Victoria Pittman, MPAP, PA-C
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Poor Sensitivity Tests: What Are They Good For?
Each shift, we order and interpret various diagnostic tests that inform our everyday therapeutic decisions for patients. There are many factors that go into choosing which test to order, but reliability and accuracy are paramount. Two key performance criteria that help determine the reliability of a test are sensitivity and specificity. While I’m sure many of us groan when we think back to learning biostatistics in school, it actually has real world implications (unlike that algebra we have yet to use)! Let’s focus on the sensitivity of common tests encountered in Urgent Care and how it impacts patient care and satisfaction.
But first…a (very quick I promise) biostats review.
What Is Sensitivity, Again?
Sensitivity is the true positive rate for a test. A test is highly sensitive if it is positive for most patients who have the certain disease the test is looking to detect. For example, a urine pregnancy test is a highly sensitive test, meaning that it is most likely to be positive in someone who is pregnant. A highly sensitive test can be helpful when ruling something out. For example, if you have a female patient with a negative pregnancy test, it is not likely that they are pregnant. If a test has low or poor sensitivity, we cannot always trust that a negative result has definitively ruled out a disease. In other words, a test with low sensitivity will have a lot of false negatives and more opportunities for missed diagnosis and management.
Specificity, on the other hand, is the true negative rate for a test. A test is highly specific when it is positive only in a very small number of people who do not have the disease (in other words, the false positive rate is exceedingly low). A highly specific test can be helpful when ruling in a disease. For example, a chest x-ray is highly specific for a rib fracture. Meaning that if you see a rib fracture on a chest radiograph, that patient has a rib fracture.
A popular mnemonic to remember sensitivity and specificity is SpPIn and SnNOut:
SpPIn → high specificity, positive test result, rules in
SnNOut → high sensitivity, negative test result, rules out
Ideally, we’d like a test that has both high sensitivity and high specificity, but no test is 100% perfect. Take the example of a chest x-ray and diagnosis of rib fracture. We know that a chest x-ray is highly specific for a rib fracture (if a rib fracture is present, the patient most likely has a rib fracture). However, a chest x-ray is poorly sensitive for rib fractures. If a patient with blunt chest trauma presents to an Urgent Care and there are no rib fractures identified on the chest x-ray, this may be a false negative result. Understanding the reliability of a chest x-ray for detecting rib fractures is helpful for your decision making and management of the patient.
While it’s too much to address in this article, sensitivity and specificity do not exist in a vacuum. It’s important to look at the sensitivity and specificity of a test in the context of other important variables like the study population, likelihood ratios, and pretest probability.
Poorly Sensitive Tests in Urgent Care: They Are Everywhere!
It’s shocking how the bread-and-butter diagnostic tests we use daily in Urgent Care fall into the category of a poorly sensitive test. One such test is the rapid nasal influenza swab. The Infectious Disease Society of America guidelines recommend using influenza testing to help inform management decisions like antiviral medications. Rapid nasal influenza tests have a sensitivity between 50% and 85%. This means that up to 50% of the time you have a negative rapid flu result, it could be a false negative. Similarly, rapid nasal COVID tests have a sensitivity as low as 66%. These rapid nasopharyngeal tests, however, are convenient, have a quick turnaround time, and are cost-effective for the patient. Another example of a poorly sensitive, but commonly used diagnostic test in Urgent Care is a plain abdominal film, or “KUB.” This diagnostic study may be used to evaluate for ureteral or renal calculi or evidence of a small bowel obstruction. In a resource limited environment like Urgent Care, this test is readily available and inexpensive. The sensitivity of this imaging study depends on what you’re looking for but, for both the diagnosis of small bowel obstruction (SBO) and kidney stone, the sensitivity of a KUB is as low as 50%,. That means that if you’re ruling out kidney stone or SBO based on a negative KUB, you’re missing about 50% of these diagnoses, which has a significant impact on patient outcomes.
Knowing that poorly sensitive tests are common (and unavoidable), what does that mean for your next Urgent Care shift?
Practical Tips for Using Poorly Sensitive Tests
Low-sensitivity tests aren’t going away, so we need to understand how to use these tests to help our decision-making. In this month’s Urgent Care Reviews and Perspectives episode, Drs. Kelly Heidepriem and Lance Shaull dive deep into low-sensitivity tests. Listen in today!