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Giải quyết các vấn đề lâm sàng: Cơ sở từ y học chứng cứ GiẢNG VIÊN : GS TS BS LÊ HOÀNG NINH. Nội dung bài học. Mục tiêu Các kỹ năng cần của y học chứng cứ trong thực hành chăm sóc bệnh nhân: Kỹ năng đặt câu hỏi đúng về tình huống lâm sàng của bệnh nhân - PowerPoint PPT Presentation
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Giải quyết các vấn đề lâm sàng: Cơ sở từ y học chứng cứ
GiẢNG VIÊN : GS TS BS LÊ HOÀNG NINH
Nội dung bài học
• Mục tiêu • Các kỹ năng cần của y học chứng cứ
trong thực hành chăm sóc bệnh nhân:– Kỹ năng đặt câu hỏi đúng về tình huống lâm sàng
của bệnh nhân
– Kỹ năng tìm kiếm các chứng cứ hiện có trên y văn
– Kỹ năng đánh giá các chứng cứ trên y văn
– Kỹ năng ứng dụng chứng cứ trên bệnh nhân của thầy thuốc
Mục Tiêu
• Định nghĩa y học chứng cứ (EBM) • Tại sao thầy thuốc phải dùng y học chứng cứ
– Compare with expert-based medicine – How are we misled by:
• Surrogate outcomes • Personal observation • Pathophysiologic reasoning
• Mô tả các công cụ y học chứng cứ• Xây dựng câu hỏi lâm sàng tốt
Y học chứng cứ là gì?
“ sử dụng chứng cứ tốt nhất hiện có vào thực hành chăm sóc bệnh nhân”
Cái gì là quan trọng khi đọc y vănCái cần có ở y văn là
1. Các kết quả có liên quan tới bệnh nhân của bạn 2. Trả lời được câu hỏi về chăm sóc bệnh nhân mà bạn đang gặp
khó khăn3. Có thể làm bạn thay đổi thực hành chăm sóc bệnh nhân của
bạn4. Là chủ đề mà bạn đang quan tâm theo dõi5. Là cái mà bạn cần biết rõ hơn, chi tiết hơn, cụ thể hơn6. Bạn cần về POEM or DOE
• Patient-oriented evidence ( POEM: bằng chứng hướng tới bệnh nhân) that matters vs disease-oriented evidence ( DOE : bằng chứng hướng tới bệnh )
Y học chứng cứ“Evidence-based medicine (EBM) requires the
integration of the best research evidence with our clinical expertise and our patient’s unique values and circumstances”
EBM, 2006, Straus et al
Y học chứng cứ đòi hỏi lồng ghép bằng chứng tốt nhất với kinh nghiệm lâm sàng và tình
trạng , hoàn cảnh, điều kiện thực của bệnh nhân
Giá trị của việc học EBM:
( một thử nghiệm ngắn hạn) • Một thử nghiệm có nhóm chứng về giảng dạy đánh giá y
văn được thực hiện trên sinh viên y khoa• Nhóm thử nghiệm được học với thầy hướng dẫn đã qua
khóa huấn luyện lâm sàng về:– Đánh giá các thử nghiệm lâm sàng– Đánh giá các bài báo về test chẩn đoán và điều trị
• Nhóm chứng được học với các thầy bình thường không qua các khóa huấn luyện kể trên
Bennett et al. JAMA. 1987;257:2451-2454.
Giá trị việc học EBM: ( một thử nghiệm ngắn hạn tt)
• Sinh viên nhóm thử nghiệm có quyết định chẩn đoán và điều trị đúng tốt hơn và họ có thể lập luận, bình luận trước khi ra các quyết định của họ
• Những sinh viên trong nhóm chứng thường ra các quyết định không đúng trong chẩn đoán và điều trị.
• Sinh viên trong nhóm chứng thường dễ chấp nhận những đề nghị từ những nhân vật có thẩm quyền.Bennett et al. JAMA. 1987;257:2451-2454.
The Patient
• Patient is a 27-year-old woman with severe right lower quadrant pain. – initial peri-umbilical pain x 2 days migrating yesterday to
current site.
• Loss of appetite. No vomiting, diarrhea; no bowel movement
• no known infectious exposure/
suspicious ingestions, or recent travel
Hướng dẫn truyền thống trong thực hành y khoa
• Pathophysiology and pharmacology– Foundation of medical practice– Do what “makes sense”
• Expert opinion– In training: learning at the bedside from the master clinician– In practice: lectures and seminars with thought leaders
• Clinical experience– Successes, outcomes, and adverse events
in our own practice
Lồng ghép chứng cứ
• Kinh nghiệm lâm sàng:– Trải nghiệm – Cân nhắc, xem xét
• Bối cảnh bệnh nhân– Chất lượng cuộc sống – Chi phí – Những yếu tố khác…
Chứng cứ hiện có tốt nhất
Kinh nghiệm
lâm sàng
Hiện trạng và Bối cảnh bệnh nhân
Chất lượng cuộc sống
Chi phí
ĐiỀU TRỊ VÀ CHẨN ĐOÁN
Vấn đề của bệnh nhân
• Bệnh nhân nử 27 tuổi đau bụng dữ dội vùng bụng ¼ phải , dưới.
– initial peri-umbilical pain x 2 days migrating yesterday to current site.
• Biếng ăn
• Không nôn mửa, tiêu chảy, không có nhu động ruột
• Không rõ tiếp xúc với nguồn nhiễm trùng, suspicious ingestions, or recent travel
Xét Nghiệm
VS BP 120/78 P 16 T 39
Chest CTA. CV RRR s M/R/G
ABD: NML exam x decreased bowel tones and definite right lower quadrant pain, specifically at McBurney’s point.
no heptomegaly nor splenomegaly (enlarged liver or spleen). She has no rebound pain or involuntary
chuẩn 5 “A”
1. Ask the right question
2. Acquire the evidence
3. Appraise the evidence
4. Apply the evidence
5. Assess its impact
Concern:
• Case discussion: 27 year old woman with right lower quadrant (RLQ) abdominal pain
• Background information available from textbooks-– What typically presents as RLQ pain
– What is the clinical course of the different diagnoses
– Specifically, what is typical presentation of appendicitis
• Foreground information– How good is a CT scan for appendicitis?
Đặt Câu Hỏi Lâm Sàng
Câu Hỏi Lâm Sàng• Câu hỏi lý tưởng:
– Focused enough to be answerable
– Pertinent to clinical scenario
– Framed as Population receiving an Intervention (test or treatment) [as Compared to other test/treatment or placebo] associated with Outcome (disease or improvement)
PICOS
P roblem/population
I ntervention
C omparison
O utcome
S tudy design
Examples of tough questions
• Should I screen men for prostate cancer?
• Who is a good candidate for hormone replacement therapy?
• Are angiotensin receptor blockers now first-line for hypertension?
Examples of better questions• Would a PSA test reduce mortality in a 65 year-old
asymptomatic man? • What is the reduction in fracture risk associated
with hormone replacement therapy? • Is losartan more effective than atenolol at
preventing cardiovascular events in middle-aged hypertensive diabetic women?
PICOS
PICOS for confirmatory diagnosis of appendicitis
P: 27 year old woman with symptoms suggestive of appendicitis
I: CT Scan
C: Ultrasound
O: Accurate diagnosis without undue delay
S: ??
Hệ quả quan trọng• Hệ quả hướng tới bệnh nhân:
outcomes patients actually care about – Death (overall or disease-specific)– Heart attacks, strokes, amputations, bed sores, broken hips, renal
failure, etc.– Ability to perform activities of daily living
Versus• Hệ quả hướng tới bệnh:
– Biochemical, physiologic, pharmacologic, or laboratory measures
Comparing DOE and POE
Shaughnessy AF, Slawson DC. Getting the Most from Review Articles: A Guide for Readers and Writers. American Family Physician 1997 (May 1);55:2155-60.
ExampleDisease-Oriented
Evidence
Patient-Oriented Evidence that
Matters CommentAntiarrhythmic Therapy
Drug X PVCs on ECG
Drug X increases mortality
POE contradicts DOE
Type 2 Diabetes Aggressive Tx with insulin or oral agentscan keep BS low
Aggressive Tx does not reduce mortality or prevent most complications
POE contradicts standard teaching
ProstateScreening
PSA screening detects prostate cancer early
Does PSA screening mortality?
DOE exists, but POE is unknown
Background versus foreground information
• Case discussion: 27 year old woman with right lower quadrant (RLQ) abdominal pain
• Background information available from textbooks-– What typically presents as RLQ pain
– What is the clinical course of the different diagnoses
– Specifically, what is typical presentation of appendicitis
• Foreground information– How good is a CT scan for appendicitis?
Steps of EBM-5 A’s
• Ask• Acquire• Appraise• Apply• Assess
“Finding Evidence”: Sources (I)
• Primary research database (articles)– PubMed (aka MEDLINE), Pyschlit, CCTR
• Secondary research databases (synthesis)– Cochrane Library, Clinical Evidence, InfoPOEMS,
UpToDate
• Tertiary resources (meta search engines, databases of databases)– TRIP+ (Translating Research Into Practice),
PrimeEvidence
“Finding Evidence”: Sources • PubMed
– 16 million peer reviewed biomedical articles indexed (note can use PubMed limits to search on particular populations, study types, etc.)
• Cochrane Library– ~3000 clinical systematic reviews (gold standard database)
• Clinical Evidence– ~2500 tsystematic reviews of treatment classified by likelihood of benefit
• InfoPOEMS (www.infopoems.com)– ~3000 regularly updated entries, Patient Oriented Evidence the Matters
(POEM), 100+ journals monitored• UpToDate
– 70,000 pages, evidence based clinical information resource, ~3000 authors, 350+ journals monitored, peer reviewed
• TRIP+– Meta-search of 55 sites of evidence based information
“Finding Evidence”: Searching1. Chuyển câu hỏi lâm sàng thành câu hỏi đúng dễ tìm y văn (e.g. PICOS)
2. Chọn nguồn dữ liệu mà bạn muốn tìm (e.g. PubMed)
3. Áp dụng bộ lọc để khu trú y văn cần tìm (e.g. PubMed limits linked to PICOS
such as gender, age, study type limits)
4. Đánh giá kết quả (e.g. using systematic review worksheet)
5. Xem xét xem liệu bạn có đủ thông tin để ra quyết định không
6. Nếu chưa đủ bạn phải đi lại các bước 1-3 cho đến khi bạn có được câu trả lời
hoặc quyết địnhlà không đủ chứng cứ hoặc có đủ chứng cứ để ra quyết định
ĐÁNH GIÁ CHỨNG CỨ
Assess the Evidence• Kết quả là có giá trị?
– Validity is defined as relative không có sai lệch hệ thống và yếu tố gây nhiễu• Kết quả gì?
– Hệ quả là gì và được đo lường bằng cách nào?– Độ lớn của hệ quả ?– Các Kết quả có ý nghĩa thống kê ?
• Kết quả áp dụng trên bệnh nhân được không?– Does my patient resemble those in the study?– Were all outcomes relevant to my patient evaluated?– Are there other factors (eg, cost, availability) that limit applicability to my
patient?
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001
Đánh giá chứng cứ (t.t)
• Phân biệt nghiên cứu quan sát và thực nghiệm observational and experimental studies
• Phân biệt 2 major study designs (randomized controlled trial and cohort study) :– How the study is designed– Advantages and disadvantages of design– How to assess validity– How to assess results– How to assess applicability
Nghiên cứu thực nghiệm và nghiên cứu quan sát
• In experimental studies, the investigator controls subjects’ exposure to intervention
– Example: randomized controlled trial (RCT)
• In observational studies, investigator does not control the exposure; it occurs naturally or is initiated by patients or their physicians
– Examples: cohort study, case-control study
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
RCTs
Generally held to be the optimal methodology for determining benefit or harm
Eligible Patients
Randomization
Treatment
Control Outcome
Outcome
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
RCTs: ích lợi
• Treatment and control groups are likely to have similar distribution of known and unknown prognostic factors (potential confounders)
• Outcomes are determined prospectively in a standardized, systematic fashion
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
RCTs: Disadvantages
• Costly to perform• Size limitations make detection of rare events
difficult (eg, adverse medication effects) • Eligibility restrictions may reduce applicability to
real patients• Cannot be ethically performed if exposure is
expected to cause harm (eg, smoking)
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
RCTs: Disadvantages• Costly to perform• Size limitations make detection of rare events
difficult (eg, adverse medication effects) • Eligibility restrictions may reduce applicability to
real patients• Cannot be ethically performed if exposure is
expected to cause harm (eg, smoking)
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Đánh giá giá trị các RCTs• Was randomization concealed?• Were patients analyzed in groups to which they were
randomized?• Were patients in treatment & control groups similar with
respect to prognostic factors?• Were patients, clinicians, outcome assessors, and data
analysts aware of allocation?• Were groups treated equally?• Was follow-up complete?Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice . Chicago, IL: American
Medical Association; 2001.
Đánh giá kết quả RCT• Magnitude of result: How large was the treatment effect?
– Relative risk and odds ratio– Absolute risk reduction and number needed to treat (NNT)
• Statistical significance– P value– Confidence interval: How precise was estimate of treatment
effect?• Clinical significance
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Calculating the Risk Ratio and Number Needed to Treat (NNT)
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Treatment(n = 1000)
Control(n = 1000)
100 have the outcome
120 have the outcome
Risk = 0.1, or 10%(100/1000)
Risk = 0.12, or 12%(120/1000)
Risk ratio = 0.1/0.12 = 0.83, or 83%
Absolute risk reduction = 0.12 - 0.1 = 0.02, or 2%
NNT = 1/0.02 = 50
Đánh giá tính ứng dụng RCT
• Were the study patients similar to my patient?– Eligibility criteria– “Table 1” data (baseline characteristics)
• Were all clinically important outcomes considered?• Are the likely treatment benefits worth the potential
harm and costs?Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago,
IL: American Medical Association; 2001.
Cohort Studies
• Similar to RCTs, except that assignment to intervention is not random
Eligible Patients
Choice or Happenstance
Exposed
Not Exposed
Outcome
Outcome
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Lợi ích nghiên cứu đoàn hệ • Outcomes are determined prospectively in a
standardized, systematic fashion• Often includes a larger, more diverse population
than those eligible for or included in RCTs• Can be used to assess effects of harmful exposures
(eg, smoking)
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001
Hạn chế nghiên cứu đoàn hệ
• Costly to perform• Size limitations make detecting rare events
difficult• Exposure and control groups are likely to differ in
factors that may affect outcomes• Control of confounding through statistical analysis
may be inadequateGuyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Đánh giá giá trị nghiên cứu đoàn hệ
• Were the exposed and control groups similar in all known determinants of outcome? – Did the analysis adjust for potential differences?
• Were the outcomes measured in the same way in the groups being compared?
• Was follow-up sufficiently complete?
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Đánh giá kết quả NC Đoàn Hệ
• How strong is the association between exposure and outcome?– Risk ratio or odds ratio– Absolute risk increase or number needed to harm (NNH)
• Statistical significance– P value– Confidence interval: How precise was estimate of risk?
• Clinical significance
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Đánh giá tính ứng dụng Cohort Study
• Were the study patients similar to the patient under consideration in my practice?
• Should I attempt to stop the exposure?
Guyatt et al. Users' Guides A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001 to the Medical Literature:.
Nghiên cứu bệnh-chứng
• In contrast to RCTs and cohort studies, participants are selected based on the presence of the outcome rather than the exposure
• Exposure status is determined retrospectively
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Case-Control Studies: Design
Select Subjects: Cases (diseased)
Controls(nondiseased)
Exposed Not ExposedObserve: Exposed Not Exposed
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Lợi ích Case-Control Studies
• Much more efficient for investigation of rare outcomes
• Take less time to perform than RCTs or cohort studies
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Hạn chế Case-Control Studies
• Retrospective assessment of exposure may be inadequate (recall bias)
• Can be performed only after outcomes have occurred (ie, after damage has already occurred)
• Selection of appropriate controls may be difficult• Control of confounding through statistical analysis may
be inadequate
Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.
Các bước EBM-5 A’s
• Ask• Acquire• Appraise• Apply• Assess
Applying EBM
Clinical Expertise
Patient Values and Preferences
Quality of Life
Costs
Best Available Evidence
Xem xét kết quả nghiên cứu điều trịVALIDITY• Clearly focused question? • Randomization• Blinding- subjects, providers, investigators• Groups similar at start and treated the same throughout?• Followed in randomized groups and accounted for at end? (intention to treat)• Enough subjects to minimize chance differences?
REUSLTS AND PRECISION1. What are results? How presented?2. Certainty & precision? (95% CI’s)
APPLICABILITY1. Can the results be applied to my patient?2. All important outcomes addressed?3. Should there by change in policy?
“Therapy”: Intention to treat
• Subjects are analyzed in the groups they were randomized to.– Maintains randomization– Better reflects real world outcomes– Measures efficacy (“Will this work?”)– Detects issues about intervention other than
effectiveness “In the best possible circumstances, do they work?”
“Therapy”: Bias
• Randomization helps lessen patient bias
– Self-selection
• Blinding helps lessen patient and investigator bias
“Điều trị ”: kết quả thế nào ?
• RR OR RRR
• ARR
• NNT / NNH
• P value/ CI
• Clinically significant?
“Điều trị”: Diễn đạt các kết quả
Risk = outcome event rate
= number having event number receiving the intervention
Relative risk = risk in intervention group(RR) risk in control group
Relative risk reduction (RRR) = 1 - RR
“Điều trị ”: Diễn đạt các kết quả
Absolute risk reduction (ARR) = difference in risk (control –
intervention)
“Diễn đạt các Kết quả
Number-needed-to-treat (NNT) = 1/ARR
NNT: là số bệnh nhân cần được điều trị nhằm ngăn ngừa một biến cố, một hệ quả có thể xảy ra trong một thời khoảng nhất định nào đó
“Thí Dụ về N. C Điều Trị
• Một thử nghiệm điều trị bệnh ung thư bằng một loại thuốc mới , sau 4 năm theo dõi cho thấy tử vong như sau:
• Nhóm thử nghiệm: 30%• Nhóm chứng : 50%• Tính RR, RRR, ARR, NNT?
“ Thí dụ .NC Điều Trị”
RR = risk of death in experiment/control groups
= 30%/50% = 0.6 or 60%
RRR = 1 - RR = 1-0.6 = 0.4 or 40%
ARR = risk of death in control – experimental groups
= .50 -.30 = 0.2 or 20%
NNT =1/ARR = 1 ÷ 0.2
= 5 bệnh nhân cần điều trị bằng thuốc mới để ngừa tử vong trong 4 năm
“Therapy”: Relative Versus Absolute Benefits
• Consider – July 3, 2002: Worldcom stock rose 120%
(relative increase)– The stock rose from: $0.10 $0.22 (absolute
increase)
“NC Điều Trị”: Giảm Nguy Cơ tương đối –Giảm nguy cơ tuyệt đối
• Baseline risk 10/100 5/100 RRR = 50% ARR = 5% NNT = 20
• Baseline risk 1/100 0.5/100RRR = 50% ARR = 0.5% NNT = 200
• Baseline risk 0.1/100 0.05/100RRR = 50% ARR = 0.05% NNT = 2000
“Therapy”: RRR Lipid Trials 4S WOSCOPS CARE AFCAPS
for acute myocardial infarction
RRR 27 31 25 40(%)NNT 19 42 40 435(5 year)
“NC Điều Trị”: Khoảng Tin Cậy 95% • Any statistic only an estimate of the “true value” of that
statistic. • Confidence Interval (CI) gives range within which that “true
value” probably lies. • 95% CI - if we repeated the experiment with similar
populations an infinite number of times, the results would fall within the CI 95% of the time. 95% certain that the “true value” will fall within the 95% CI range.
• CI gives us an idea of the precision of the result, since the narrower the CI is, the more certain we can be that the experimental value is close to the “true value”.
• And, generally, the larger the sample size, the narrower the CI
• CI =idea of significance, e.g.– If the 95% CI for the ARR includes 0, no difference between the
experimental and control groups.
– If the 95% CI for the RRR or Odds Ratio includes 1, no difference between the experimental and control groups.
• Similar to P values (e.g., P<0.05) =statistically significant
• CI gives a sense of the size of the differences found in the study.
• e.g., research study - 50% of patients treated with Drug A are cured, compared with 45% of patients treated with Drug B.
• ARR I s thus 5%.• Statistical analysis P<0.001, statistically significant.• But if 95% CI of ARR is 0% to 10%, indicates result is not clinically
significant (includes “0%” - “no difference”).
“NC Điều Trị”: ý nghĩa thống kê
ý nghĩa lâm sàng
• Are the results clinically important?
– Duration of pharyngitis: 8.1 days to 7.4 days
– Weight: 279 lbs to 266 lbs after 3 months
– Survival increased from 4.5 mos to 5.2 mos with 100% mortality at
12 months
– Claudication: Increase in walking distance by 34 ft.
“Bình Luận về Chẩn Đoán”: Bộ Công cụ
TÍNH GIÁ TRỊ :• Câu hỏi có tập trung, rõ ràng không? • Chuẩn có phù hợp không?• Chuẩn tham khảo & test áp dụng cho mọi đối tượng không? (verification
bias)• Kết quả có bị ảnh hưởng bởi sự giải thích kết quả không? (review bias)• Tình trạng bệnh được váo cáo và các thay đổi có không? (spectrum bias)• Phương pháp kiểm có đủ chi tiết để làm lại được không?
KẾT QUẢ VÀ TÍNH CHÍNH XÁC1. Kết quả gì?2. Có chắc và chính xác không ? Certainty & precision? (95% CI’s)
TÍNH ƯNG DỤNG1. Kết quả có áp dụng cho bệnh nhân khác không?2. Nguồn lực địa phương có đủ để triển khai những kết quả nầy không?
“Chẩn đoán ”: Bảng 2X2 Các đặc trưng của test chẩn đoán
• Sensitivity
• Specificity
• Predictive Value
• Likelihood Ratios
DIAGNOSTIC TEST
D I S E A S EPresent Absent TOTALS
Test positive
True
Positive
False
Positive
All positive
Test negative
False
Negative
True
Negative
All negative
TOTALSAll with
disease
All without disease
Entire population
T
E
S
T
“Diagnosis”: What are the Results?
(+) (-)
(+)a b
(-)c d
D I S E A S E
T
E
S
T
“Diagnosis”: What are the Results?
Pt has diseaseDz (+)
Dz (-)
Test(+)a b
Test(-)c d
“Diagnosis”: What are the Results?
Dz (+) Pt does not have disease Dz (-)
Test(+)a b
Test(-)c d
“Diagnosis”: Sensitivity
Sensitivity is proportion of people with disease who have a positive test
Dz (+) Dz (-)
Test(+)a b
Test(-)c d
Sensitivity = (a/a+c) =(TP/TP+FN)
TP
FN
“Diagnosis”: What are the Results?
Dz (+) Pt does not have disease Dz (-)
Test(+)a b
Test(-)c d
“Diagnosis”: Specificity
Specificity is proportion of people without disease who have negative test
Dz (+) Dz (-)
Test(+)a b
Test(-)c d
Specificity = (d/b+d) =(TN/FP+TN)
FP
TN
“Diagnosis”: Tradeoffs of sensitivity & specificity labeling diabetes
Blood sugar
• 70• 100• 130• 160• 200
Sensitivity Specificity
98.6% 8.8%88.6% 69.8%64.3% 96.9%47.1% 99.8%27.1% 100%
“Diagnosis”: Choosing a test
• SnNout-
A sensitive test, if negative, rules out a disease
• SpPin-
A specific test, if positive, rules in a disease
“Diagnosis”: Sensitivity & Specificity
• Useful for picking a test (test properties)– Screening- prefer sensitive test– Diagnosis – prefer specific test
• Less help in making diagnosis
“Diagnosis”: What are the Results?
• In patients, what you know are their test results- you are trying to determine whether they actually have the disease.
• Positive Predictive Value :
– Of all who tested positive for a disease, the proportion that actually has it
• Negative Predictive Value :
– Of all who tested negative for a disease, the proportion that actually does not have it
“Diagnosis”: What are the Results?
(+) (-)
(+)a b
(-)c d
D I S E A S E
T
E
S
T
“Diagnosis”: Positive Predictive Value
Proportion of people with a positive test who have a disease
PPV = a/a+b=
TP/TP+FP
Dz (+) Dz (-)
Test(+) TPa
FPb
Test(-)c d = true positives
over all positives
“Diagnosis”: Negative Predictive Value
Proportion of people with a negative test who don’t have a disease
NPV= d/d+c=
TN/TN+FN = true negatives
over all positives
Dz (+) Dz (-)
Test(+)a
FPb
Test(-) FNc
TNd
“Diagnosis”: What are the Results?
PPV dependent on prevalence, even when using the same test
• Example: Prevalence of a particular disease in a population is 50%. Sensitivity= 90% Specificity= 95%
PPV: = a/a+b
= 95%
Dz (+) Dz (-)
Test(+)a b
Test(-)c d
100 100 200
90
10 95
5
“Diagnosis”: PPV & prevalence
PPV dependent on prevalence using same test
• Example: Prevalence of a particular disease in a population is 5%. Same test, same sensitivity & specificity– Sensitivity= 90% Specificity= 95%
PPV: = a/a+b
= 47%
Dz (+) Dz (-)
Test(+)a b
Test(-)c d
10 190 200
9
1 180
10
“Diagnosis”: PPV & NPV
• Useful for diagnosis– Probability of disease after (+ ) or (–) test
• Drawbacks:– Sensitive to prevalence of disease– Prevalence of disease in general population may not be
the same as that of patients you see in clinic/ER.– Not all test results can be categorized as “+” or “-”.
For these reasons, some consider PPV & NPV “Old School”.
“Diagnosis”: What are the Results?
Likelihood Ratios
• Likelihood Ratio is how much more likely is it that someone with this finding has the disease, compared to someone who doesn’t. It does NOT vary with prevalence.
Technically, the + LR is how much more likely someone is to get any positive test result if they have disease, compared to someone who doesn’t.
“Diagnosis”: Likelihood ratio
LR = SENSITIVITY 1 - SPECIFICITY
“Diagnosis”: What do all the numbers mean?
The Likelihood Ratio is a diagnostic weight;
It tells you by how much a given diagnostic test result will raise or lower the probability of having the disorder.
Pretest Probability: the chance that the pt has disease, prior to ordering any tests. This is often an estimation based on clinical experience
Post-test Probability: the chance that the pt has disease, given the results of the test
“Diagnosis”: What are the Results?
What do all the numbers mean?
A LR of 1.0 means the post-test probability is exactly the same as the pretest probability.
A LR >1.0 increases the probability of having the disorder.
A LR<1.0 decreases the probability of having the disorder.
“Diagnosis”: What are the Results?
Likelihood ratios >10 or <0.1 generate large changes from pre- to post-test probability and are generally considered significant.
Strong evidence to rule in/rule out a diagnosis.
Likelihood ratios of 5-10 and 0.1-0.2 generate moderate changes in probability.
Moderate evidence to rule in/rule out a diagnosis.
Likelihood ratios of 2-5 and 0.2-0.5 generate small changes.
Minimal evidence to rule in/rule out a diagnosis
Likelihood ratios 0.5-2 usually have little effect
Figure 1a: Likelihood Ratio Nomogram
LRs = Diagnostic Weights
-45%0.1
-30%0.2
-15%0.5Values between 0 and 1 DECREASE probability of disease
01.0
+15%2
+30%5
+45%10Values greater than 1 INCREASE probability of disease
Change in probability
Likelihood Ratio
From Steve McGee, Evidence Based Physical Diagnosis
Table 2a: Likelihood Ratios of Tests for the Diagnosis of Appendicitis
Likelihood ratio
• DVT– Homan’s sign +LR 1.5– Doppler + LR 39
• ANEMIA– Conjunctival rim pallor +LR 16.7
Summary Diagnostic Test
D I S E A S EPresent Absent TOTALS
Positive True
Positive
False
Positive
ALL Positives
Negative False
Negative
True
Negative
ALL Negatives
TOTALSAll with
disease
All without disease
Entire population
T
E
S
T
Sensitivity Specificity
PPV
NPV
Common pitfalls
• Results reported as relative risk – (ex. Migraines, CVA & OC)
• Results that came from recalculating the data after trial was done – (Post-hoc analysis) (ex. Hot study)
• Over-interpreting results• Relying on just one study (ex. Mg for heart dz),
a poor study, or wrong type of study (ex. HRT)• Confusing statistical significance with clinical
significance (ex. Drugs for BPH)• Not looking at CI’s • Not considering who funded study
Steps of EBM-5 A’s
• Ask• Acquire• Appraise• Apply• Assess
Core of EBP
“Supposing is goodbut finding out
is better.”
Mark Twain