Why is the Bayesian method not employed for diagnosis in practice

James Oldham james.oldham at HEALTH.NSW.GOV.AU
Sun May 27 05:49:46 UTC 2018


Dear all
There is a difference between the treating doctor who makes a probabilistic diagnosis and evidence providers such as radiologists who comment on phenomena the observe and interpret it - taking Bayesian prior probability calculations explicitly into their offerings. I think the testing physician and their patients and family as a collective Judge relying on evidence provided by expert witnesses. So this final stage will be informed by Bayesian logic but not ruled by it and certainly not trying to make a prediction- just best information to hand
James Oldham
Child Psychiatrist
NSW

from  James Oldham - sent from my phone. (Please accept apologies odd sentence construction and creative additions courtesy of Apple autocorrect)

On 27 May 2018, at 13:25, Stefanie Lee <stefanieylee at GMAIL.COM<mailto:stefanieylee at GMAIL.COM>> wrote:

An interesting paper from Blackmore and Terasawa, JACR 2006 on the complexity of CT interpretation and how clinical probability may be factored into the decision to call a test result positive or negative:

"In this paper, we present an example of how health utility assessment can be used to guide the optimum interpretation of an imaging test."

"Radiologists have the ability to alter the sensitivity and specificity of their interpretations. For objective positivity criteria, different thresholds can be chosen to consider test results positive. For example, with appendicitis, one important factor in CT interpretation is the size of the appendix. Using a lower size threshold (eg, 6 mm) to consider an appendix abnormal will result in higher sensitivity for appendicitis, at the expense of lower specificity. Using a higher size measurement to consider the appendix abnormal (eg, 8 mm) would result in lower sensitivity but higher specificity. For subjective criteria such as periappendiceal fat stranding, the process is the same but less explicit. Individual radiologists can alter how much increase in the density of the fat is necessary to be considered abnormal “stranding.”"

"In general, when a disease is rare and there are substantial costs to a false-positive diagnosis, interpreting a test with high specificity will maximize patient benefit. In contrast, when a disease is common and there are substantial costs for a false-negative diagnosis, interpretation at high sensitivity will maximize utility."

"Our prior meta-analysis indicated that radiologists interpret CT scans with approximately equal sensitivity and specificity. The current analysis indicates that this is an appropriate threshold at the intermediate probability of disease at which CT is commonly used today. If CT is to be used in populations at higher or lower probabilities of disease, then different imaging thresholds will be appropriate."

On 15 May 2018 at 13:18, Jain, Bimal P.,M.D. <BJAIN at partners.org<mailto:BJAIN at partners.org>> wrote:
In this attached paper, I discuss that the prescribed Bayesian method is not employed for diagnosis in practice because probability of a diseases is considered evidence for it in a given patient in this method, which is incorrect as a probability is a frequency in a population. This leads to all sorts of errors in practice which I discuss.
The correct method of diagnosis, which is employed in practice, consists of hypothesis generation and verification in which evidence is assessed by a likelihood ratio which locates it in the given patient of interest.
Please review and comment on this paper.

Thanks

Bimal

Bimal P Jain MD
Northshore Medical Center
Lynn MA 01904.

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<Blackmore Optimizing Interpretation JACR 2006.pdf>

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