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Your Captain Speaking: One Test Is One Test
“Samantha, we’ve seen not only patients but also friends and family members emotionally crushed when they’ve learned the results of medical tests. As a paramedic, I’ve explained that one test is just one test, and life-changing decisions should not be made based on one test.”
“Dick, maybe helping them understand some basic statistical concepts with those tests can help.”
Medical tests come in many forms, and the results can be joyful or soul-crushing. During the current COVID-19 pandemic, a positive test for the virus might fill us with uncertainty, fear, or anger. Cancer tests result in these feelings as well, big-time. But patients should fully understand what tests results are telling them. Let’s take a look at sensitivity, specificity, false positives, and false negatives in medical tests.
Snapshots of the Moment
No test is perfect in its results, and many may give results in shades of gray rather than clear positive/negative binaries.
Many tests are snapshots of conditions as they existed when the test was taken. If we perform a finger stick to check blood sugar, we get a reading based on the blood we sampled at that moment. A 12-lead ECG is a momentary glimpse of the electrical activity of the heart. Some tests, such as the A1C with blood sugar, measure results over time and might provide a known lookback period.
Taking multiple tests, as with home pregnancy tests, can help identify false positives and false negatives. A false positive in a pregnancy test would indicate you’re pregnant when you’re not. Remember, it’s only one test! A false negative would tell you there’s no baby when there is.
Sensitivity and specificity must exist in balance. Sensitivity is how well a test can identify true positives, and specificity is how well it can detect true negatives. Higher sensitivities will usually mean lower specificities, and vice versa. High sensitivity is desired in tests like cancer screenings, as we don’t want to mislead patients by telling them they don’t have cancer when they do.
COVID-19
Consider at-home COVID-19 self-tests. After taking the nasal swab and mixing in the test fluid, we wait 15 minutes. A colored control line should appear to show the test is working. If no second line appears, the test is negative, while the appearance of a full second line indicates positive for COVID-19. But there are no absolutes from one test. Manufacturers try to balance the yin and yang of sensitivity and specificity. Does this mean you should repeat the same at-home test? Instead take a different test, such as a PCR test, which is the current gold standard and highly specific, to confirm the diagnosis.
Essentially, the at-home COVID-19 test is a very good screening test, and if your result is positive, isolate for at least five days from the start of symptoms or test date and follow all other CDC recommendations. The SARS-CoV-2 PCR test via nasopharyngeal swab has been found to be 87% sensitive and 97% specific.1 These numbers are really good for any test! And while that’s a terrific balance between the two factors, notice they are not perfect. This is why the PCR test is the gold standard. It’s specific to SARS-CoV-2 and its different mutations. The numbers for home COVID-19 tests aren’t nearly as good. These tests can also be impacted by the user not following directions correctly.
PPV and NPV
“Dick, can you make my head stop spinning and give us an easy way to evaluate a test’s reliability?”
“Yep, let’s talk PPV!”
PPV, or positive predictive value, is the probability a positive test result accurately predicts the presence of disease. Negative predictive value (NPV) is the likelihood a negative test result accurately reflects a disease’s absence. The COVID-19 PCR test was also found to have a PPV of 98% and an NPV of 80% among symptomatic patients.2
What this says is that 98% of the time a patient has a positive PCR test and symptoms, they have the virus. That’s not perfect, but it’s really good. The NPV tells us a patient with symptoms who takes a PCR test but is free of the virus will show negative 80% of the time.2 This second number isn’t fantastic, but remember, one test is only one test, and none of them are perfect.
Final Takeaway
The availability of COVID-19 tests right now is strained. The CDC updates its guidelines frequently, so your best course is to go there for answers to your questions. Take prudent action after a test to err on the side of caution, but follow up and confirm. Tests are not perfect, how they are collected and processed may not be perfect, but we can and should take reasonable actions in the meantime and not ignore them.
References
1. George B, McGee J, Giangrosso E, et al. What Is the Predictive Value of a Single Nasopharyngeal SARS-CoV-2 PCR Swab Test in a Patient With COVID-Like Symptoms and/or Significant COVID-19 Exposure? Open Forum Infectious Diseases, 2020 Oct; 7(10): ofaa399.
2. Health News Review. Understanding medical tests: sensitivity, specificity, and positive predictive value. HealthNewsReview.org, www.healthnewsreview.org/toolkit/tips-for-understanding-studies/understanding-medical-tests-sensitivity-specificity-and-positive-predictive-value/.
Dick Blanchet, BS, MBA, worked as a paramedic for Abbott EMS in St. Louis, Mo., and Illinois for more than 22 years. He was also a captain with Atlas Air for 22 years and an Air Force pilot for 22 years.
Samantha Greene is a paramedic and field training officer for the Illinois Department of Public Health Region IV Southwestern Illinois EMS system, a paramedic and FTO for Columbia (Ill.) EMS, and a full-time paramedic at the St. Louis South City Hospital emergency department.
