Sensitive Testing

Sensitivity, Specificity, Positive Predictive Value, Negative Predictive Value


These terms provide the basis with which we evaluate all tests in medicine, however they are all often misinterpreted or confused for each other.  

Last night at Journal Club Ben Hunter gave a great synopsis and I have recorded a recap of it here



All these values interpret the percentage of the time a test is correct in a certain population


Sens–   % correct in population with the disease

Spec– % correct in population without disease

PPV — % correct in population with positive test

NPV — % correct in population with negative test


PPV sounds like a dream.  Exactly how worthwhile is a positive test.  But it is deceptive.  PPV and NPV are highly dependent on prevalence.  Here is an example.  

Two populations

Group A — Pts < 40 with chest pain.   1% prevalence of MI

Group B  — Pts 65-75 with chest pain, SOB, nausea, diaphoresis, HTN, DM, ESRD. 50% prevalence of MI.

I am going to evaluate a test where I take blood from the patient, throw it in the trash, and say “Your test is Negative.”  Lets see how it works in these patients

Group A

Everyone gets a negative test result.  In 100 patients, 99 times this is correct so NPV is 99%

Next group

Group B

Everyone gets a negative test result.  In 100 patients, 50 times this is correct so NPV is 50%

This shows how NPV is affected tremendously by prevalence.

The real power of a test is in LR, which is a combination of sensitivity and specificity. This is not affected by prevalence and is a whole other lecture.








Headaches? Back Pain? Who do we MR? oh my….

Dr Greg Ruppel talked today about the back pain and headache causes which need a MRI.  


Here are my one line takeaways


Cauda Equina

Got to do a rectal exam, especially in the risky patients

Spinal Epidural Abscess

Pain is the most common complaint, when motor symptoms start its too late

Spinal Epidural Hematoma

This is why you ask every back pain patient about anticoagulation


Doesn’t always go with osteo, Back pain, worse with motion, better if still…sound familar?  These people should, but not always, have signs of infection.


Not every back pain with active ca needs an MRI

Cerebral Venous Sinus Thrombosis

Think about this in every HA.  

Red flags different from your normal HA red flags.

Negative D-dimer can help in low risk patients.   

Carotid/Vertebral Artery Disection

Even minor trauma can trigger this.  localized HA with some sort of neuro symptom.  MRA is better than CTA

Brain Abscess

These people will be sick.  Head CT anyone with focal neuro signs before LP.

Cutaneous Abscess Shownotes

What is an abscess?


Natural History

    Most of these are Staph Aureus, MRSA increasing.  
Pertinent History

Ask about DM, Liver or Kidney disease, transplant, chronic steroid use, HIV, History of recurrent abscesses .  These are important comorbidities which affect healing.  Though I take issue with unqualified HIV being placed here.  On HAART, CD4 >400 shouldn’t be different than the general population.


Diagnosis of abscess can sometimes be tricky.  Pain can limit your ability to detect fluctuance and diffuse cellulitis can hide a pocket of pus. Here is a guide to  Ultrasound diagnosis 

Decision to I&D

This is the treatment—-get the pus out.

Here is a study looking at practice variation amount EM providers


Physician judgement call.  Some locations, large sized abscesses, etc… may not be able to get adequate anesthesia with local.  


Local abscess roof anesthesia vs. field block

? buffered lidocaine direct injection ?–I could not find any papers looking at this.

warmed lidocaine?


I&D technique

Loop Drain Technique


To pack or not to pack??

  Here is a small RCT.  Showed a difference in rating pain and pain med use.  No difference in healing.  

Post I&D antibiotics?

These guys from Penn did a great review in Annals 2006.  Very in-depth systematic review. 

I’m going to go ahead and say—

  • Healthy patient, minimal to no cellulitis, no abc
  • DM, steroids, uncontrolled HIV, mostly cellulitic—treat for MRSA

Closure?  What the F***?

Here is a great synopsis of some guy’s from Stony Brook and their systematic review of the literature.  Points out some flaws in generalizing this data to our ED practice. Includes link to the original article.

Singer et al from Stony Brook did their own trial on primary closure.  Here is the paper. Thanks Google Foam.  

              -A few words of my own about this study.  

  • Very small–N=56
  • Excludes DM, HIV, chronic steroids
  • Excludes systemic symptoms, >5cm cellulitis
  • Overall underpowered; but definitely makes you think maybe this could work…



            This just goes to show you even a simple condition that you thought you knew how to treat has controversy.