The Case Report and Case Series

In the previous parts we met studies each of which compared two groups of people. The RCT did this by drawing lots, the cohort by following people over time, the case-control study by going back into the patients’ past, and the cross-sectional study by measuring everything at one moment. In this part we reach the lowest rung of the evidence ladder. Here there is no comparison at all. A clinician has seen something in one or a few patients and has reported it.

What a case report is

A case report is a description of one patient’s situation: what problem they presented with, what treatment they received, and what the outcome was. When several patients with a similar situation are reported together, it is called a case series. For example (a hypothetical one), a dentist sees an unusual gingival reaction in three consecutive patients after using a new restorative material and describes these three cases in one article. That is a case series.

These reports differ from the studies we have seen so far in two main ways. The first difference is that they have no comparison group. In the previous parts we said that to understand the effect of a factor, we need a group that did not receive that factor. A case report has no such group and only says what happened in these few patients. The second difference is that they were not planned in advance. In a designed study, the researcher decides from the outset who will be examined and what will be measured. In a case report, the event happens first and the clinician then decides to report it. For this reason these observations are called unsystematic.

Why they are valuable nonetheless

Despite these two weaknesses, the case report has a use in three situations that other studies cannot replace.

The first situation is rare diseases. When a disease occurs so rarely that no large study can be designed for it, the only way to come to know it is for every clinician who sees a case to report it. The same function serves when a common disease shows a new aspect that had not been seen before.

The second situation is the introduction of a new treatment or method. When someone tries a method for the first time, no study about it exists yet. The case report is the first place where that method is introduced to others.

The third situation is the reporting of adverse effects. When a drug or a material produces a reaction in a patient that was not expected, reporting that case can be the first warning to everyone else.

There is one more advantage. A case report carries a great deal of detail. In a trial with hundreds of patients, each patient is reduced to a few numbers. In a case report, the full course of one patient is described, and these details sometimes reveal something that is not visible in the overall numbers.

From case report to stronger study

When one clinician reports a case and other clinicians then report similar cases, a hypothesis takes shape: perhaps this factor is related to this outcome. The case report cannot prove this hypothesis. But it can draw the attention of the scientific community so that someone designs a stronger study to test it, of the kind we saw in the previous parts. The case report raises the question, and stronger studies answer it.

Why it sits at the bottom of the ladder

The reason is the same two differences we stated at the beginning. Suppose a new treatment has given a good result in five patients. We do not know what would have happened had these five not been treated; perhaps they would have recovered on their own. We do not know how similar these five are to the patients we see in our own practice. Observations gathered without a control and without a plan cannot say what happens in a larger population of patients. For this reason, the result of a case report cannot be generalised to other patients. A case report says this can happen. It does not say this usually happens or that this treatment works.

Summary

A case report is the description of one patient, and a case series the description of several similar patients. They have no comparison group and were not planned in advance, so their result cannot be generalised to other patients. But for rare diseases, new methods and unexpected adverse effects they are the first and sometimes the only source, and they generate hypotheses that stronger studies later test.

With this part we have seen every type of primary study, from the RCT to the case report. The next part turns to the work that is done on top of all of these: pooling the results of several studies on one question, in the form of the systematic review and meta-analysis.