EMS DOCS’ PERSPEC TIVES ON STREET MEDICINE
SENSITIVITY & SPECIFICITY
From headache to helping hand
By Neal J. Richmond, MD, FACEP
Many of us get headaches and go cross-eyed when we see statisti- cal concepts like sensitivity and
specificity in print. Usually, these appear in
articles about a diagnostic test’s ability to
tell you when a condition or disease state is
present (i.e., sensitivity) or, conversely, to tell
you when that condition or disease state isn’t
there (i.e., specificity).
An initial 12-lead ECG, for example, has a
sensitivity of about 68% for diagnosing an acute
ST-elevation myocardial infarction (STEMI).
That means that a little more than two-thirds of patients who eventually rule-in for
MIs are going to have ST elevations on their
What this translates to on the street is that a
nondiagnostic ECG (no ST elevations) probably shouldn’t be used to withhold aspirin in
a suspected MI, or to support a patient’s decision to refuse transport; otherwise, there’s a
reasonable chance that you and your patient
are going to get into trouble about one-third
of the time (32%).
On the flip side, an ECG’s ability to detect a
STEMI has a specificity of about 97%. In other
words, if the patient isn’t having a STEMI, then
there’s only a very small chance that they’re
going to have ST elevations that look like
they’re having one (3%).
What this all means is that a 12-lead ECG
is a good way to rule-in an MI when ST-elevations are present (68%), but it’s a bad way to
rule one out when those changes aren’t (32%).
There’s also a small chance that you might see
ST-elevations when the patient isn’t actually
having a STEMI (3%).
Fortunately, we’re all trained to rely on more
than just the 12-lead ECG. We also base our
clinical decision-making on patient history
and risk factors, review of systems and physical exam.
TAKING IT TO THE STREET
Translating sensitivity and specificity from the
more abstract realm of statistics or research to
the reality of the street can provide some unex-
pected benefits. These concepts not only help us
to more appropriately interpret ECGs, but they
may also give us a more accurate perspective on
our histories and physical exams.
When we arrive on scene, our patients aren’t
usually standing there with signs saying, “Hey,
I’m having a pulmonary embolism,” or, “I think
this pleuritic chest pain and shortness of breath
must be a spontaneous pneumothorax.”
Instead, all we really have to rely on is our
history, our stethoscope and a few diagnostic
tests to narrow down the differential diagnosis
and figure out what’s going on. To make things
even more complicated, all this must happen
while we’re managing a complex scene and
trying to keep our “task times” under the radar.
Hanging this all together is a little like being
a judge in court. We have evidence and it’s our
job to weigh it—either to make a treatment
decision or appropriately advise a patient of the
risks and consequences of refusing transport.
PUTTING IT TO THE TEST
By keeping a file on the sensitivity and specificity of history and physical exam findings
in the backs of our heads, on-scene clinical
decision-making becomes more evidence-based,
efficient and accurate.
For example, tachypnea is typically thought
of as one of the hallmark presenting symptoms of pulmonary embolism (PE). Unfortunately, the sensitivity and specificity of a
respiratory rate greater than 20 are 53% and
What that means is that a little more than
half of patients with a PE will present with elevated respiratory rates (53%) and, conversely, a
little more than half who don’t have PEs will
also be tachypneic.
We could probably flip a coin instead of
counting the respiratory rates in these patients:
“Heads they have a PE, tails they don’t.”
The point, though, is not to get cynical, but
instead to use this information wisely. If we use
the absence of tachypnea to rule out a PE, then
we’re going to miss the boat in half our cases.
Similarly, most cases of PE are thought to
arise from deep venous thromboses (DVT) of
the lower extremity. The sensitivity and specificity of the physical exam for detecting calf
or thigh swelling, erythema, edema, tenderness
or a palpable cord are 47% and 77%, respectively—again, a virtual coin toss.
If you use the absence of these signs to rule
out a DVT, you’re going to miss one in about
half of cases. On the other hand, a little more
than a quarter of your patients who do have
these findings won’t actually have DV Ts or PEs.
COIN TOSSES ARE OK
There’s nothing wrong with a little uncertainty,
as long as you know that’s what you’re dealing with.
No individual symptom, sign or test should
guide our clinical decision-making, but an
understanding of the sensitivity and specificity of these diagnostic elements can turn what
would otherwise be a coin toss into a more
sophisticated and effective approach to critical thinking. JEMS
1. Ioannidis JP, Salem D, Chew P W et al. Accuracy and clinical
effect of out-of-hospital electrocardiography in the diagnosis
of acute cardiac ischemia: a meta-analysis. Ann Emerg Med.
2. Stein PD, Beemath A, Matta F et al. Clinical characteristics of
patients with acute pulmonary embolism: data from PIOPED
II. Am J Med. 2007;120( 10):871.
Neal J. Richmond, MD, FACEP, is board certified in emergency medicine and medical
director for the MedStar Mobile Healthcare
System in Fort Worth, Texas.
Learn more from Neal Richmond at the
EMS Today Conference in Charlotte, N.C.,
on Feb. 21–23, 2018. EMSToday.com