breast cancer. The inference for causation derives completely from previous biological knowledge regarding the estrogenic effects of BPA.
Despite limitations of case series data, they may be highly suggestive of an important new association, disease process, or unintended side effect of a medication or treatment.
Example 3.2. In 2007, a case series described three cases of male prepubertal gynecomastia. The report included detailed information on each subjects’ age, body size, serum levels of endogenous steroids, and known exposures to exogenous hormones. It was discovered that all three otherwise healthy boys had been exposed to some product containing lavender oil (lotion, shampoo, soap), and that in each case, the gynecomastia resolved upon discontinuation of the product. Subsequent in vitro studies demonstrated endocrine-disrupting activity of lavender oil. This novel case series data may lead to further investigations to determine whether lavender oil, a common ingredient in commercially available products, may be a cause of gynecomastia.
Example 3.3. A vaccine designed to prevent rotavirus infection was found to cause weakening of the intestinal muscle layers in animals. Following release of the vaccine, a number of cases of intussusception (when one portion of the bowel slides into the next) were reported in children who received the vaccine, with some fatal cases. The strong biological plausibility underlying this initial association, and knowledge that intussusception is otherwise rare in infants, was highly suggestive of a causal relationship and the vaccine was removed from the market.
Information referenced from B. Kestenbaum, Epidemiology and Biostatistics: An Introduction to Clinical Research, DOI 10.1007/978-0-387-88433-2_3, © Springer Science+Business Media, LLC 2009. The scope of our information is limited to chiropractic as well as to spinal injuries and conditions. To discuss the subject matter, please feel free to ask Dr. Jimenez or contact us at 915-850-0900 .
Referenced by Dr. Alex Jimenez
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