LAUSR

Headaches are a prevalent, but highly nonspecific, complaint. Although most patients seek symptom relief, reducing the pain is never the sole aim of a consultation.1 Understanding whether the pain is the first manifestation of dangerous disease is equally essential; headaches offer the opportunity to detect early potentially lifethreatening disorders.2 Secondary headaches divide into two categories.3 Diagnosing those in the first group requires no additional examinations (e.g., “headaches attributed to a substance or its withdrawal”). Conversely, additional examinations are indispensable to diagnose those in the second group (e.g., “headaches attributed to intracranial neoplasia”). Diagnostic tests investigating the presence of underlying diseases are available, but their use warrants consideration given their price and the sheer number of headache patients. Therefore, diagnosing a headache comprises pondering the available evidence and the need for more evidence. Red Flags may simplify the latter. Red Flags can be interpreted like screening tests identifying patients whose headache diagnosis is not known yet as having an elevated risk of a secondary headache. The primary strategy is to use readily available information to infer the unknown. Different publications share a similar understanding of Red Flags but offer only conceptualized definitions.4,5 A possible operationalized definition that accommodates all published Red Flags, therefore, would consist of the following. Definition: In patients with an undetermined headache, a sign or symptom is a Red Flag if a secondary headache is more likely in its presence than in individuals who do not screen positive for the Red Flag. The disadvantage of this definition is that the probability of a secondary headache in the presence of the Red Flag might be tiny and marginally bigger than the prior probability (see Example 1 in the Supporting Information). Moreover, there is a risk of this definition being applicable only under certain circumstances (e.g., in primary but not tertiary care). Above, we defined Red Flags as a symptom or sign. However, using them to predict the probability of a secondary headache implies a second meaning of the term. It is also a diagnostic test that reports the probability of a secondary headache, given the presence of one defined symptom or sign. Thus, it is a measure of conditional probability. Let SH+ be the event of the presence of a secondary headache, and RF+ the event of the presence of a Red Flag. Then, Bayes’ theorem describes the relationships of the probabilities of the investigated events as follows. According to Bayes’ theorem, three factors determine the conditional probability of a secondary headache in the presence of a Red Flag.