Bayesian decision making in primary care or how to stop people dying of chicken pox

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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

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Before we startWhat is primary health care?
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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

4
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

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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

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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

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Case historyA patient with query chicken pox
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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.

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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.

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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?

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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 cant
  it?
• Cecilia Gould
• Crouch End Surgery coffee break, July 1989

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Bayesian decision-making
Pre-test odds of disease X
TEST Y
Post-test odds of disease X
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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
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Bayesian decision-making

• Assume Disease X Patient is seriously ill

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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
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A patient with chicken pox

• I asked
• 1. How old is the child? Answer 15

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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
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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?

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A patient with chicken pox

• He said
• For one thing, this family have been on my list
  for 17 years and theyve never asked for a visit
  before.
• For another thing, they go to the most orthodox
  synagogue in Golders Green.

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A patient with chicken pox

• And theres one more thing I dont like about
  this case. It wasnt the mother who rang, it was
  the father. In that family, the father never
  does the kids health.

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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.
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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
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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

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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 Im at home with
  the 3 kids.
• My husband and I are both working and its not
  convenient to take time off.

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Illness scripts chicken pox visit

• My 15 year old daughter definitely has chicken
  pox. Ive 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

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A patient with chicken pox

• I didnt go for my swim. I didnt 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.

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There were 14 relatives in the room, lined up in
silence. All the siblings had been woken up and
were standing staring at me.
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Narrative drama

• Consulting room is a stage
• The illness story is not told but enacted
• The patients performance is the clue to
  diagnosis
• Cheryl Mattingly. Healing Dramas and Clinical
  Plots The Narrative Structure of Experience.
• New York Cambridge University Press, 1998.

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On examination by torchlight, the child was
conscious and co-operative, and had a typical
chicken pox rash.
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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
couldnt get up, or even sit up.
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On direct questioning, she said I just dont
feel well. Maybe Im a bit faint. No, I havent
fainted or blacked out but its muzzy and I feel
quite scared that somethings wrong.
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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.
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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.
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As I left the room, the father thanked me
profusely for saving his daughters life.
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A patient with chicken pox

• We didnt 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

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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

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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

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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

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A note on stories

• Story
• Actors
• Setting
• Plot
• Trouble
• Surprise
• Kenneth Burke 1945
• A grammar of motive
• after Aristotle 528

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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

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Conclusion Stories and Bayes

• GPs may be alert to subtle aspects of the
  patients 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 dont take the hunches of
  experienced GPs as evidence may be missing a
  trick

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PS The fascinoma paradox

• Doctors learn to manage common problems by
  discussing uncommon ones
• When you hear hoofbeats, dont think zebras
• Kathryn Montgomery Hunter. Dont think zebras
  uncertainty, interpretation, and the place of
  paradox in clinical education. Theoretical
  Medicine 1996 17 225-241

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Case 2A patient with depression
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A story from general practice

• TG a locum GP
• Mrs Christine Morgan a bank clerk

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ACT ONE
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THE LETTERS
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• Dear Trudy
• Thanks for seeing this 54 year old lady with
  depression. She has recently been through a
  divorce and is losing the family home. She has
  had some suicidal ideation but denies concrete
  plans.
• She works as a bank clerk but is currently off
  sick. She has one daughter who lives locally but
  with whom she has little contact, and a four year
  old grandson. She has been prescribed Prozac
  but I wonder if she is taking it.
• Thanks for seeing her with a view to counselling.
  I have also referred her to Prescription for
  Exercise.

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• Dear Ali
• Thanks for seeing this 54 year old lady with
  depression and mild obesity. She went through a
  divorce recently and now feels the time has come
  to work on her physical shape and meet new
  people.
• She has no physical contra-indications except the
  usual low cardiorespiratory fitness. She is
  taking HRT and an antidepressant. Her blood
  pressure is normal.
• She is off sick right now but when she returns to
  work she will need to fit the sessions in around
  her flexi-time. Im grateful to you for
  organising this.

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ACT TWO(3 weeks later)
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Textbook medical consultation

• Take a history
• Examine the patient
• Order investigations
• Establish a differential diagnosis
• Prescribe treatment
• Refer if indicated

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The narrative approach

• Takes a holistic view of the problem
• Sees illness as part of a life story
• Places the patient as narrator subject
• Uses the storytelling (and listening) as part of
  the treatment
• The doctors role is partly to suggest
  alternative storylines

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A STORYLINE HYPOTHESIS
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Her marital relationship
Her daughter and grandson
Her physical body
Her friends
ACT ONE A fragmented, inconsistent, unfinished,
unhappy story
Her leisure activities
Her work
Dear Trudy letter
Dear Ali letter
STORYLINE OPTION Psychiatric illness
STORYLINE OPTION Shaping up and meeting people
ACT TWO The back to work plot
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The referral as a twist in the plot

• Summarises the story so far
• Focuses on some aspects at the expense of others
• Attributes causality to events
• Interprets behaviour and assigns motives
• HENCE
• Changes the direction of the story

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• Dear Trudy
• Thanks for seeing this 54 year old lady with
  depression. She has recently been through a
  divorce and is losing the family home. She has
  had some suicidal ideation but denies concrete
  plans.
• She works as a bank clerk but is currently off
  sick. She has one daughter who lives locally but
  with whom she has little contact, and a four year
  old grandson. She has been prescribed Prozac
  but I wonder if she is taking it.
• Thanks for seeing her with a view to counselling.
  I have also referred her to Prescription for
  Exercise.

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• Dear Ali
• Thanks for seeing this 54 year old lady with
  depression and mild obesity. She went through a
  divorce recently and now feels the time has come
  to work on her physical shape and meet new
  people.
• She has no physical contra-indications except the
  usual low cardiorespiratory fitness. She is
  taking HRT and an antidepressant. Her blood
  pressure is normal.
• She is off sick right now but when she returns to
  work she will need to fit the sessions in around
  her flexi-time. Im grateful to you for
  organising this.

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Case 2 Summary

• Conventional medicine draws a linear and rational
  sequence of history-taking, examination,
  investigation, provisional diagnosis, referral
  and treatment.
• An alternative view is to see the illness as an
  unfinished story. A referral can be a crucial
  twist in the plot, and may offer the patient a
  range of storyline options.

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Thank you for your attention

• Handouts available from Marcia Rigby
• m.rigby_at_pcps.ucl.ac.uk