Mục tiêu Mục tiêu Các kỹ năng cần của y học chứng cứ trong thực hành chăm sóc bệnh nhân



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Chuyển đổi dữ liệu13.11.2018
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Mục tiêu

  • 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


Định nghĩa y học chứng cứ (EBM)

  • Định nghĩa y học chứng cứ (EBM)

  • Tại sao thầy thuốc phải dùng y học chứng cứ

  • Mô tả các công cụ y học chứng cứ

  • Xây dựng câu hỏi lâm sàng tốt



  • “ sử dụng chứng cứ tốt nhất hiện có vào thực hành chăm sóc bệnh nhân”



Cái cần có ở y văn là

  • Cái cần có ở y văn là

    • Các kết quả có liên quan tới bệnh nhân của bạn
    • Trả lời được câu hỏi về chăm sóc bệnh nhân mà bạn đang gặp khó khăn
    • Có thể làm bạn thay đổi thực hành chăm sóc bệnh nhân của bạn
    • Là chủ đề mà bạn đang quan tâm theo dõi
    • Là cái mà bạn cần biết rõ hơn, chi tiết hơn, cụ thể hơn
    • 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 )


“Evidence-based medicine (EBM) requires the integration of the best research evidence with our clinical expertise and our patient’s unique values and circumstances”

  • “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



Một thử nghiệm có nhóm chứng về giảng dạy đánh giá y văn được thực hiện trên sinh viên y khoa

  • 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.



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

  • 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.


Patient is a 27-year-old woman with severe right lower quadrant pain.

  • 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



First, do no harm.

  • First, do no harm.

  • How do we know

  • that we are not?



Pathophysiology and pharmacology

  • Pathophysiology and pharmacology

  • 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


Kinh nghiệm lâm sàng:

  • Kinh nghiệm lâm sàng:

    • Trải nghiệm
    • Cân nhắc, xem xét
  • Bối cảnh bệnh nhân

    • Chất lượng cuộc sống
    • Chi phí
    • Những yếu tố khác…






Bệnh nhân nử 27 tuổi đau bụng dữ dội vùng bụng ¼ phải , dưới.

  • 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



VS BP 120/78 P 16 T 39

  • 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



Ask the right question

  • Ask the right question

  • Acquire the evidence

  • Appraise the evidence

  • Apply the evidence

  • Assess its impact



Case discussion: 27 year old woman with right lower quadrant (RLQ) abdominal pain

  • 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?




Câu hỏi lý tưởng:

  • 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)


P roblem/population

  • P roblem/population

  • I ntervention

  • C omparison

  • O utcome

  • S tudy design



Should I screen men for prostate cancer?

  • 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?



Would a PSA test reduce mortality in a 65 year-old asymptomatic man?

  • 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 for confirmatory diagnosis of appendicitis

  • 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 hướng tới bệnh nhân:

  • 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:





Case discussion: 27 year old woman with right lower quadrant (RLQ) abdominal pain

  • 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?




Primary research database (articles)

  • 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


PubMed

  • 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


Chuyển câu hỏi lâm sàng thành câu hỏi đúng dễ tìm y văn (e.g. PICOS)

  • Chuyển câu hỏi lâm sàng thành câu hỏi đúng dễ tìm y văn (e.g. PICOS)

  • Chọn nguồn dữ liệu mà bạn muốn tìm (e.g. PubMed)

  • 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)

  • Đánh giá kết quả (e.g. using systematic review worksheet)

  • Xem xét xem liệu bạn có đủ thông tin để ra quyết định không

  • 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





Kết quả là có giá trị?

  • 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


Phân biệt nghiên cứu quan sát và thực nghiệm observational and experimental studies

  • 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


  • 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.


Generally held to be the optimal methodology for determining benefit or harm

  • Generally held to be the optimal methodology for determining benefit or harm



Treatment and control groups are likely to have similar distribution of known and unknown prognostic factors (potential confounders)

  • 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.



Costly to perform

  • 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.



Costly to perform

  • 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.



Was randomization concealed?

  • 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.



Magnitude of result: How large was the treatment effect?

  • 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.





Were the study patients similar to my patient?

  • 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.



Similar to RCTs, except that assignment to intervention is not random

  • Similar to RCTs, except that assignment to intervention is not random



Outcomes are determined prospectively in a standardized, systematic fashion

  • 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



Costly to perform

  • 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 inadequate

  • Guyatt et al. Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Chicago, IL: American Medical Association; 2001.



Were the exposed and control groups similar in all known determinants of outcome?

  • 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.



How strong is the association between exposure and outcome?

  • 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.



Were the study patients similar to the patient under consideration in my practice?

  • 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:.



In contrast to RCTs and cohort studies, participants are selected based on the presence of the outcome rather than the exposure

  • 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.





Much more efficient for investigation of rare outcomes

  • 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.



Retrospective assessment of exposure may be inadequate (recall bias)

  • 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.









VALIDITY

  • VALIDITY

  • 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 PRECISION

  • What are results? How presented?

  • Certainty & precision? (95% CI’s)

  • APPLICABILITY

  • Can the results be applied to my patient?

  • All important outcomes addressed?

  • Should there by change in policy?



Subjects are analyzed in the groups they were randomized to.

  • 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?”


Randomization helps lessen patient bias

  • Randomization helps lessen patient bias

    • Self-selection
  • Blinding helps lessen patient and investigator bias



RR OR RRR

  • RR OR RRR

  • ARR

  • NNT / NNH

  • P value/ CI

  • Clinically significant?



Risk = outcome event rate

  • 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



  • Absolute risk reduction (ARR)

  • = difference in risk (control – intervention)



Number-needed-to-treat (NNT) = 1/ARR

  • 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



Một thử nghiệm điều trị bệnh ung thư bằng một loại thuốc mới , sau 4 năm theo dõi cho thấy tử vong như sau:

  • 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?



RR = risk of death in experiment/control groups

  • 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



Consider

  • Consider

    • July 3, 2002: Worldcom stock rose 120% (relative increase)
    • The stock rose from: $0.10  $0.22 (absolute increase)


Baseline risk 10/100 5/100

  • Baseline risk 10/100 5/100

  • RRR = 50% ARR = 5% NNT = 20

  • Baseline risk 1/100 0.5/100

  • RRR = 50% ARR = 0.5% NNT = 200

  • Baseline risk 0.1/100 0.05/100

  • RRR = 50% ARR = 0.05% NNT = 2000



4S WOSCOPS CARE AFCAPS

  • 4S WOSCOPS CARE AFCAPS

  • for acute myocardial infarction

  • RRR 27 31 25 40

  • (%)

  • NNT 19 42 40 435

  • (5 year)



Any statistic only an estimate of the “true value” of that statistic.

  • 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.

  • 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”).


Are the results clinically important?

  • 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.


TÍNH GIÁ TRỊ :

  • 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 XÁC

  • Kết quả gì?

  • Có chắc và chính xác không ? Certainty & precision? (95% CI’s)

  • TÍNH ƯNG DỤNG

  • Kết quả có áp dụng cho bệnh nhân khác không?

  • Nguồn lực địa phương có đủ để triển khai những kết quả nầy không?



Sensitivity

  • Sensitivity

  • Specificity

  • Predictive Value

  • Likelihood Ratios











Sensitivity is proportion of people with disease who have a positive test

  • Sensitivity is proportion of people with disease who have a positive test





Specificity is proportion of people without disease who have negative test

  • Specificity is proportion of people without disease who have negative test



Blood sugar

  • Blood sugar

  • 70

  • 100

  • 130

  • 160

  • 200



SnNout-

  • SnNout-

  • A sensitive test, if negative, rules out a disease

  • SpPin-

  • A specific test, if positive, rules in a disease



Useful for picking a test (test properties)

  • Useful for picking a test (test properties)

    • Screening- prefer sensitive test
    • Diagnosis – prefer specific test
  • Less help in making diagnosis



  • 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 :





Proportion of people with a positive test

  • Proportion of people with a positive test

  • who have a disease

  • PPV =

  • a/a+b=

  • TP/TP+FP



Proportion of people with a negative test

  • 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



PPV dependent on prevalence, even when using the same test

  • 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%



PPV dependent on prevalence using same test

  • 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%



Useful for diagnosis

  • 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”.



Likelihood Ratios

  • 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.



LR = SENSITIVITY

  • LR = SENSITIVITY

  • 1 - SPECIFICITY



  • 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



What do all the numbers mean?

  • 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.



  • 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









DVT

  • DVT

    • Homan’s sign +LR 1.5
    • Doppler + LR 39
  • ANEMIA

    • Conjunctival rim pallor +LR 16.7






Results reported as relative risk

  • 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





“Supposing is good

  • “Supposing is good

  • but finding out

  • is better.”

  • Mark Twain






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