Pertussis (Whooping Cough)

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

SymptomsIncreased Disease Probability (Positive Likelihood Ratio)
Posttussive Emesis1.8× (1.4-2.2)
Paroxysmal Cough1.1× (1.1-1.2)
SignsIncreased Disease Probability (Positive Likelihood Ratio)
Inspiratory Whoop1.9× (1.4-2.6)

Negative Findings (Patient Doesn't Have This)

Sign / SymptomDecreased Disease Probability (Negative Likelihood Ratio)
Paroxysmal Cough0.52× (0.27-1.0)
Posttussive emesis0.58× (0.44-0.77)
SignsDecreased Disease Probability (Negative Likelihood Ratio)
Inspiratory Whoop0.78× (0.66-0.93)


  • Outcome: Pertussis

  • RCE Conclusion: In a non-outbreak setting, data to determine the diagnostic usefulness of symptoms classically associated with pertussis are limited and of relatively weak quality. The presence or absence of posttussive emesis or an inspiratory whoop modestly change the likelihood of pertussis; therefore, clinicians must use their overall clinical impression to decide about additional testing or empirical treatment.


  • None of the above LR's would dramatically change pretest probability

  • All patients in the 3 included studies were enrolled in outpatient clinics


Rodrigo Kong, MD and Shahriar Zehtabchi, MD


February 15, 2013

What are Likelihood Ratios?

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)