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Medical Statistics Joan Morris (j.k.morris@qmul.ac.uk) Professor of Medical Statistics Goldsmiths Lecture 2014
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Aims To give a brief description of some different areas of medical statistics –Folic acid and Neural Tube Defects –Screening for Heart Disease
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Folic Acid and Neural Tube Defects
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Can folic acid reduce neural tube defects (e.g. spina bifida) ? MRC Vitamin trial - randomised controlled trial
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Randomised Controlled Trial A clinical trial is an experiment in which a treatment is administered to humans in order to evaluate its efficacy and safety Controlled = a comparison group Randomised = allocated to groups on basis of chance e.g. tossing a coin (ensures fair comparison)
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Can folic acid reduce neural tube defects (e.g. spina bifida) ? MRC Vitamin trial - randomised controlled trial Large: 1817 women who had had a previous NTD, 33 centres, 7 countries
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Folic Acid vs Placebo for Neural Tube Defects Lancet 1991 Neural Tube Defects YesNoTotal Folic Acid Yes6587593 No21581602 Risk of NTD in treated group = Risk of NTD in control group = Relative Risk of NTD in treated group compared to control group = 1% 3.5% 1%/3.5% = 0.29
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Folic Acid vs Placebo for Neural Tube Defects RR = 0.29 95% Confidence Interval : 0.10 to 0.76 P = 0.008
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Can folic acid reduce neural tube defects (e.g. spina bifida) ? Results : Women who did not receive folic acid were 3 times more likely to have a second NTD pregnancy Impact : Women are advised to take folic acid PRIOR to becoming pregnant
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Statisticians Involvement Planning the study – how large Analysing the results Stopping the study early (Independent Data Monitoring Committee)
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What Dose ? Women in MRC trial had had a previous NTD pregnancy and were given 4mg folic acid per day Current recommendation is 0.4mg folic acid per day
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Dose Folic Acid Serum Folate Level Risk of NTD pregnancy ?
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Dose Folic Acid Serum Folate Level Risk of NTD pregnancy
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Folic Acid and NTD Dose Response
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Interpretation The same proportional increase in serum folate has the same proportional reduction in NTD All women benefit from taking folic acid. There is not a threshold effect
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Conclusions Women planning a pregnancy should take 5mg folic acid tablets daily, instead of the 0.4mg dose presently recommended (THE LANCET Vol 358 December 15, 2001)
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MRC Trial
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Fortification (0.2mg/day)
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Use of Statistics in Screening Screening is the identification, among apparently healthy individuals, of those who are sufficiently at risk from a specific disorder to benefit from a subsequent diagnostic test, procedure or direct preventive action. Screening for Heart Disease
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Relative odds of major IHD event by fifths of the distribution of haemostatic and lipid markers for all men (——) and for men free of IHD at baseline examination ( ∘ ––– ∘ ). Yarnell J et al. Eur Heart J 2004;25:1049-1056 The European Society of Cardiology
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AffectedUnaffected Biomarker : ZZ
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AffectedUnaffected Screen negative Screen positive Biomarker : ZZ
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Screen negative Screen positive Biomarker : ZZ False positives False negatives
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Risk Factor Unaffected Affected Good test Screening for a medical disorder
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Risk Factor Unaffected Affected Poor test Screening for a medical disorder
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Is Cholesterol any good for screening ? AffectedUnaffected Risk screen converter screen http://www.wolfson.q mul.ac.uk/rsc/
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Detection Rate False Positive Rate
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4.2mm Hg
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7.5mm Hg
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Are there any good screening tests ? Antenatal screening for Down’s syndrome
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Quadruple test markers Total hCG Inhibin-A AFP uE 3 Down’s syndrome Unaffected Down’s syndrome Unaffected Down’s syndrome Unaffected
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1:10 8 1:10 6 1:10 4 1:10 2 1:1 10 2 :1 10 4 :1 Down’s syndrome Unaffected Distribution of risk in Down’s syndrome and unaffected pregnancies using AFP, uE3, total hCG and inhibin-A measured at 14-20 weeks (+ maternal age) Risk of a Down’s syndrome pregnancy at term
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Method : Monte Carlo Simulation Generate a population of 500,000 people aged 0-89 years. [Use Office for National Statistics Population Data for England and Wales] Assign risk factors (eg diabetes, smoking, blood pressure) [Use Health of the Nation Survey] Calculate a persons risk [Use Framingham risk equations] Assign deaths according to people’s risks
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Conclusion Age is as good at predicting heart disease as measuring conventional risk factors Therefore treatment should be offered on the basis of age
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Treatment to Prevent Heart Disease Blood Pressure Lowering Drugs –What dose –Which drug
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Several studies looking at the same thing Each study may be relatively inconclusive because of too much uncertainty (too small) Meta-analysis : statistical method of combining and presenting results from several studies Can indicate more robust results
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Blood pressure reduction (mmHg)
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Major influence for prescription of combination therapy as first line of action 1 Drug Standard dose 3 Drugs Half standard dose 1 Drug Standard dose 3 Drugs Half standard dose 7 mm Hg 20 mm Hg10% 4% Reduction in blood pressure People reporting side effects
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BMJ 2009;338:b1665
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A reduction in blood pressure of 20mm Hg halves the risk of a CHD event or stroke regardless of the person’s original blood pressure or their level of cardiovascular risk. This means that everyone at sufficient cardiovascular risk will benefit from a reduction in blood pressure, even if they don’t have a high blood pressure. For example all people with diabetes should be offered treatment. BMJ 2009;338:b1665
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Involvement of Statistician Study design for clinical trial Analysing data from clinical trial Meta analysis from several trials Monte Carlo simulation using results above
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Conclusion As much about collection, interpretation and presentation as calculation Making sense out of uncertainty
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