Confirmation bias is not always bad

Phillip Benton, MD, JD pgbentonmd at AOL.COM
Fri May 27 04:50:37 UTC 2016


No apology needed – your English is so much better than our Spanish.

Good job with your TB case. I shows what I like to call "physicians instinct," but in reality it comes from experience and simple reasoning (plus being honest, and smart enough to ask for help when you are unsure). On the other hand sometimes you just know it just has to be the diagnosis, and pursue it like an Argentine Dogo pursues a wild boar in the jungle.

Another explanation is in the old (?Hippocratic) aphorism: 'Uncommon manifestations of common diseases are more common than common manifestations of uncommon diseases.' This was EBM before we knew what to call it. 

Again, good job on saving that patient.                                                                                                                                                                                                               Phil Benton, MD                                                                                                                                                                                                                                                    Atlanta, GA

-----Original Message-----
From: Jesus Rivas <jesus.rivasce at GMAIL.COM>
Sent: Wed, May 25, 2016 5:26 pm
Subject: [IMPROVEDX] Confirmation bias is not always bad

Hello friends,

First, excuse my poor English.                                                                                                               

I’d like talking to you about a case I saw in my hospital a few weeks ago. It’s about a 62 years´old woman with a 15 days history of cough and intermittent hemoptisis. The first presumptive diagnosis was lung Tuberculosis. Three tests of acid-fast bacilli in sputum and gastric aspirate were negative. A Chest-X-ray and CT-scan showed a right lower lobe cavitary lung lesion consistent with bronchiectasis. Ceftriaxone was initiated. The physician  insisted on TB diagnosis and ordered a fourth test in sputum and a fiberoptic bronchoscopy. That sputum smear was positive (4 bacilli per 100 fields) before FOB be done. 

In endemic regions like Peru, it´s not difficult to identify typical pictures of TB (probabilistic and prototypical-based reasoning). However, if we´re lacking a rapid positive mycobacterial test, diagnosis and treatment of TB can be omitted o delayed, a serious situation which has been considered as the main factor of nosocomial propagation of TB. Thanks to doctor’s insistence on his first clinical suspicion in spite of repeated negative tests (confirmation bias) cases like this (paucibacillary tuberculosis) are not missed.

I agree with Monteiro’s opinion that we should call these situations as “active pursuit of supportive data” and use the label “bias confirmation” only in the event that the end result is an error1. In defining clinical reasoning the experts give different weight to several aspects: cognitive, memory, experience and narrative skill. I think that in some cases like this there is also a little room for behaviors like “clinical stubbornness”. 

(1)Teach Learn Med. 2013;25 Suppl 1:S26-32. doi: 10.1080/10401334.2013.842911.

Jesús Rivas-Ceballos, MD

Departamento de Medicina. Hospital Grau, Essalud

Lima, Perú



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