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Bayesian decision making in primary care – or how to stop people dying of chicken pox. Trisha Greenhalgh Professor of Primary Health Care UCL. Before we start: What is primary health care?. Hospital medicine.
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Bayesian decision making in primary care – or how to stop people dying of chicken pox Trisha Greenhalgh Professor of Primary Health Care UCL
Hospital medicine “Distinguishing the clear message of the disease from the interfering noise of the patient as a person.” Marshall Marinker. ‘The mythology of Hilda Thompson’ In Greenhalgh T and Hurwitz H (eds) ‘Narrative Based Medicine’. London: BMJ Books, 1998
Primary health care “In secondary care diseases stay, but patients come and go, whereas in primary care patients stay but diseases come and go." Iona Heath ‘The mystery of general practice’. London: Nuffield Provincial Hospitals Trust, 1995
Primary health care “First-contact care, delivered by generalists, dependent on teamwork, which is accessible, comprehensive, co-ordinated, population-based, and activated by patient choice.” Pat Gordon and Diane Plamping ‘Extending Primary Care’. Oxford: Radcliffe, 1996
Primary health care “Doing simple things well, for large numbers of people, few of whom feel ill.” Julian Tudor Hart ‘A new kind of doctor’. London: Merlin Press, 1998
A patient with chicken pox It was Saturday morning. I was on call from 8.30 am. I got a call from one of my partners, Dr B, at 5.45 am. He was on holiday 200 miles away but had been called on his mobile phone by Health Call. One of his patients had rung Health Call and demanded a visit by Dr B. No other doctor would do.
A patient with chicken pox The family had a child with chicken pox. She had been seen the day before by another partner, Dr R, who has 24 years’ experience in general practice and is also a clinical assistant in dermatology. She had said it was “definitely chicken pox” and prescribed fluids, analgesia and calamine.
A patient with chicken pox The child had apparently deteriorated and the parents were worried. They had decided that only Dr B would know what to do. Dr B (who was many miles away) asked me to go round immediately and examine the child. I was not yet on call and keen to go for my early morning swim before surgery. What should my next move be?
Intermission: getting by as a GP You only need to answer three questions: • Are they ill or are they not ill? • If ill, can I deal with it or does someone else need to be involved? • If someone else, can it wait 12 weeks or can’t it? Cecilia Gould Crouch End Surgery coffee break, July 1989
Bayesian decision-making Pre-test odds of disease X TEST Y Post-test odds of disease X
Bayesian decision-making Parent phones up to say “I think my child has chicken pox” Pre-testY odds of disease X O.5 TEST Y Dr R examines child Post-testY odds of disease X O.97 Swab to virology TEST Z Post-testZ odds of disease X O.99
Bayesian decision-making Assume Disease X = Patient is seriously ill
Bayesian decision-making Parent phones up asking for visit to child with chicken pox Pre-testP odds of serious illness O.0005 TEST P INSERT QUESTION HERE Post-testP odds of serious illness O.005 INSERT QUESTION HERE TEST Q Post-testQ odds of serious illness O.5
A patient with chicken pox I asked: 1. “How old is the child?” [Answer: 15]
Bayesian decision-making Parent phones up asking for visit to child with chicken pox” Pre-testP odds of serious illness O.0005 TEST P How old is the child? [High risk age group] Post-testP odds of serious illness O.005 INSERT QUESTION HERE TEST Q Post-testQ odds of serious illness O.5
A patient with chicken pox I asked: 1. “How old is the child?” 2. “Why the $#*! are you so convinced that these guys are not time wasters?”
A patient with chicken pox He said: “For one thing, this family have been on my list for 17 years and they’ve never asked for a visit before. For another thing, they go to the most orthodox synagogue in Golders Green.”
A patient with chicken pox “And there’s one more thing I don’t like about this case. It wasn’t the mother who rang, it was the father. In that family, the father never does the kids’ health.”
Probability • Of calling the doctor out at night: • 1 in 17 years (1 in 6205) • Of using the telephone on the Sabbath: • 1 in 10,000? • Of father rather than mother negotiating: • 1 in 100? Estimate the index of parental concern.
Bayesian decision-making Parent phones up asking for visit to child with chicken pox” Pre-testP odds of serious illness O.0005 TEST P How old is the child? How old is the child? [High risk age group] Post-testP odds of serious illness O.005 How worried are the parents? TEST Q Post-testQ odds of serious illness O.5
The illness script theory • We start by learning detailed rules about the cause, course and treatment of diseases • As we gain knowledge we convert these rules to stereotypical stories (‘scripts’) • We refine our knowledge by accumulating atypical and alternative stories via experience and the oral tradition (grand rounds etc) • Knowledge is stored in our memory as stories
Illness scripts: chicken pox visit • “My febrile child should stay indoors.” • “I think my child has meningitis.” • “This is the first ever illness in my first baby” • “My husband has got the car and I’m at home with the 3 kids.” • “My husband and I are both working and it’s not convenient to take time off.”
Illness scripts: chicken pox visit • “My 15 year old daughter definitely has chicken pox. I’ve seen chicken pox in my other kids and this is different. I think my daughter is going to die.” DOES NOT FIT KNOWN ILLNESS SCRIPT
A patient with chicken pox I didn’t go for my swim. I didn’t even stop for a bath or breakfast. I drove straight to the house, where all the lights were off. The father, dressed in Orthodox Jewish style complete with long black coat and hat, came out to meet me and apologised that the lights were on a time switch which he could not override. I got a torch out of the car boot.
There were 14 relatives in the room, lined up in silence. All the siblings had been woken up and were standing staring at me.
Narrative drama • Consulting room is a ‘stage’ • The illness story is not told but enacted • The patient’s performance is the clue to diagnosis Cheryl Mattingly. ‘Healing Dramas and Clinical Plots: The Narrative Structure of Experience’. New York: Cambridge University Press, 1998.
On examination by torchlight, the child was conscious and co-operative, and had a typical chicken pox rash.
She was post-pubescent and somewhat overweight. Her BP was 90/50 and pulse 100. She was possibly overbreathing (we all were). She said she couldn’t get up, or even sit up.
On direct questioning, she said “I just don’t feel well. Maybe I’m a bit faint. No, I haven’t fainted or blacked out but it’s muzzy and I feel quite scared that something’s wrong.”
I examined her respiratory system. She had a respiratory rate of 20 and no focal signs. That was a shame because I was hoping there would be.
I found no other physical signs. So I decided to lie about the chest findings. I admitted her to Coppetts Wood Hospital by blue light ambulance.
As I left the room, the father thanked me profusely for saving his daughter’s life.
A patient with chicken pox We didn’t hear anything for a month, and then got a discharge summary to say the child had had chicken pox with disseminated intravascular coagulation. The child had initially been admitted to Intensive Care for 5 days. The parents had been told she was lucky to have survived
Hospital medicine “Distinguishing the clear message of the disease from the interfering noise of the patient as a person.” Marshall Marinker. ‘The mythology of Hilda Thompson’. In Greenhalgh T and Hurwitz H (eds) ‘Narrative Based Medicine’. London: BMJ Books, 1998
Primary care at the interface “Inferring the indistinct signal of serious disease from the complex, fuzzy and largely unclassifiable ‘noise’ made by the patient and the family in their cultural setting.” Trish Greenhalgh RFH Grand Round, January 2003
A note on stories "Neither biology nor information science has improved upon the story as a means of ordering and storing the experience of human and clinical complexity. Neither is it likely to." Kathryn Montgomery Hunter ‘Doctors' stories - the narrative structure of medical knowledge’. Princeton: Princeton University Press, 1991
A note on stories Story = Actors + Setting + Plot + Trouble + Surprise Kenneth Burke 1945 “A grammar of motive” [after Aristotle 528]
A note on stories Medical students learn to “take a history” – i.e. to distort and sanitise the illness narrative to fit a standardised formula. B and M-J Good. ‘Fiction and historicity in doctors’ stories’. In Mattingly C and Garro L. ‘Narrative and the cultural construction of illness and healing.’ Berkeley: University of California Press, 2000
Conclusion: Stories and Bayes GPs may be alert to subtle aspects of the patient’s narrative (including the enacted drama of the acute illness). These hunches, which draw on personalised and contextualised tacit knowledge about the patient, and the accumulated ‘illness scripts’ of professional experience, can be articulated through dialogue Hospital doctors who don’t take the hunches of experienced GPs as “evidence” may be missing a trick
PS: The fascinoma paradox • Doctors learn to manage common problems by discussing uncommon ones • “When you hear hoofbeats, don’t think zebras” Kathryn Montgomery Hunter. “Don’t think zebras”: uncertainty, interpretation, and the place of paradox in clinical education. Theoretical Medicine 1996; 17: 225-241
A story from general practice • TG – a locum GP • Mrs Christine Morgan – a bank clerk
