Streptococal Pharyngitis

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

SymptomsIncreased Disease Probability (Positive Likelihood Ratio)
Strep exposure in the past 2 weeks1.9× (1.3-2.8)
Myalgias1.4× (1.1-1.7)
No cough1.1-1.7
History of sore throat1.0-1.1
Reported fever0.97-2.6
Duration <3d0.72-3.5
Signs on Physical ExamIncreased Disease Probability (Positive Likelihood Ratio)
Tonsillar exudates3.4× (1.8-6.0)
Pharyngeal exudates2.1× (1.4-3.1)
Tonsillar or pharyngeal exudates1.8× (1.5-2.3)
Any Exudates1.5-2.6
Tonsillar swelling/enlargement1.4-3.1
Palatine petechiae1.4× (0.48-3.1)
Ant. Cervical lymph node tenderness1.2-1.9
Measured temp >37.8 C1.1-3.0
Male sex0.87× (0.72-1.05)
No coryza0.86-1.6
Measured temp >=38.3 °C0.68-3.9
Pharynx injected0.66-1.63
Ant. Cervical lymph node swollen/enlarged0.47-2.9
Centor Criteria Points (Ignoring Age Modification)Increased Disease Probability (Positive Likelihood Ratio)
4 Points6.3×
3 Points2.1×
2 Points0.75
0 Points0.16
1 Points0.3

Negative Findings (Patient Doesn't Have This)

SymptomsDecreased Disease Probability (Negative Likelihood Ratio)
Duration <3d0.15-2.2
Reported fever0.32-1.0
No cough0.53-0.89
History of sore throat0.55-1.2
Nausea0.91× (0.86-0.97)
Strep exposure previous 2 wk0.92× (0.86-0.99)
Myalgias0.93× (0.86-1.0)
Signs on Physical ExamDecreased Disease Probability (Negative Likelihood Ratio)
Pharynx injected0.18-6.42
Measured temp >37.8 C0.27-0.94
No coryza0.51-1.4
Measured temp >=38.3 C0.54-1.3
Ant. Cervical lymph node swollen/enlarged0.58-0.92
Ant. Cervical lymph node tenderness0.60× (0.49-0.71)
Tonsillar swelling/enlargement0.63× (0.56-0.72)
Any Exudates0.66-0.94
Tonsillar exudates0.72× (0.60-0.88)
Tonsillar or pharyngeal exudates0.74× (0.66-0.82)
Pharyngeal exudates0.90× (0.75-1.1)
Palatine petechiae0.98× (0.92-1.1)
Male sex1.1× (0.93-1.2)


The Bottom Line: Clinician judgment performs better when looking at the constellation of findings to determine diagnosis.


  • Authors identify Centor scoring method as being validated with an ROC of 0.79 but this is for an adult population that is not where Strep throat incidence peaks (children 24-36% vs 5-24% adults).

  • There are 9 clinical prediction rules for strep throath. Not all references provide LRs or ROCs and most have limitations for their use).


Khaled Hassan, MD and Shahriar Zehtabchi, MD


February 25, 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!)