Clinical Decision Rules (CDRs), also known as Decision “Instruments” or “Aids,” are evidence-based tools to assist the practitioner in decision-making for common complaints. In the Emergency Department (ED) setting, these decision aids are often used to help identify patients who might be at higher risk for serious conditions such as pulmonary embolism (PE) or intracranial hemorrhage (ICH), or they are used to prevent overuse of unnecessary testing, which is how many of the orthopedic rules are applied. 

CDRs, despite being called “Rules,” are not meant to replace critical thinking from experienced practitioners. In fact, many CDRs have been directly compared against clinician gestalt or clinical practice, and they are not always better [1,2]. Additionally, some rules incorporate clinician gestalt, whereas the rule cannot even be applied unless the pre-test probability (based on the physician’s judgment of the likelihood of the disease) is below a pre-determined threshold [3]. Also, for a CDR to be useful to a practitioner, it must be practical. If a CDR is developed that has too many complicated variables, it is unlikely to be applied in a busy clinical environment  [4].

Another caveat to the application of CDRs is that they must be applied appropriately.  CDRs evolve through a process of derivation to validation to impact analysis of the tool. After the tool is derived (level 4 evidence), the tool is validated in a limited patient setting (level 3 evidence), then a broader validation setting (level 2 evidence), and finally, the impact of the tool is assessed (level 1 evidence) [5]. These levels are important to caution the novice learner against applying every CDR which has been derived and published automatically into their clinical practice. The tool must be validated in a patient population with similar characteristics to the practitioner’s patient population. For example, the tool may not perform the same (have the same sensitivity and specificity) if the prevalence of disease is different between the study and actual patient populations. Also, the practitioner must be familiar with a particular tool’s inclusion and exclusion criteria. If not, the tool could be misused. For example, if the tool was derived and validated for a patient population over the age of 18, it should not be inappropriately applied in a pediatric setting.

The practitioner must also understand the purpose of the CDR and whether it is a one-way or two-way rule. As noted by Green, for example, the Ottawa Ankle Rules are intended to be a two-way rule; if the patient meets the criteria, you do an X-ray; if they don’t meet the criteria, you do not do an X-ray [6]. There are two paths you can take after you apply your CDR. Alternatively, the pulmonary embolism rule-out criteria (PERC) demonstrate a one-way rule. This tool was developed to identify a subset of patients at very low risk for PE so that no further testing is needed. If the patient is “PERC positive,” this should not imply that further testing for PE, such as a D-dimer or CT angiogram of the chest, should be done. Whether or not additional testing should be done remains up to the practitioner and depends on many variables, including whether an alternate diagnosis is much more likely. PERC was designed to help “rule out” the diagnosis of PE, not “rule in.” This rule only guides you down one path: potentially, to do no testing; it makes no judgment as to what you should do if the patient is “PERC positive.”

In addition to CDRs, many risk stratification tools or scales have been used for serious conditions such as pulmonary embolism (PE) and acute coronary syndrome (ACS). Others have more recently been developed for use in the ED setting for common conditions such as congestive heart failure (CHF), chronic obstructive pulmonary disease (COPD), and transient ischemic attack (TIA) to identify patients at higher risk for acute severe complications [7]. From a practical perspective, the ED physician will often use these risk stratification devices to help determine which patients require admission. However, these tools are less prescriptive in that they are not rules that suggest what a practitioner should or should not do, but rather, they help the physician more objectively look at the risk for an individual patient. Then the practitioner must decide what level of risk they are comfortable with in regards to inpatient or outpatient management, which may greatly depend on the resources available in those environments. Most of the risk stratification tools encompass multiple variables with more complicated scoring systems; as they are not easily memorized, most of these would typically be used by ED physicians with real-time access to a computer or smartphone with appropriate apps.

Given the many pitfalls noted above of CDRs, the goals of using evidence-based medicine to reduce practice variability, maximize cost-effective use of resources, and help identify and diagnose high-risk conditions are important.  It is equally important that the ED physician critically appraise these tools and selectively apply them in appropriate ways [8]. The remainder of this chapter will use case scenarios to review the most commonly used CDRs in the ED setting.

The useful FOAM reference MDCalc.com provides a summary of the most common tools that are being used with easy-to-use online calculators and additional information on inclusion and exclusion criteria, evidence basis for the tools, as well as pearls and pitfalls for each tool.